Chapter 1 - General Provisions
Purpose and Scope of Application. Pursuant to Article 85 and Note 1 to Article 86 of the Labour Law of the Islamic Republic of Iran, this Regulation has been drawn up with the aim of making the work environment safe and preventing work-related accidents against hazards arising from electricity, through the earthing system, and for the purpose of (electric shock, fire, and explosion resulting from unsafe conditions of electricity distribution), and ultimately safeguarding the country's human resources and material resources in this sector. For the design, implementation, maintenance, and measurement of the earthing system, this Regulation is mandatory for all employers of workplaces subject to the Labour Law that meet the conditions set out in Chapters 9 through 23 of this Regulation.
Definitions and Terms. In this Regulation, the following definitions and terms are used with the meanings set out below:
1. Insulation Monitoring Device (IMD) / earth fault detector: a device that measures the insulation resistance between a phase conductor and the earth electrode and, if the resistance falls below a specified limit, indicates this by means of visual and audible alarms.
2. Air Termination: part of a lightning (thunderbolt) protection system responsible for intercepting the lightning strike and discharging it to earth.
3. Functional earthing: the connection of the neutral point of a system (neutral) to earth, which causes any potentially faulty circuits to be disconnected; as a result, the system's insulation is preserved and overvoltages are limited, thereby ensuring the correct operation of appliances, electrical devices, and circuits.
4. Protective earthing: the connection of a conductive enclosure to the neutral and to earth so that, in the event of a fault, the faulty circuit is rapidly disconnected, thereby ensuring the safety of a person in contact with electrical equipment and extraneous conductive parts, and also limiting the risk of fire. (For further details on the various types of protective earthing systems, see Annex A.)
5. Fault: a condition of a circuit in which current flows through an abnormal or unforeseen path. This current may result from a defect in insulation or from the clips used on the insulation of conductors.
6. Earth Electrode: a conductor or group of interconnected conductors that provide an electrical connection to earth.
7. Miscellaneous electrodes: conductive components of installations and equipment made of copper, iron, steel, and similar materials, which are used in buildings and their associated installations for special purposes and are used in bonding to reduce the overall resistance. The metal sheaths and armouring of cables, which are normally used to provide a path for conducting fault current to the source's neutral point at the transformer location, may be regarded as a miscellaneous electrode, provided that they are buried underground for a length of at least 300 m.
8. Incubator: a neonatal incubator is a device for the special care of newborns at birth and of infants born prematurely. The incubator device provides facilities for maintaining the health of infants, including adjustable temperature, humidity, and oxygen; the physician may adjust these parameters according to the infant's needs.
9. Earth Potential: the electrical potential produced relative to the mass of the earth, or relative to the surface of the ground surrounding the earth electrode, when electric current flows from the electrode into the earth.
10. Peat (organic soil): soil in which undecomposed organic matter of varying thickness is mixed with a small amount of mineral matter (clay, silt, and sand) and is observed as a black layer developed beneath the soil layers. The undecomposed organic matter in peat soil generally consists of the branches and leaves of trees in low-lying, gently sloping forested areas, or the roots, stems, and leaves of herbaceous plants in rangeland areas.
11. Switchboard: an assembly of electrical inputs and outputs and automatic overcurrent devices housed within a box or cabinet; some types also include switches for controlling lighting, heating, or power circuits.
12. Electrical Installation: an assembly of interrelated electrical equipment that fulfils a specific purpose or purposes, having coordinated technical characteristics.
13. Temporary electrical installation: an installation used to supply electricity during the period of construction, repair and maintenance, remodelling, or demolition of buildings, structures, and equipment, and other similar activities; it may be used until the completion of the works. In the case of temporary lighting for celebrations and similar events, this period is 90 days.
14. Electrical Equipment: all circuits, appliances, devices, and consumer units, and any other similar equipment used as part of, or in connection with, an electrical installation.
15. Current Equipment: equipment used for converting electrical energy into another form of energy.
16. Main Earthing Terminal (earth busbar): a terminal or busbar provided for connecting protective conductors - including bonding conductors for equipotential bonding and conductors related to functional earthing (if any) - to the earthing system.
17. Direct Contact: contact of a person with parts that are normally live, such as contact with a live busbar, a phase conductor, or a neutral conductor.
18. Indirect Contact: contact of a person with accessible electrically conductive parts that have become live, and that under normal conditions should not be live.
19. Isolating Spark Gap (ISG): in the event of sudden overvoltages, the electrical resistance of this device decreases and it discharges the voltage; as a result, equipotential bonding for lightning protection is provided.
20. (Solid) Short-Circuit Current: an overcurrent resulting from a fault of negligible impedance between conductors having different potentials under normal operating conditions.
21. Overload Current (of a circuit): an overcurrent occurring in a circuit that has no electrical fault.
22. Earth Leakage Current: current flowing to earth, or to other conductors whose electrical circuit has a path to earth. Where capacitors are used in circuits, this current may also include a capacitive component.
23. Minimum Ignition Energy (EMIE): the minimum energy capable of igniting a mixture of a specified flammable substance with air or oxygen. This energy is measured by standard methods.
24. Barrier (protective barrier): a barrier that prevents direct contact with hazardous voltages, such as the fencing of a high-voltage transformer substation.
25. Electrochemical Corrosion: the formation of an electrical cell by dissimilar metals in an electrolytic medium (for example, copper and steel, where copper forms the positive pole relative to steel, causing rapid corrosion).
26. Chemical Corrosion: the effect of chemical substances in the soil on the metal of the earthing conductor.
27. Ground Potential Rise (GPR): this phenomenon occurs when a large electric current enters the earth. It usually results from a fault current at electricity distribution substations or from a lightning strike. In this condition, currents of large magnitude enter the earth through the earthing system, which not only raises the potential of the earth but also raises the potential of the surrounding soil. This phenomenon causes large currents to enter the soil along the path of the fault current, leading to electric current flowing in earthed conductive bodies, including pipes and copper wires, and even in the bodies of persons.
28. Dimmer: a device in an electrical circuit used to vary the light intensity of lamps in lighting installations.
29. Dry-Niche Luminaire: a luminaire intended for installation in the floor or wall of a swimming pool, tub, or fountain, in a niche that is sealed against water ingress.
30. Wet-Niche Luminaire: a luminaire intended for installation in an enclosure fixed in the structure of a swimming pool or fountain, which will be completely surrounded by water.
31. No-Niche Luminaire: a luminaire intended for installation above or below water without a niche.
32. Earth: the conductive mass of the earth, whose electrical potential at any point is conventionally taken as zero. Any intentional or accidental conductive connection of an electrical circuit or equipment to the earth, or to certain conductive bodies that act in place of the earth, is deemed to constitute earth.
33. Protective Earth: in this method, conductive bodies are connected to the neutral and to earth so that, in the event of a fault, the faulty circuit is rapidly disconnected; this ensures the safety of persons who, by virtue of their job duties, are in contact with electrical system equipment, as well as other members of the public who are end users of the energy, and also limits the risk of fire.
34. Remote Earth: a location on the ground surface, at a distance from the earth electrode, at which the potential is not significantly affected by the rise in the potential of the earth electrode caused by the current flowing through it.
35. Functional Earth (isolated): in this method, connecting the neutral point of the system to earth causes any potentially faulty circuits to be disconnected; as a result, the system's insulation is preserved, the correct operation of appliances and electrical devices is ensured, overvoltages are limited, and the correct functioning of appliances and circuits is thereby assisted.
36. Effective Earth: an intentional connection to earth through an earthing connection or connections of sufficiently low impedance, and having adequate current-carrying capacity, so as to prevent the occurrence of voltages that could give rise to a hazard to the equipment connected to it, or to a person.
37. Earthing (earthing system): an assembly of one or more electrodes together with earth wires and bonding conductors capable of being connected to the main terminal, implemented to provide effective earthing.
38. Lightning Protection System (LPS): the complete system used to protect a structure and its internal components against the effects of lightning.
39. Fuel Cell System: an electrochemical system that uses fuel to generate electric current. In such cells, the primary chemical reaction used to generate electricity is not combustion.
40. Wiring System: an assembly composed of cables, wires, and busbars, together with the parts supporting them, including concealed and surface-mounted conduit, ducting, trays, and channels.
41. Photovoltaic System (PV): a supply system that converts sunlight directly into electricity. A photovoltaic system consists of one or more solar panels together with converters and other electrical and mechanical hardware.
42. Shielding (cable shield): an earthed metallic layer on a cable that prevents the effect of the cable's electric field from extending beyond it, or protects the cable against the effects of external electrical influences.
43. Protection Level (Protection Class) of Lightning Protection Equipment (lightning rods/air terminals): the protection class consists of defining a zone within which the probability of a direct lightning strike corresponds to a specified percentage, determined on the basis of lightning current parameters. Under IEC 62305, four protection classes are defined - I, II, III, and IV - where Class I has the highest protection level (99%), and the other classes have, respectively, 75% for Class II and 50% for Classes III and IV.
44. Cable Tray: a supported base for cables, having upturned edges, uncovered, and which may have pressed-out perforations.
45. Competent Person: consultants and technical-protection and safety service providers who, pursuant to the Regulation on Technical-Protection and Safety Consultants and Service Providers, hold a Certificate of Competency in the electrical-safety discipline issued by the Research and Training Center for Technical Protection (Occupational Safety) and Occupational Health.
46. Normal Operating Conditions: a condition in which plant equipment operates within its design parameters. The release of very small quantities of flammable material may be regarded as part of normal operation, including release from the mechanical packing of pumps.
47. Intracardiac (Cardiac) Procedures: procedures in which electrical conductors are placed within the patient's heart or in contact with it, and are accessible outside the body. Such conductors may include insulated wires such as pacemaker electrodes or intracardiac electrodes for electrocardiography, or insulated tubes containing a conductive fluid.
48.1 Classification of Hazardous Locations - Class I Locations: locations in which flammable gases, vapours produced from flammable liquids, or vapours produced from explosive liquids are present, or may be present, in the air in sufficient quantities to produce explosive or ignitable mixtures.
48.2 Classification of Hazardous Locations - Class II Locations: locations that are hazardous owing to the presence of explosive dust, ignitable fibres, or explosive suspended particles.
49.1 Classification of Hazardous Locations by Hazard Zone - Zone 0 (continuously hazardous): a location in which flammable concentrations of gases or flammable vapours are present continuously, or for long periods, in the environment.
49.2 Classification of Hazardous Locations by Hazard Zone - Zone 1 (occasionally hazardous): a location in which flammable concentrations of gases or flammable vapours are likely to occur owing to normal operation, repair or maintenance work, leakage or equipment failure, operational error, or electrical equipment failure; or a location that is naturally adjacent to a Zone 0 (continuously hazardous) location, unless this connection is prevented by positive-pressure ventilation from a clean air source and effective safety safeguards against ventilation failure are provided.
49.3 Classification of Hazardous Locations by Hazard Zone - Zone 2 (rarely hazardous): a location in which flammable concentrations of gases or flammable vapours are not likely to occur under normal operating conditions, or, if they occur, will exist only for a short period, or where leakage may occur as a result of the puncture or failure of tanks or systems, or the abnormal operation of handling equipment or process equipment, or ventilation failure; or a location that is naturally adjacent to a Zone 1 (occasionally hazardous) location, unless this connection is prevented by positive-pressure ventilation from a clean air source and effective safety safeguards against ventilation failure are provided.
50. Insulation: the covering or separation of live parts with a material that prevents the flow of electric current, or reduces it, so as to provide the necessary safety against electric shock.
51. Insulation (of a Cable): insulating material used in the construction of a cable, whose primary function is to withstand voltage.
52. Patient Care Space: all spaces within a medical facility in which patients are examined or treated.
53. Extraneous Conductive Part (EXCP): a conductive part in which there is a likelihood of the appearance of a potential, in particular earth potential, and which does not form part of the electrical equipment. Extraneous conductive parts include: metal pipes for gas, oil, water, compressed air, and sewage; pipes and ducts and other services; heating and cooling systems; air-ventilation ducts and pipes; accessible metal parts of a building; lightning conductors; and metal warehouse racking.
54. Exposed Conductive Part (ECP): an exposed part of equipment that is accessible and does not normally carry current, but which may become live under fault conditions. The neutral conductor is included among these parts; however, by convention, the PEN conductor (combined earth-neutral conductor) is not regarded as a live part.
55. Live Part: any current-carrying conductive part, such as a wire, terminal, or other component of electrical equipment, that lacks proper insulation.
56. Simultaneously Accessible Parts: wires or conductive parts that can be touched at the same time in particular positions. These parts include live enclosures, exposed conductive parts, extraneous conductive parts, earth wires, and earth electrodes.
57. Circuit-Breaker: an automatic device for making and breaking a circuit, capable of making or breaking currents under normal circuit conditions, and of carrying, for a short duration, or breaking currents under abnormal conditions such as a short circuit.
58. Caravan: a caravan or mobile home is a non-motorized vehicle used for habitation or work, whose width does not exceed 2.6 m and whose length does not exceed 12 m.
59. Cable Channel: an enclosure or covering located on or in the ground, in some cases ventilated, whose dimensions do not permit the entry of persons, but which allows access to the conductors or cables along their entire length.
60. Switch (load-break switch): a mechanical switching device capable of making, carrying, and breaking the circuit's electric current under normal conditions. Normal conditions may include a condition of specified overloads. This device can also withstand, for a specified time, currents under abnormal circuit conditions, such as a short circuit.
61. Potential Gradient (at a point on the ground): the potential difference measured per unit length at a point, in the direction in which the potential has the greatest value.
62. Sensitive Electronic Equipment: equipment and devices whose operating voltage is much lower than that of other electrical devices, and which are therefore more sensitive to, and more vulnerable to, sudden voltage changes caused by lightning, static electricity, short-circuit faults, and the like. Sensitive computers, Programmable Logic Controllers (PLC), and Distributed Control Systems (DCS) are examples of such sensitive equipment.
63. Patient Environment: any space in which intentional or unintentional contact can occur between a patient and a medical electrical equipment system, or between a patient and other persons who touch parts of the medical equipment.
64. Isolated Circuit: a circuit that is separated (isolated) from the AC line to prevent electric shock.
65. Circuit (of an installation): an assembly of electrical equipment supplied from a single source and protected against overcurrents by a single protective device.
66. Distribution Circuit (of an installation): a circuit that supplies a switchboard.
67. Intrinsically Safe Circuit: with respect to hazardous locations, a circuit designed and constructed for use in flammable and explosive hazardous environments such that the energy (thermal or electrical) is limited to a level below that required for sparking, ignition, or explosion, and consequently no spark or thermal effect is capable of igniting a mixture of flammable or explosive substances in air under test conditions.
68. Joint: a device for connecting two cables, forming a continuous circuit.
69. Total Earthing Resistance (of an earth electrode): the resistance between the main earthing terminal and the general mass of the earth. The electrode resistance is the resistance of the volume of soil surrounding the electrode, referred to as the resistance area of the earth electrode.
70. Mutual Resistance of Earth Electrodes: the voltage variation in one electrode arising from current variations in another electrode, expressed in ohms.
71. Neutral Resistance: the total resistance of the neutral conductor, which may include several earthing electrodes near a transformer or generator substation, earth connections of cables with metal sheaths, earth connections of overhead lines at the beginning and end of each main line, and the like.
72. Group 0 Medical Locations: locations in which medical electrical equipment that is supplied from the mains and is in contact with the body is not used.
73. Group 1 Medical Locations: locations in which medical electrical equipment is used for purposes other than intracardiac (cardiac) applications.
74. Group 2 Medical Locations: locations in which medical electrical equipment is used for intracardiac (cardiac) applications.
75. Zoning of Swimming Pools, Fountains, and Similar Structures: the zones of pools, tubs, fountains, and similar structures are described in accordance with paragraphs 702-30-102, 702-30-103, and 702-30-104 of Iranian National Standard No. 702-7-1937.
76. Mobile Unit: a unit refers to a vehicle, or to a mobile or relocatable structure, that includes all or part of an electrical installation. Television and radio units, medical service units, advertising units, firefighting units, units using specific information technology, temporary accommodation units used during incidents, food delivery units, and similar units are included among mobile units.
77. Switchgear and Control Gear (upstream or downstream of a switchboard): all electrical components and devices provided for making and breaking an electrical circuit and for protecting consumers of electrical equipment, for one of the purposes of protection, control, isolation, or performing switching operations; this includes all switches, contactors, residual current devices (RCDs), and motor protective switches.
78. Insulation Monitoring Device: this device monitors the unearthed system between an active phase conductor and earth. If the resistance between the two conductors falls below a specified value (typically 50 kOhm), this device shall give a warning or disconnect the power supply.
79. Residual Current Device (RCD): a mechanical switching device, or an assembly of devices, that opens the contacts when the residual current (the difference between the circuit current and the device's reference current) reaches a specified value under specified conditions.
80. Transfer Voltage: at high-voltage substations and power plants, owing to the very high short-circuit level, a severe current on the scale of several tens of kiloamperes can be injected into the earth grid at the time of a fault, producing a considerable voltage in the form of GPR in the earth grid relative to remote earth. On this basis, extraneous conductive parts that extend outward from the boundary of the substation or power plant - such as railway rails, metal water and gas pipes, cable trays, cable shields, and the like - can carry voltages of high GPR magnitude to the outside. This transfer of voltage is called Transfer Voltage, and it can be hazardous to persons who are outside the earth-grid area of the substation or power plant.
81. Safety Extra-Low Voltage (SELV): an AC voltage with an r.m.s. value of less than 50 V, or a DC voltage, ripple-free, of less than 120 V, between conductors, or between a conductor and reference earth, in an electrical circuit that is galvanically separated from the electrical power supply system by means of a separate-winding isolating transformer.
82. Touch Voltage: the voltage that appears, when an insulation fault occurs, between simultaneously accessible parts such as conductors, conductive enclosures, extraneous conductive parts, and the like. (See Figure 1(a).)
83. Step Voltage: the voltage on the surface of the ground that a person can experience across a distance of 1 metre between their feet, without being in contact with any earthed object. (See Figure 1: Touch voltage and step voltage - (a) Touch voltage; (b) Step voltage.)
84. Prospective Touch Voltage: the maximum touch voltage likely to appear in an electrical installation in the event of a short circuit of negligible impedance.
85. High Voltage: AC - 1000 V and above; DC - 1500 V and above.
86. Low Voltage: AC - voltage less than 1000 V; DC - voltage less than 1500 V.
87. Earthing Conductor (earth wire): a protective conductor that connects the installation's main earth terminal to the earth electrode or to other earthing parts.
88. Bypass Conductor: a conductive cable that establishes a direct electrical connection between the shell of a tank and its floating roof.
89. Protective Conductor (PE): a conductor that connects the various parts of a system to one another, and ultimately to earth, for the purpose of main equipotential bonding.
90. Neutral Conductor: a wire connected to the neutral point of the system (earth zero) that is capable of transmitting electrical energy.
91. Earth Conductor (earthing wire): the part of the earthing system that connects the earth electrode to the main earthing terminal.
92. Metal-Sheathed Conductor for Earthing Purposes: a type of wiring system in which one or more insulated wires are covered along their entire length by a metal tape or sheath, and which functions like a PEN conductor.
93. PEN Conductor (combined earth-neutral conductor): a conductor that jointly performs the function of both the earthing conductor and the neutral conductor.
94. Bonding Conductor: a strand of copper wire, a copper strip, a piece of rebar, or a metal beam or column, installed in the roofs and columns of a building in accordance with the bonding plan.
95. Supplementary Bonding: complements the main bonding and is carried out for the purpose of equalizing the voltage of points that are simultaneously accessible. This type of bonding is also called auxiliary or local bonding.
96. System Bonding: the connection of the various components of the earthing system to one another for the purpose of equipotential bonding of the different parts of the installation; the assembly of these connections is also called the bonding network.
Chapter 2 - Specific Requirements for Types of Earthing Systems
Section 1 - TN Systems
The TN-C earthing system may be implemented only under the following conditions: 1. the continuity of the PEN conductor is maintained at all times and is not interrupted under any circumstances; 2. the environment is free of dust; and 3. the environment is free of flammable and explosive substances and gases.
In the TN-C earthing system, the neutral conductor shall never be interrupted.
To prevent electromagnetic interference, the use of the TN-C earthing system is prohibited in buildings containing sensitive electronic equipment (precision instruments).
All accessible earth electrodes shall be interconnected through an earthing busbar.
In the TN earthing system, a single common conductor may be used for both the protective (PE) and neutral purposes only if the cross-sectional area of the common conductor is not less than 10 mm². Note: If the cable is of the concentric type, a minimum cross-sectional area of 4 mm² may be used for the common conductor, provided that the continuity of the sheath conductor is maintained at all connections of the concentric cable.
The use of separate earth electrodes is prohibited in all TN earthing systems.
Where either the TN-S or TN-C-S earthing system is implemented, reconnecting the protective and neutral conductors after the point of separation is prohibited.
The use of a residual current device (RCD) as protection in TN-C earthing systems is prohibited.
Where an RCD is used in a TN-C-S system, the RCD shall be installed after the point of separation of the protective and neutral conductors.
In the TN-C-S earthing system, the neutral and earth conductors shall be connected at the main electrical distribution board.
Section 2 - TT Systems
A residual current device (RCD) shall be used in the TT system.
Where several exposed-conductive-parts are simultaneously connected to a common RCD, they shall not be connected to independent, separate earth electrodes.
In the TT system, the neutral conductor shall be regarded as live, and the neutral conductor shall also be disconnected simultaneously with the disconnection of the phase conductors.
In the TT system, connecting the earth conductor and the neutral conductor is prohibited.
Where safety-related systems, including fire detection and fire-fighting systems, are connected in the circuit ahead of the main protective device, the neutral conductor shall be disconnected together with the phase conductor, to protect persons against direct and indirect contact with the neutral conductor.
Section 3 - IT Systems
The use of the IT system for public electricity distribution networks is prohibited.
In production workplaces — including furnaces and metal-smelting facilities, power plants, munitions manufacturing industries, control circuits, underground mines, and certain hospital departments — where a power outage would cause extensive damage, the IT earthing system shall be implemented.
In the IT earthing system, to prevent exposed-conductive-parts from becoming live at a voltage equal to the phase-to-phase voltage, these systems shall be equipped with an insulation monitoring device (IMD).
In the IT earthing system, all exposed-conductive-parts (ECP) shall be earthed (connected to earth) independently of the power source.
To prevent intermittent random earth faults in the IT earthing system, the neutral point of the source shall be connected to earth through an electrical resistance whose value satisfies the following relation: Rs<1/3ωCO, where Rs is the electrical resistance between the neutral point of the source and earth, ω is the angular frequency of the line current, and CO is the capacitance of the faulty phase line.
Chapter 3 - Requirements for Electrodes Used in the Earthing System
The material of the electrode and its earthing conductor shall be such that the soil type does not cause corrosion of them.
The provisions of Chapter 4 concerning the burial location of electrodes and the interconnection of electrodes shall be complied with.
Section 1 - Plate Electrodes
For the installation of plate electrodes, copper plates with dimensions of at least 0.5 × 0.5 m and a minimum thickness of 2 mm, or galvanized steel plates with dimensions of at least 0.5 × 1 m and a minimum thickness of 3 mm, shall be used.
Earthing electrodes shall be cut before galvanizing, and after any cutting or drilling performed on them, the cut area shall be re-galvanized.
A mesh plate made of strip or wire (mesh grid) may be used in place of a solid plate as a plate earthing electrode.
The dimensions of the earth electrode mesh shall be determined by the designer, taking into account the soil type, burial depth, the magnitude of the current injected into the earth electrode, the permissible voltage gradient, and other factors affecting earthing resistance, based on physical relations as well as experience.
The earth electrode pit for placing plate electrodes shall be excavated at a location with the lowest surface level, where the likelihood of reaching moisture at the shallowest possible depth exists.
Plate earthing electrodes shall be installed vertically in the soil.
The main earthing conductor shall be connected to the electrode at a minimum of two separate points.
If the main earthing conductor and the electrode plate are made of two different materials, the connection point of the two different materials shall be coated with a sealing, impermeable material such as bitumen, to eliminate the electrolyte and prevent galvanic corrosion.
The main earthing conductor shall be connected to the copper plate electrode by one of the following methods, in order of priority: 1. brazing with brass or silver; 2. exothermic welding (Cadweld); 3. mechanical connection of the same material as the plate, using a minimum of two clamps.
The distance from the top edge of the plate electrode to ground level shall not be less than 600 mm.
Section 2 - Vertical (Rod) Electrodes
The minimum burial depth of a vertical electrode in the natural soil bed of the area shall not be less than 2 m.
Copper-clad steel rod earthing electrodes shall be manufactured using electroplating technology, and the use of molecular bonding technology is prohibited.
At the time of installation, the vertical earthing electrode (pipe or rod) shall be sound and free from any scratches, rust, bending, or dents.
In the absence of an obstruction, rod earthing electrodes shall be installed vertically.
The spacing between rod earthing electrodes shall be at least equal to the sum of the lengths of the two electrodes.
The diameter of rod electrodes made of copper and of copper-clad steel shall be 12 mm and 16 mm respectively, and for rods made of galvanized steel the diameter shall be 16 mm.
Section 3 - Horizontal Electrodes
The minimum cross-sectional area of a copper strip used as a horizontal earthing electrode shall not be less than 50 mm², and its thickness shall not be less than 2 mm.
The minimum cross-sectional area of a hot-dip galvanized steel strip used as a horizontal earthing electrode shall not be less than 100 mm², and its thickness shall not be less than 3 mm.
The minimum cross-sectional area of stranded copper wire used as a horizontal earthing electrode shall not be less than 25 mm².
The diameter of none of the individual strands making up the copper wire used as a horizontal earthing electrode shall be less than 1.7 mm, and the use of flexible conductors as a horizontal electrode is prohibited.
Horizontal electrodes shall be installed at a depth below the frost depth of the area.
Section 4 - Coil Electrodes
For the coil electrode, No. 50 bare wire shall be used, wound into a ring shape with an outer radius of 40 cm, comprising 5 turns (placed at the bottom of the earthing pit).
Section 5 - Ring Electrodes
If, for existing installations, a horizontal peripheral electrode (ring electrode) is used as the earth electrode, the burial depth of the electrode shall be at least 1 m.
The minimum dimensions of the various types of steel earthing electrodes shall be in accordance with Table P-P-1 of Annex P.
The minimum dimensions of the various types of copper earthing electrodes shall be in accordance with Table P-P-2 of Annex P.
Section 6 - Incidental Electrodes
The metal parts of the reinforced-concrete foundation may be used as an effective earthing electrode, subject to the following conditions: 1. it shall not be used as the down-conductor of a lightning protection earthing system; 2. no moisture insulation/waterproofing shall be applied between the entire building foundation and the surrounding ground bed, and the foundation concrete shall be in direct and complete contact with the soil; 3. this type of electrode shall not be used alone as an earthing system. Note: If, during construction of the building, reinforcing bar (rebar) is used as the conductor embedded in the concrete, its diameter shall be at least 10 mm; and if bare copper wire is used, its cross-section shall be at least 25 mm².
The use of reinforced concrete foundations as an earthing electrode is prohibited in foundations with a depth of less than 0.8 m.
A reliable electrical connection shall be established between all metal components that are considered part of the earthing electrode.
The use of underground installations for oil and petroleum products, gas, compressed air, sewage, or municipal water piping as an earth electrode or protective conductor is prohibited.
The minimum cross-sectional area of the protective conductor shall be in accordance with Table P-P-3, Annex P. Note: If a protective conductor is used in common for two or more circuits, its size shall be selected based on the size of the larger phase conductor.
If the sheathing of the protective conductor (PE) differs from that of the phase conductor and the protective conductor is protected against mechanical damage, its minimum cross-sectional area shall be 2.5 mm² for copper; and if the protective conductor is not protected against mechanical damage, its minimum cross-sectional area for copper shall be 4 mm². In both cases, this value shall not be less than 16 mm² for aluminium.
The use of cable trays, cable ladders, and similar items as a protective conductor is permitted, provided that continuity is maintained along their entire length and both ends are connected to earth on the load side and the source side.
Chapter 4 - Soil Resistivity and Electrode Installation Location
The installation location of the earthing electrode shall be selected according to soil type, in the following order of priority (See Annex B for further details on the priority of earth electrode installation location by soil type): 1. Swampy ground; 2. Clayey ground or grassland; 3. Clayey ground mixed with a small amount of sand; 4. Clayey ground mixed with pebbles, gravel, and sand; 5. Wet or damp gravel, or peat.
The installation location of the electrodes shall be selected such that drainage at that location is low.
The installation of earthing electrodes is prohibited in waterlogged ground; riverbeds and seasonal watercourses; the paths of flowing and underground water; the probable path of alluvium contaminated with agricultural fertilizers; sewage wells and their effluent infiltration zone; and also in fill soil. Note 1: If fill soil is present at the earthing electrode installation location, excavation shall be carried out to a greater depth to reach undisturbed, virgin soil. Note 2: Dumping construction debris, stones, and gravel around the electrode is prohibited.
The installation location of the earthing electrode shall be selected such that no sewage well or water well falls within its voltage zone.
If an unused water well exists at the site, it may be used for installing the earthing electrode, provided that from then on it is used solely for this purpose.
In rocky and craggy elevated terrain where deep excavation is not possible, to implement the earthing system, grooves shall be created, a copper strip placed inside them, the strip covered with suitable soil-conditioning material (a suitable electrolyte), and all connections welded together beneath the soil.
In areas where soil resistivity is high, the use of electrolyte is permitted under the following conditions: 1. The additive compounds (electrolyte) used in the electrode shall be designed and manufactured such that their performance is safe and reliable for persons and the environment surrounding the components used. 2. All additive compounds for earthing shall be tested in accordance with Iranian National Standard INSO 18499-7 and shall meet the required standard. Note: In the event of a revision of the aforementioned standard, the latest version shall be valid. 3. The materials used for the electrolyte shall have the following characteristics: 3-1. Low specific resistance; 3-2. Very low corrosiveness; 3-3. Long service life; 3-4. Environmental compatibility; 3-5. High moisture absorption; 3-6. pH in the neutral to slightly alkaline range (between 8 and 8.5); 3-7. Resistance to being washed away by surface and underground water; 3-8. Suitable adhesion to the earth electrode.
If more than one electrode is used, to achieve the lowest equivalent electrical resistance, the minimum distance between two electrodes shall be such that they do not fall within each other's voltage zone.
The main terminal of the earthing system shall be accessible, so that during earthing-related measurements it is possible to disconnect the installation from that system.
The earthing electrode shall be installed at a depth such that freezing temperatures cannot penetrate to it.
Every earthing system shall have an identification record issued by technical protection and safety consultants and service providers, containing its complete specifications, including the type and material of the electrode(s) and their dimensions; the date of installation, location, and soil type; the initial measured resistance value, subsequent periodic measurements, and annual changes; and documentation of the earthing system's inspection.
Chapter 5 - System Bonding
Section 1 - Main Bonding
For carrying out main bonding, the following shall be connected to one another by separate conductors on the building's main earthing busbar: - the protective conductor - the neutral conductor - metal water pipes - metal gas pipes downstream of the meter - the main metal pipes and ducts of other installations such as central heating and ventilation systems, ducts, and chimneys - the main and auxiliary earthing electrodes - all main metal parts of the building, including the metal structural frame, reinforced-concrete rebar, columns, frames, and metal doors and windows - the cabin rails and counterweight rails of traction elevators - the jacks of hydraulic elevators - the lightning protection system - the sockets of all lamp holders - outlets - connections - and similar fittings. Note 1: For bonding to the lightning protection system, suitable protective equipment shall be used along the path to the main earthing busbar. Note 2: If cathodic protection is provided for buried metal equipment (pipes and tanks), voltage-limiting protective equipment with a surge gap shall be used for bonding.
All protective conductors of socket-outlet and lighting circuits shall be bonded to the earthing system.
Extraneous conductive parts that originate outside the building and continue into its interior shall be bonded to the earthing system at the nearest possible point to the building's point of entry.
If the water inlet pipe to the building is non-metallic but metal piping is used inside the building, bonding of the metal section to the main earth is mandatory.
If the various sections of metal piping present in the workplace are separated by dielectric joints, bonding of all metal sections to the main system is mandatory.
In an electrical panel whose door contains live conductors, both the panel and its door are considered exposed conductive parts (ECP) and shall be bonded to the earthing system. In this case, the panel and its door shall also be bonded to one another.
Where an electrical panel has accessible live parts and its door is considered an enclosure, bonding of the panel door to its core is mandatory.
All mechanical installations and metal components buried in the soil in the vicinity of the earthing electrode shall be bonded to the earthing system.
When bonding the building's metal structural frame, rebar, columns, and metal beams, the electrical connection of all parts of the frame to one another shall be ensured.
Connection of the tie beams and the peripheral metal beams of the stairwell and elevator frame to the bonding network of the earthing system is mandatory.
In areas of the workplace where the floor is routinely wetted, one of the tie beams or metal columns shall be bonded to the earthing system.
Metal equipment that may become live during operation shall be connected to the earthing system before use.
All electrical equipment and metal parts of mobile equipment in the workplace, including hoists, cranes, and similar equipment, shall be connected to the earthing system. Note: Crane rails and the elevator frame shall be bonded to the earthing system.
All columns located at the outer corners of the workplace structure shall be bonded to the earthing system.
All parts of metal railings and stairs shall be bonded to the earthing system.
All internal and external metal parts of site containers and metal cabins shall be connected to the earthing system.
All components of the earthing system's bonding network shall be connected to one another by reliable electrical connections.
At every point where a discontinuity exists in the earthing system's bonding network, a reliable electrical connection shall be established between the two sections.
At every corner of the structure where bonding-network components meet from two sides, a reliable electrical connection shall be established between the two sides.
At every branch taken from a component of the earthing bonding network (such as T-junctions), a reliable electrical connection shall be established.
At every point where the longitudinal and transverse components of the earthing bonding network cross one another (crossings), a reliable electrical connection shall be established.
In steel-frame structures where beams and columns are connected to one another by bolts and nuts, a copper jumper wire shall be used to create a reliable electrical connection between the metal components of the earthing system's bonding network.
The connection of the jumper wire between the metal components of the earthing system's bonding network and steel components shall be made by thermite welding or by using a cable lug, and this connection shall always be accessible, inspectable, and repairable.
The connecting wire between metal parts of the earthing system's bonding network shall have the curvature (slack) necessary for expansion and contraction of the connected parts, and after being fastened, it shall not remain in a taut (stretched) condition.
One of the connection points of the bonding network to the building's earthing system shall be located on the column nearest to the main electrical panel (the installation location of the main earthing busbar).
The overall bonding resistance of the earthing system, measured between the farthest points, shall not exceed 0.2 Ω.
Every bonding conductor of the earthing system shall bear a label or tag identifying the conductor's function and indicating which points it connects to one another.
At all points where, under operating conditions, the earthing system connection may need to be disconnected for repair (such as water and gas pipes), a warning label prohibiting disconnection of the earthing system's bonding conductor shall be installed.
Section 2 - Supplementary Bonding
In all wet, damp, and moist environments and locations where water splashing or the formation of water vapor (steam) is possible — including bathrooms, kitchens, pantries, restaurants, and washing, degreasing, and metal-plating areas, and similar locations — supplementary bonding to the earthing system shall be implemented.
Supplementary bonding for equipotential bonding purposes shall cover the following items: 1. The metal enclosure of all permanently installed electrical equipment; 2. Extraneous conductive parts of any kind; 3. Accessible metal parts in buildings, including the metal structural frame.
In bathrooms, swimming pools, and sanitary facilities, both the hot-water and cold-water metal pipes shall be connected to the supplementary bonding conductor path.
The supplementary bonding terminal in damp locations shall be installed outside that location, on the wall of one of the adjacent spaces.
In locations where supplementary bonding to the earthing system is required, the supplementary bonding terminal shall be installed in the form of a surface-mounted copper busbar or terminal block inside an inspectable box designated as the supplementary bonding box.
Supplementary bonding shall be implemented for all electrical equipment and extraneous conductive parts located on the roof of the workplace, including cooling towers, antenna masts, metal railings, and metal roof sheeting.
Supplementary bonding shall be implemented for electrical panels located adjacent to one another.
The minimum cross-sectional area of the supplementary bonding conductor to the earthing system shall be selected in accordance with Table P-P-4, Annex P.
The cross-sectional area of the supplementary bonding conductor connecting the metal enclosures of two items of electrical equipment shall not be smaller than the smallest protective conductor of the supply circuits connected to the conductive enclosures of the two items of equipment concerned.
The cross-sectional area of the supplementary bonding conductor between the enclosure of the electrical equipment and the extraneous conductive part shall not be smaller than half the cross-sectional area of the protective conductor of the equipment's supply circuit.
To eliminate any potential difference, in locations where the use of intrinsically safe systems is necessary, a local busbar shall be provided, and a local bonding network shall be established comprising cable shields, instrumentation equipment, metal parts of equipment and enclosures, cable trays and cable ladders, and all extraneous conductive parts.
If earthing the cable shield at more than one point is a safety requirement, and this requirement conflicts with electromagnetic compatibility (EMC) considerations, double-sheathed cables (having two shields) shall be used. In this case, the outer sheath shall be earthed at multiple points to satisfy the safety requirements, and the inner sheath shall be earthed at one end only to satisfy the EMC requirements.
After completion of any permanent or temporary addition or modification work on existing installations, the equipotential bonding arrangements of the consumer circuit and the earthing system shall be reviewed and re-measured.
Branch connections from the main earthing busbar shall be provided for auxiliary equipment, including control and relay panels, metal structural components, and fire-fighting installations.
Branch connections shall be run from the main earthing busbar to each item of installation equipment individually, and where several items of electrical equipment are located next to one another, connecting them in series (daisy-chaining) is prohibited.
For composite electrical equipment such as conveyor belts, packaging systems, and tunnel kilns, electrical continuity of their metal parts shall be ensured, and they shall be bonded to the earthing system at not fewer than two points. Bonding to extraneous conductive parts surrounding this equipment shall likewise be ensured.
Bonding of the earthing system to auxiliary electric generators is required, in accordance with the manufacturer's instructions.
In the earthing system, continuity of the earth connections shall be ensured throughout the entire system — from the equipment enclosure to the earth pit and between the electrodes — and factors such as corrosion or connections made on painted surfaces shall not impair this continuity.
Chapter 6 - Earthing and Bonding of Electrical Substations
Section 1 - Workplaces Without a Substation, Supplied from the Public Low-Voltage Network
Workplaces that do not have a substation, that receive electricity as a single-phase supply (phase + neutral) or a three-phase supply (3 phases + neutral) from the power grid, and that do not have access to the source's neutral and earth busbars, shall have an TN-C-S earthing system at their main electrical panel.
In the TN-C-S earthing system, the main power distribution panel shall have two separate busbars — neutral and earth — connected to each other by a connecting conductor (jumper), with the incoming neutral wire connected to the neutral busbar and the earthing conductor connected to the earth busbar.
In the TN-C-S earthing system, the electrical resistance of the earthing electrode at the point where the neutral and the earthing connection are separated shall be less than 2 Ω. Note: In conditions such as overhead distribution systems, where there is a risk of the neutral connection breaking, selecting lower resistance values for the earthing electrode is preferable.
Section 2 - Workplaces with a Single Substation Supplied from the Medium-Voltage Network
In workplaces with a dedicated ground-based or overhead substation with a single transformer, the earthing system shall be TN-S or TN-C-S.
In workplaces with a dedicated single-source substation, the TN-C system may be used only if the connections along the protective earthed neutral conductor (PEN) path are permanent and there is no possibility whatsoever of the PEN protective earthed neutral wire or cable being disconnected or broken.
If, owing to the operational requirements of the equipment, the use of a TT earthing system is unavoidable, residual current protective devices (RCDs) shall be installed at the point where the earth conductor is separated from the TN arrangement to form the TT arrangement.
Section 3 - Workplaces with Two or More Parallel Power Sources
In electrical systems with multiple sources, directly connecting the transformer or generator neutral to the earthing connection is prohibited.
In the TN arrangement of systems with multiple parallel sources, the connecting conductor from the neutral to the earth connection point shall be fully insulated, and no electricity-consuming equipment shall be connected to it.
In electrical systems with multiple sources, the connecting conductor between the neutrals of the sources and the PE protective conductor shall be interconnected only at the main electrical panel. Note: Reconnecting the PE protective conductor downstream of the main electrical panel is permitted.
In the earthing system, use of a common earth for the high-voltage side of the substation and the low-voltage side of the transformer is prohibited.
To prevent transferred voltage arising from ground potential rise (GPR) being carried from high-voltage substations and power-plant earth grids to remote areas by extraneous conductive parts (such as railway rails, metal water and gas pipes, cable shields, cable trays, or metal cable ladders, etc.), the continuous electrical connectivity of the extended conductor inside and outside the zone of influence of the earth grid shall be interrupted at the point where such equipment leaves the earth grid's zone, by creating a gap or by using electrically insulating joints and connectors. As needed, the conductor in question shall further be earthed at several points along the remaining route so as to ensure that no hazardous transferred voltage is carried from the high-voltage earth grid to the remote area. Note: If metal objects capable of transferring voltage to surrounding buildings and grounds are already present near the high-voltage electrode and the substation's metal structure, one of the methods of multiple earthing along the route, or insulating along the route, shall be applied to prevent the transfer of hazardous voltages to remote areas.
In workplaces where two or more substations supply a single hall that has a metal structural frame, bonding between the two substations shall be provided, in addition to bonding of the metal structural frame.
In substations where both high-voltage and low-voltage exist together, where it is possible to install two independent earth electrodes, the distance between the high-voltage and low-voltage earth electrodes shall not be less than 20 metres, and there shall be no conductive connection between the two earth electrodes within that distance.
If, in a substation, insulating separation of the low-voltage switchgear panels from the high-voltage switchgear panels, transformer, and metal structure of the substation has been provided, the enclosure of the low-voltage switchgear panel and the low-voltage neutral point (neutral) shall be connected to the low-voltage earth electrode, which shall be at least 20 metres from the substation structure, and all enclosures of the high-voltage switchgear panels, the transformer, and all metal parts of the substation structure shall be connected to the high-voltage earth electrode (inside or adjacent to the substation).
Where it is possible to install two independent earth electrodes at a substation, but insulating separation of the low-voltage switchgear panels from the high-voltage equipment and the metal structure of the substation cannot be achieved, the enclosure of the low-voltage switchgear panel, the enclosures of the high-voltage switchgear panels and transformers, and all metal parts of the substation structure shall be connected to the high-voltage earth electrode, and the low-voltage neutral point shall be connected, using a cable with a non-metallic sheath, to the low-voltage earthing electrode located at least 20 metres from the substation.
Where it is not possible to install two independent earth electrodes at a substation, a single common earth electrode shall be used for both high-voltage and low-voltage purposes. In this case, the total resistance of the earth electrode with respect to the general mass of earth shall not exceed one ohm.
If any of the high-voltage incoming or outgoing lines of a substation, or even part of such lines within a distance of less than 3 kilometres from the substation, is of the overhead-line type, then, in addition to the line being equipped with a surge arrester, two separate earth electrodes that lie outside each other's voltage zone of influence shall be used for the low-voltage and high-voltage purposes.
Where a substation has two separate earth electrodes for high voltage and low voltage, and the enclosures of the low-voltage switchgear panels are connected, together with the enclosures of the high-voltage switchgear panels and equipment, to the high-voltage earth electrode, while the low-voltage neutral point is connected to the low-voltage earth electrode, the low-voltage switchgear panels shall be selected with an insulation grade higher than the rated voltage between phase and neutral.
Chapter 7 - Selection and Installation of the Earthing Conductor
The main earthing conductor between the earth electrode and the main earthing busbar shall be of copper and shall have a cross-sectional area of at least 35 mm².
A separate test and inspection box of suitable dimensions, permanently marked with the earth symbol, shall be provided for each earth electrode.
Any branching from the connecting conductors between the earth electrode and the test box is prohibited; all branch connections and the formation of the earthing system's ring network shall be made downstream of the test box.
The route of the wires connecting the earth electrode to the main earthing busbar shall be the shortest possible route, and these wires shall, as far as possible, be direct and free of bends.
Bare aluminium or copper-clad aluminium shall not be used in contact with the ground, whether as an electrode or as an earthing conductor. Nor shall these materials be used as an earthing conductor in damp environments.
Bare wires and insulated wires with a green-and-yellow outer sheath, or a green/yellow combination, shall be used only for equipment earthing conductors and bonding conductors.
The earthing conductor wire shall possess the necessary mechanical strength and resistance to chemical and electrochemical corrosion.
A bare earthing wire shall not be routed through metal pipes.
In cables where the cross-sectional area of the neutral wire is half the cross-sectional area of each phase wire, the cross-sectional areas of the earthing wire (protective conductor, PE) and the neutral wire shall be equal.
An earthing busbar shall be provided in the main distribution board, such that the earthing wire runs from the electrode to this busbar and is then distributed from the main terminal to the various parts of the installation.
A neutral busbar shall be provided in the main distribution board and shall be mounted on an insulating support.
In a TN-C-S system, downstream of the point where the neutral and earth conductors are separated, reconnecting the neutral and earth conductors is prohibited.
The material of the connections between the main earthing wire and the electrode shall be compatible with the materials used in the electrode and the earthing wire, so that galvanic corrosion is minimised.
Connections and clamps associated with the earthing system shall be mechanically robust and shall provide a secure connection.
Where a strip conductor is used as the earthing conductor and is connected to equipment, the strip shall not be drilled for a bolt whose diameter exceeds one-third of the width of the strip.
Bonding conductors used for equipotential bonding shall have adequate mechanical strength, and their electrical resistance shall be negligible.
The minimum cross-sectional area of the main bonding conductor shall be as follows:
1. Copper conductor: 6 mm²;
2. Aluminium conductor: 16 mm²;
3. Steel conductor: 50 mm².
The minimum cross-sectional area of protective conductors and supplementary bonding conductors shall be as set out in Table P-P-5 of Annex P.
Chapter 8 - Measurement of the Electrical Resistance of the Earth Electrode
Before measuring the electrical resistance of the earth electrode, the earthing installation shall be completely disconnected from the power supply source or the network, and shall remain disconnected from the supply until the end of the test.
Before measuring the electrical resistance of the earth electrode, the earthing conductor shall be disconnected from the earth electrode.
The use of safety electrically insulating gloves and footwear is mandatory while measuring the electrical resistance of the earth electrode.
Measuring the electrical resistance of the earthing system is prohibited on stormy, rainy, or snowy days, and at times when lightning is likely to occur.
In view of the effect of climatic variation on the resistance of the earthing system in rocky, vegetation-free areas or areas with a long freezing period, the resistance measurement shall be carried out under the worst existing conditions, unless the stability of the earthing system's resistance is ensured throughout the year by corrective measures such as continuous irrigation.
If, during measurement of the electrical resistance of the earthing system, the current and voltage probes are out of sight of the tester, or if the test leads are located in an area accessible to the public, these points shall be kept under continuous observation and monitoring — for as long as the test signal is applied or the remote potentials exceed 50 volts — by an attendant who maintains wireless communication with the operator of the measuring instrument (the tester).
When measuring the resistance of the earth electrode, to minimise self-inductance, the conductors (leads) shall be completely unwound from their reels.
When measuring the resistance of the earth electrode, the measuring electrodes (probes) shall, as far as possible, be kept away from extensive buried metal objects, such as metal water piping or cable armouring.
Note – Where there is a likelihood of buried metal objects, such as metal water pipes, along the measurement path, a further measurement, preferably perpendicular to the previous path, shall be carried out; where the two values differ, the higher value, which represents the smallest error caused by buried metal, shall be taken as the basis for the measurement.
Before proceeding to measure the electrical resistance of the earth electrodes, any voltages potentially present on the earthing system shall be measured, and it shall be confirmed, with reference to the instrument's instruction manual, that these are within acceptable limits.
For the measurement of the electrical resistance of earth electrodes, before disconnecting an electrode from the earthing system, it shall be confirmed that no large currents are flowing through it.
For the measurement of the electrical resistance of earth electrodes, all earth electrodes of a system shall not be disconnected simultaneously, unless the relevant installation has already been completely de-energised beforehand.
The instrument used to measure the electrical resistance of the earthing connection shall hold a valid calibration certificate.
The use of a two-point test method, such as the clamp method [see Annex T, Section T-5], is prohibited for earthing systems with low resistance.
Use of the clamp method for measuring earthing resistance is permitted only where a closed-loop path exists for the flow of the test current and the resistance of this loop is negligible in comparison with the resistance of the system.
The report on the measurement of electrical resistance and inspection of the earthing system shall include the following:
1. Particulars of the competent person;
2. Particulars of the workplace;
3. Date of measurement;
4. Environmental conditions (soil and weather);
5. Particulars of the measuring instrument and its calibration records;
6. Particulars of the workplace supply system, including voltage, current, and the like;
7. Type of earthing system;
8. Location details and the type and arrangement of the electrodes;
9. Result of the current and voltage measurement taken before measuring the earthing resistance;
10. Direction of the measurement;
11. Diagram of the measurement results (depending on the measurement method);
12. Final result of the measurement.
After completion of the measurement of the electrical resistance of the earth electrode, it shall be confirmed that the earthing conductor has been fully reconnected.
At construction sites and other workplaces where temporary electrical installations are used, a periodic inspection programme shall be established for the earthing system of the temporary electrical installation equipment, such that the installation and maintenance of all cable assemblies, of sockets that are not part of the building's or structure's permanent wiring, and of all equipment connected to the supply by a plug, are checked by a competent person.
In checking the earthing of temporary electrical installations, the following shall be tested:
1. Electrical continuity of all equipment earthing conductors;
2. Correct connection of all sockets.
Tests of the earthing system of temporary electrical installations shall be carried out before their first use, after any damage to the installation, after any item of equipment returns from repair and before it is put back into service, and periodically at intervals of less than 3 months.
Chapter 9 - Earthing in Flammable and Explosive Locations
The permissible electrical resistance value for the earthing system in flammable locations shall satisfy the following relationship: [Formula]
Where: C: the value of the capacitance to earth; EMIE: the minimum ignition energy of the environment; I: the electric current magnitude.
In cases where the earthing system is used solely to prevent the accumulation of static electricity, the minimum cross-sectional area of the earthing conductors and bonding conductors, to provide adequate mechanical strength, shall be 4 mm² for aluminium and 2.5 mm² for copper, and the electrical resistance of the earthing system shall not exceed 1 MΩ.
Note – Standards also recognise the use of a steel conductor. In view of the very high resistance value permitted for the earth connection, the use of an iron conductor of suitable cross-section, possessing adequate mechanical strength, is permitted.
To reduce or eliminate the risk of electric shock to persons from contact with electrical devices (even when switched off) owing to the accumulation of static electrical charges, the metal enclosures of such equipment shall at all times be provided with a suitable earth connection.
For structures containing flammable vapours, flammable gases, or liquids that release flammable vapours, a ring earth electrode or ring earthing conductor shall be provided.
Metal petroleum and gas storage tanks shall be earthed by one of the following methods:
1. The tank shall be connected, without insulating connections, to an earthed metal piping system;
2. A vertical cylindrical tank shall be sited on soil or concrete and have a diameter of at least 6 metres, or shall be sited on paving of bituminous asphalt and have a diameter of at least 15 metres;
3. The tank shall be earthed by at least two earth electrodes, positioned such that the distance between successive electrodes around the outer perimeter of the tank is less than 30 metres.
Note – The earthing of a tank sited on flooring made of an insulating membrane shall be established by the method in paragraph (3) of this Article.
In underground structures containing explosive materials, an additional lightning protection system (LPS) shall be provided for protection against lightning.
Only those petroleum and gas storage tanks whose metal structure satisfies all of the following conditions do not require an external lightning protection system:
1. It is electrically continuous;
2. It is sealed tightly enough to prevent the escape of liquids, vapours, and gases;
3. Its thickness is at least 4.8 mm, such that it is able to withstand a direct lightning strike.
At locations where a large number of petroleum and gas tanks are situated close together with no other structures or installations between them, earthing each tank at a single point only is sufficient. In such cases, all tanks shall be connected (bonded) to one another.
If the largest dimension (diameter or length) of combustible-material tanks is equal to or less than 20 metres, they shall be earthed at one point; otherwise, they shall be earthed at two points.
In petroleum and gas storage tanks with a floating roof, the floating roof shall be effectively bonded to the main shell of the tank. This bonding shall be established by means of parallel connections at intervals of 1.5 metres around the perimeter of the tank.
In petroleum and gas tanks, all accessories of movable covers (hatches, manholes, pressure safety equipment, and other openings) on the external floating roof shall be bonded to the external floating roof.
The metal seal shoe around the floating roof of a petroleum and gas tank shall be bonded to the floating roof at both sections of the shoe by one of the following two methods:
1. Inherently, through suitable design and construction;
2. By using at least one Class I lightning-protection conductor, or an equivalent thereof.
Where a non-metallic seal is used on the roof of petroleum and gas tanks, shunts meeting the following conditions shall be installed:
1. Shunt material: flexible stainless steel with a cross-sectional area of at least 20 mm², or other conductive materials of equivalent cross-section that are corrosion-resistant;
2. Minimum shunt width: 50 mm;
3. Maximum spacing between shunts around the outer perimeter of the floating roof: 3 metres;
4. The shunt shall follow the shortest and most direct path possible between the metal part of the roof and the tank shell;
5. The length of shunt used shall be the minimum possible, sufficient only to permit assembly of the floating-roof components, while ensuring that contact with the shell is maintained throughout all horizontal and vertical movements of the floating roof;
6. The point of contact between the shunt and the shell shall be submerged, at least 0.03 m below the surface of the liquid inside the tank.
In tanks whose roofs use a non-metallic seal, shunts installed on the deck of the floating roof shall be replaced with submerged shunts when existing tanks are refurbished.
The fixed bonding connection of the floating roof of a petroleum and gas tank to the shell shall be made by a direct electrical connection using at least two bypass conductors with a cross-sectional area at least equal to that of the main conductor.
The end-to-end electrical resistance of the bypass conductor used to bond the floating roof of a petroleum and gas tank to its shell, including all of its connections, shall not exceed 0.3 ohms.
The bypass conductor bonding the floating roof of a petroleum and gas tank to its shell shall be of the minimum length possible, sufficient to permit movement of the floating roof.
For the bonding connection of the floating roof of a petroleum and gas tank to its shell, bypass conductors shall be distributed at equal intervals around the circular perimeter of the tank, with one conductor installed for every 30 metres of the tank's outer perimeter.
If petroleum and gas tanks with a floating roof are fitted with a rolling ladder on the tank roof, both sides of the ladder shall be bonded, by a flexible conductor with a cross-sectional area of at least 35 mm² and a thickness of at least 3 mm, to the tank shell and roof at two points: one between the ladder and the upper part of the tank shell, and one between the ladder and the floating roof of the tank.
Note – Where the rolling ladder is not connected to the floating roof, at least two flexible bonding conductors of the dimensions specified in this Article shall be used between the tank shell and the floating roof.
If a rolling ladder is located on the floating roof of a petroleum and gas tank, one of the bypass conductors of the bonding connection between the roof and the shell shall be installed along the length of the ladder and bonded to it.
Any conductive component in the seal section of the roof of a petroleum and gas tank that is not fully submerged in the liquid inside the tank, and any gauge, guide pole, or component that penetrates from the floating roof into the tank, shall be electrically insulated from the tank shell. The dielectric strength of this insulation shall be at least 1 kV.
Electrodes made of materials that are cathodic relative to the structure under protection shall be connected to one another by means of DC decoupling devices, unless it is provided that both the protected structure and the electrodes are to be subjected to cathodic protection together as a single assembly.
Direct connection to the structure of electrodes made of materials that are anodic relative to the structure under protection is permitted.
Where it is not possible to use a single common earth electrode network for the central control section and the explosion-prone area, and the earthing systems of these two sections are isolated from one another, the electronic circuits between these two sections shall also be galvanically isolated from one another.
Where a TN earthing system is used in explosion-prone areas, it shall be of the TN-S type, and at no point within the area shall the neutral wire and the protective conductor be connected to one another.
Note – Under the circumstances addressed by this Article, at every point where a TN-C system transitions to a TN-S system, that point shall simultaneously be connected to the bonding system of the explosion-prone area.
Where shielded cables are used in an explosion-prone area, the shield of each cable shall be earthed at only one point, located outside the explosion-prone area.
In hazardous locations, above-ground metal pipes situated outside process units shall have an earth connection, by means of a rod or surface electrode, at least once every 30 metres.
In hazardous locations such as filling stations, the shells of tanks, metal parts of the building, cylindrical cylinders, outdoor metal installations and equipment, and metal piping shall be connected to the earthing conductor.
In hazardous locations of refineries and fuel storage depots that have a number of tanks containing explosive substances, all tanks shall be connected (bonded) to one another for the purpose of equipotential bonding.
In structures having Zone 0 (continuously hazardous) and Zone 1 (occasionally hazardous) areas, spark gaps (ISG) and insulating sections shall be relocated to outside the hazardous areas.
Closed metal tanks whose interior spaces are defined as Zone 0 (continuously hazardous) shall have a wall thickness that is adequate, in accordance with Table P-P-6 of Annex P, to ensure that the rise in internal surface temperature at the point of a lightning strike does not give rise to a hazard; otherwise, the installation of air-termination devices is mandatory.
In Class I locations, bonding shall not rely solely on the metallic connections between pipes; the two sides of such metallic connections shall be bonded to one another by a bonding wire (jumper).
Metal equipment located in Class I locations shall, in addition to being separately connected to the earthing system, be bonded to other earthing systems that exist for electrical or protective purposes.
In Class I, Zone 2 locations (rarely hazardous), the various parts of the enclosures of large motors and generators shall be bonded to one another, and the enclosure (frame) shall be connected to earth.
In hazardous locations, all portable connecting cords shall have earthed sockets.
For mobile equipment situated in a Class II location (solely where combustible fibres or explosible suspended particles are present), such as mobile cranes and hoists used for materials handling, mobile cleaning equipment for textile machinery, and similar equipment, the following shall be observed:
1. The power source shall be electrically separated from all other systems, and its earth connection shall likewise be separate from other systems.
2. This power source shall also be equipped with an acceptable earth-fault detector that, in the event of a fault occurring in the earthing system, either issues a warning signal and automatically de-energises the connecting conductors, or issues a visual and audible warning for as long as power remains connected to the conductors and the earth fault persists.
All extraneous-conductive-parts associated with intrinsically safe circuits, including equipment and enclosures, metal wireways, cable sheaths, and equipment integrated with them, in hazardous locations, shall be connected to the equipment earthing conductor.
Metal intrinsically safe devices in hazardous locations shall be supplementarily bonded to one another.
Where metal wireways are used for the wiring of an intrinsically safe system, bonding is mandatory at all terminating points of the wireway run, regardless of location.
In Class I, Zone 2 circuits (rarely hazardous), where multi-core cables are used for the installation of separate intrinsically safe protection systems, the conductors of each circuit shall be enclosed within a metal shield connected to earth (earthed).
Bonding shall be applied to all intervening metal wireways, junction boxes, distribution boards, and the like, between Class I locations and the earthing point for service equipment or the earthing point of a separately derived branch system.
In all vehicle repair garages classified within the group of hazardous locations, all power sockets installed at locations where electrical diagnostic equipment, hand-held electrical tools, or portable hand-held lighting equipment is used shall be provided with a residual current device (RCD).
In all vehicle repair garages, at filling stations, and at fuel storage locations classified within the group of hazardous locations, all wireways, metal armour or metal sheathing on cables, and all metal parts of fixed or portable electrical equipment that do not normally carry electric current shall be connected to the earthing system and then bonded.
All dispensers shall be equipped with one or more emergency circuit-breaking devices, installed at a distance of not less than 6 metres and not more than 30 metres from the relevant fuel dispensing equipment. The emergency circuit-breaking device shall disconnect power to all fuel dispensing equipment and to all pumps upstream of the dispenser, together with the associated supply, control, and signal circuits and other electrical equipment located within the hazardous locations classified around the fuel dispensing equipment.
At spray work stations and in painting and coating processes in which flammable or combustible materials are used, all persons and electrically conductive objects — including metal parts of the process equipment or apparatus, material containers, gas exhaust ducts, and piping systems carrying flammable or combustible liquids — shall be connected to earth.
At spray work stations and in coating processes and other processes employing electrostatic charging methods, which use atomizer and electrostatic charging equipment and the like mounted on a mechanical support, the electrodes and electrostatic atomizer heads shall be effectively insulated from earth.
At spray work stations and in coating processes, high-voltage wires shall be suitably insulated, and all components exposed to high voltage shall be permanently mounted on suitable insulators that are effectively protected, by means of an earth connection, against accidental contact.
Objects being spray-painted or powder-coated shall have an electrical connection to the conveyor belt, hangers, or any other support that is connected to earth, and hooks shall be cleaned regularly to ensure they are electrically connected to earth with a resistance not exceeding one megohm.
All electrically conductive objects in the spray-painting and powder-coating work area, except objects that must be at high voltage for the purposes of the process, shall have a suitable earth connection. This requirement covers paint containers, wash buckets, guards, hose fittings, and any other electrically conductive object or device in the area.
The handle of the spray gun shall be electrically connected to earth by means of a suitable conductor such that the earth resistance does not exceed one megohm.
All persons entering the spray-painting and coating work area shall have a suitable connection to earth by means such as wrist straps, ankle straps, and anti-static footwear, and warning notices requiring the earthing of persons shall be installed at the location.
All metal wire conduits, metal armour and metal sheathing of cables, and all metal parts of fixed or portable electrical equipment at powder-coating stations shall be connected to earth, and bonding shall be established between them.
When performing spray operations within membrane enclosures, the workpiece shall have a suitable earth connection. Likewise, the spray-painting equipment shall be connected to earth. The support fixture holding the workpiece shall be bonded to the workpiece and shall have a suitable earth connection.
Chapter 10 - Earthing Systems of Hospitals and Other Medical Locations
In hospitals, to prevent interference with sensitive electronic devices from electromagnetic waves, a TN-S earthing system shall be used.
The use of TN-C and TN-C-S earthing systems in hospital electrical installations, in medical locations, and in medical buildings, downstream of the main distribution board, is prohibited.
In medical location group 2, an IT earthing system shall be used for equipment supply circuits in the following cases:
1. Medical electrical equipment;
2. Systems used for life support;
3. Surgical applications;
4. All equipment used within the “patient environment”.
Note – In these medical locations, at least one separate IT earthing system shall be implemented for each group of rooms having a single type of function.
For isolating transformers used in the IT earthing system of medical facilities, the maximum leakage current from the output winding to earth, as well as the leakage current to the enclosure, under no-load conditions and at rated voltage and frequency, shall be 0.5 mA.
In the IT earthing system of medical facilities, for three-phase loads, a separate three-phase transformer with a line-to-line output voltage not exceeding 250 volts shall be used.
The transformer shall be installed in the vicinity of the medical location (inside or outside), and, for protection against accidental contact with live parts, shall be housed within a cabinet, enclosure, or cover.
For each IT earthing system in medical facilities, an insulation monitoring device (IMD) with a permanent visual and audible alarm system shall be provided and installed at a suitable location where it can be continuously monitored by medical staff.
The insulation monitoring device used in the IT earthing system of medical facilities shall have the following characteristics:
1. The device's internal AC resistance shall be at least 100 kΩ;
2. The test voltage shall not exceed 25 volts;
3. The test current, even under fault conditions, shall not exceed one milliampere;
4. Indication of a fault shall begin once the insulation resistance has dropped to at least 50 kΩ;
5. It shall be capable of indicating transformer overload and overtemperature conditions.
In medical locations, an equipotential bonding busbar shall be provided and installed in, or in the vicinity of, each distribution board enclosure, for the purpose of connecting equipotential bonding conductors and protective conductors.
In medical locations, the connections of the conductors to the equipotential bonding busbar within distribution boards shall be clearly visible and disconnectable from the system.
Fixed metal operating tables that do not operate electrically shall be connected to the equipotential bonding conductor of the earthing system.
In each medical location of groups 1 and 2, supplementary bonding for equipotential purposes shall be carried out between the following various metal parts located at a height of less than 2.5 metres above the floor:
1. Supplementary bonding busbar;
2. Extraneous-conductive-parts;
3. Metal screen of the isolating transformer;
4. Protective screen against interference from magnetic fields;
5. Protective screen against interference from conductive floors;
6. Conductive enclosures of equipment used in safety extra-low voltage (SELV) systems, including operating-table lamps;
7. Fixed metal operating tables that do not operate electrically.
In the earthing system of medical locations, the resistance of the conductors and connection points between the protective-conductor terminals of sockets, or the terminals of fixed installed equipment, or any type of extraneous-conductive-part, and the equipotential bonding busbar shall not exceed 0.2 ohms.
A sufficient number of dedicated earthing sockets shall be provided for earthing the frames of operating or delivery tables, intensive care rooms, anaesthesia machines, surgical instrument tables, and mobile X-ray imaging equipment.
The metal enclosures of surgical lamps, ceiling- and wall-mounted units for compressed-air, vacuum, oxygen, and natural gas system valves, metal frames of doors and windows, and all metal objects shall be permanently connected, by a wire with a cross-sectional area of at least 16 mm², to the room's earthing network.
In operating rooms, delivery rooms, intensive care rooms, and casting (plaster) rooms, a separate individual wire shall be run from the main earthing busbar of the isolated distribution board to the relevant device, for each earthing box, lamp, unit, metal frame, and accessible metal object.
In operating rooms, delivery rooms, intensive care rooms, and casting rooms, the cross-sectional area of the earthing wire of power sockets that is run in the same conduit as the phase and neutral wires shall be equal to the cross-sectional area of the corresponding phase and neutral wires.
In medical location groups 1 and 2, in open-heart surgery departments, cardiac angiography and cardiac catheterization diagnostic departments, and cardiac surgery intensive care units, equipment supply circuits located at a height of less than 2.5 metres shall, in addition to the earthing system, also be equipped with a suitable residual current device (RCD).
In open-heart surgery and cardiac catheterization operating rooms, equipotential bonding shall be established, as applicable, between the following components:
1. Tables/beds and other metal objects;
2. Pipes and metal parts of the building and mechanical installations;
3. The enclosures of distribution boards and all electrical devices and equipment;
4. Anti-static or static-conductive flooring and electrostatic shielding;
5. Protective conductors of power socket circuits.
For each patient bed in the patient bed space in intensive care units, a dedicated earthing plug shall be provided for connecting the protective conductor (PE) to the patient bed.
In the neonatal intensive care unit (NICU) and in cardiac surgery intensive care medical locations, a TN-S earthing system shall be used.
For each neonatal bed space in the neonatal intensive care unit (NICU), a dedicated earthing plug shall be provided and installed for connecting the protective conductor (PE) to the neonatal incubator.
The enclosures of all electrical appliances and touchable metal objects and equipment, and the protective conductors of socket and lighting circuits, in pantries, shower cubicles, and the research laboratory of the neonatal intensive care unit, and in shared laboratory spaces, the blood bank, the dirty utility room, the isolation room, the research laboratory of the cardiac surgery intensive care unit, and similar spaces, shall be supplementarily bonded for equipotential purposes.
The supply system for electrical installations in isolation rooms and patient bed spaces in the cardiac surgery intensive care unit shall be of the medical IT type.
Patient isolation rooms and patient bed spaces in the cardiac surgery intensive care unit shall have a dedicated isolated-supply distribution board equipped with an isolating transformer, a dedicated monitor, and indicators for leakage current level, normal condition, and fault alarm; these boards shall be supplied from the essential power supply.
For the detection of possible faults in the circuits supplying patient care areas in medical facilities, an effective earth connection shall be provided by means of a metal conduit or a cable with metal armour or a metal sheath.
In medical facilities, the earth terminals of all sockets — except sockets isolated from earth — metal socket boxes, metal boxes of appliances or metal enclosures, and all conductive surfaces of fixed electrical equipment that do not normally carry current and that could become energised at a voltage exceeding 100 volts and come into contact with a person, shall be directly connected to an insulated copper earthing conductor.
The earth terminals of the normal- and essential-power distribution board equipment for branch circuits located in the patient care area shall, in addition to their earth connection, be bonded to one another by a continuous insulated copper conductor with a cross-sectional area greater than or equal to 2.5 mm² (size 2.5).
In the patient care area, the earthing of power sockets shall not be limited to the functional earth connection (isolated earth); a protective earth connection is also mandatory.
At the patient bed location in a general patient care area, the earth terminal of all socket-outlets shall be connected to a single insulated copper equipment earthing conductor.
In critical patient care areas, the earth terminal of all socket-outlets at the patient bed location and of the operating room socket-outlets shall be connected to the main earthing busbar by an insulated copper equipment earthing conductor.
At all socket-outlets where portable medical diagnostic equipment, such as mobile X-ray equipment, may be used, an earthing conductor connected to the main earthing busbar shall be provided.
In medical locations where flammable and explosive anaesthetic agents are used, all metal cable conduits and copper-armoured cables, and all conductive parts of fixed electrical equipment that are not normally energised, shall be connected to an equipment earthing conductor installed for Class I locations.
Note: Equipment operating at a voltage below 10 volts is not required to have an equipment earthing connection.
In medical locations where flammable and explosive anaesthetic agents are used, the wiring of isolated circuits shall be installed in conduits or cables separate from the earthing wires and cables.
In medical locations where flammable and explosive anaesthetic agents are used, all conductive surfaces of equipment and luminaires, and the core and frame of isolating-type transformers supplying low-voltage circuits, shall be connected to an equipment earthing conductor.
In locations where X-ray equipment is used, all of the following shall be connected to a protective earthing conductor: 1. the metal enclosure of any capacitors present; 2. all high-voltage parts used to generate the X-ray; 3. all metal parts of the X-ray equipment and its ancillary equipment that are not normally energised.
Chapter 11 - Earthing of Exhibition Halls, Television Studios, Fairs, and Similar Events
In performance venues, television studios, and similar locations, the chassis of electronic (solid-state) dimmers shall be connected to a protective earthing conductor.
At outdoor gatherings such as festivals, celebrations, mourning ceremonies, exhibitions, and similar events, all of the following equipment supplied from a single power source shall be bonded together: 1. metal cable routes and metal-armoured cables; 2. metal enclosures of electrical equipment; 3. metal frames and metal parts of portable structures and trailers, or other equipment that houses electrical equipment or supports electrical equipment.
At outdoor gatherings such as festivals, celebrations, mourning ceremonies, exhibitions, and similar events, a TN-C-S earthing system shall be implemented.
In television studios and similar locations, all cables containing an insulated protective earthing conductor, metal cable routes, and all metal parts of appliances and equipment shall be connected to a protective earthing conductor.
Note: This Article does not apply to portable lamps, portable stage lighting, stage audio equipment, or other special portable stage equipment operating at a voltage below 150 volts DC to earth.
Chapter 12 - Earthing of Agricultural, Livestock, and Horticultural Installations
In locations within agricultural and livestock facilities and land where corrosive conditions exist (such as manure collection areas and damp locations contaminated with corrosive particles from wastewater and chemical detergents, and similar cases), the buried earthing conductor shall be insulated.
In all agricultural structures and buildings, the equipment earthing conductor shall be connected, at the electrical distribution panel, to both the enclosure and the earthed conductor (neutral). (TN-C-S system)
In livestock areas with a cement floor where damp conditions exist, all exposed metal parts that may become energised shall be bonded to the floor equipotential plate (integrated reinforcing bars providing equipotential-plate conditions) and connected to the earthing system.
For electric irrigation machines, all of the following equipment shall have adequate earthing: 1. all electrical equipment on the irrigation machine; 2. all electrical equipment attached to the irrigation machine; 3. electrical panels and sub-panels; 4. control panels or control equipment that supply and control the electrical equipment up to the irrigation machine; 5. metal cable sheaths and metal cable routes; 6. the metal structure of the machine.
Exposed metal parts of the irrigation machine and of other mobile electric agricultural machines that are not normally energised shall be earthed by means of an earthing conductor forming part of the wire, cable, or power supply route of that machine. The size of this earthing conductor shall not be less than that of the largest supply conductor in any wire.
If the irrigation machine has a stationary point, an earth electrode system shall be connected to the machine at the stationary point for protection against lightning.
Chapter 13 - Earthing of Mobile Units and Caravans
Section 1 - Mobile Units
Inside all mobile and relocatable units, only TN and IT earthing systems may be used, and the use of the specific TN-C and TT earthing systems is prohibited.
Note: The requirements governing the earthing system of the fixed installations supplying power to mobile units are those of the general part of this Regulation and the provisions relating to the TN-C-S earthing system.
Accessible conductive parts of the mobile unit shall be connected, by means of a protective bonding conductor, to the main earth terminal inside the mobile or relocatable unit.
Note: Protective bonding conductors shall be of the stranded type with high flexibility.
In an IT earthing system inside a mobile unit having a conductive body and structure, the accessible conductive parts of the equipment shall be bonded to the conductive body and structure.
Note: For a unit that does not have a conductive body, the accessible conductive parts inside the unit shall be connected to one another and to the protective conductor.
If a TN earthing system is used inside a mobile unit and is supplied by either of the two methods — connection to an independent low-voltage generator, or connection through isolating equipment such as a transformer — the conductive body and other accessible conductive parts shall be connected to the neutral point, or, if the neutral is not accessible, to a phase conductor. [Footnote 82]
Footnote 82: For further explanation, refer to clauses 4, 411, and 717 of IEC 60364-7-717.
Note 1: Connecting the conductive body to a phase conductor is prohibited if the neutral is earthed outside or inside the mobile unit.
Note 2: Where the supply system is directly connected to the network power supply, all electrical and non-electrical metal parts shall be earthed by connection to an earthing busbar in the unit's electrical panel, and shall be connected, by means of an insulated conductor within the supply cable or wiring, to the earthing system located near the mobile unit's location; alternatively, this earth connection may be established outside the mobile unit.
In mobile and prefabricated units, the main electrical circuit disconnecting device shall, for earthing purposes, have a solderless-type earthing busbar or conductor with a sufficient number of terminals for all earthing conductors.
If no service equipment or disconnecting device is provided for the mobile unit, or if the mobile unit is supplied through a socket-outlet and plug, the earthing conductor of the network terminal, or the earth electrode, shall be bonded, by means of a copper bonding conductor, to the metal chassis or to the accessible earth terminal of the mobile unit.
At the earthing busbar of the main electrical circuit disconnecting device of mobile and prefabricated units, the neutral terminal shall be insulated.
All socket-outlets used in mobile and prefabricated units shall be of the earthed type.
In damp areas of mobile and prefabricated units, such as toilets, bathrooms, kitchens, sinks, and dishwashers, supplementary bonding shall be implemented in accordance with the articles set out in Part Two of Chapter Five of this Regulation.
All metal roofs and metal exterior coverings of mobile units shall be bonded in accordance with the following conditions: 1. the metal sheets shall overlap one another and be securely connected, by means of metal clamps, to the metal or wooden frame parts; 2. the lowest sheet of the metal exterior covering shall be secured by metal clamps at the point where the body connects to the floor; in each unit, two metal straps shall be used at its two ends for this purpose.
If the internal network of the mobile unit is of the TT or TN type, or if, in an IT system, a socket-outlet is provided to supply a specific load in the mobile unit that requires power disconnection upon the first fault, the power supply to the mobile unit's electrical network shall be provided through a residual current device (RCD).
Note: The socket-outlet connecting a cable plug to the main circuit located on the exterior body of the mobile unit shall be equipped with a residual current device (RCD) with a tripping current of less than 30 milliamps.
Section 2 - Caravans
In caravan parks, the earthing system of the caravan power supply panel shall be of the TN-C-S type, and, at the point where the neutral conductor is separated, it shall be connected to a supplementary earth electrode.
At the output of the supply circuit downstream of the residual current device, no connection shall be made between the earthing conductor and the neutral conductor.
The socket-outlet connecting a cable plug to the main circuit located outside the caravan shall be equipped with a residual current device (RCD) as the circuit-disconnecting device.
The socket-outlet supplying power to caravans shall be equipped with earthing.
For earthing the metal gas, water and waste pipes in a caravan, bonding them to the chassis is permitted.
Bonding requirements shall comply with the provisions of Chapter 5 of this Regulation.
Circuit earthing conductors shall have the necessary mechanical strength and electrical continuity.
Bonding of metal roofs shall comply with Chapter 5 of this Regulation.
The caravan electrical panel shall have an earth busbar with a sufficient number of terminals for all earth conductors, and shall be connected, via the caravan supply cable, to the earth busbar of the supply panel located at the parking site.
The caravan's neutral conductor shall be insulated from the equipment earthing conductors, equipment enclosures and other earthed parts.
Chapter 14 - Earthing of Floating Structures, Piers, and Ports
Section 1 - Piers and Ports
None of the earthing conductors in the port shall be made of aluminium, or be a non-flexible cable with mineral insulation and a copper sheath.
The design of the pier supply system shall be such that each fuelling point on ships can be connected to the earthing conductor of the electrical distribution system.
Earthing of ships' fuelling points shall be established before fuelling begins and shall remain in place until the end of the fuelling stage and the disconnection of the discharge pipes from the ship.
Metal parts at the fuelling point shall have a permanent connection to the ship's fuel tank and to the protective conductor of the circuit of all protective wiring on the ship.
All metal parts on floating surfaces within the port that contain electrical equipment, or that may come into contact with electrical equipment, and all accessible metal parts, shall be bonded to earth.
When a vessel is berthed in the port and its power is supplied from the pier, an earth conductor shall be connected to the hull for the main earthing of the deck.
In the vicinity of water, all metal parts in contact with water, all metal piping and tanks, and all metal parts that do not normally carry current but may become live, shall be bonded to the earth terminal in the distribution equipment.
In the vicinity of water, near all outdoor service equipment or disconnecting devices that control equipment in or on the water, that have metal enclosures and controls accessible to persons, and that may become live, equipotential planes shall be installed to achieve the objectives of supplementary bonding. These equipotential planes shall surround the area around the equipment and shall extend directly outward from the area beneath the equipment, such that in every direction in which a person could stand and come into contact with that equipment, they extend at least 900 millimetres.
Supplementary bonding of the equipotential planes in the vicinity of water shall be carried out through a local busbar that is bonded to the main earthing system.
Section 2 - Floating Structures
Earthing of the electrical equipment and the non-electrical metal components of floating structures on water shall be established by connection to an earth busbar in the electrical panel of that same structure.
Earthing of the earth terminal in the power distribution equipment of floating structures shall be established by connecting an insulated earth electrode conductor to an earth electrode on land.
In floating structures, implementation of a TN-C-S earthing system is mandatory.
All outdoor socket-outlets within or on buildings or floating structures in the pier area, and electrical supply panels in the vicinity of water, shall be equipped with a residual current device (RCD), and shall be positioned at least 1 metre above the highest water level at high tide.
Chapter 15 - Earthing of Temporary Electrical Installations
All socket-outlets used in temporary electrical installations shall be of the earthed type, and the earth connection of the socket-outlet shall establish a reliable electrical connection with the equipment earthing conductor.
In temporary electrical installations, all branch circuits shall have a separate equipment earthing conductor.
In temporary electrical installations, the metal frames of lamps and metal-framed sockets are usable only where they have a proper earthing connection.
Chapter 16 - Earthing of Cranes, Hoists, and Concrete Pump Trucks
All accessible metal parts of cranes, monorails, workplace hoists and their accessories shall be bonded to one another, such that the entire crane structure is also capable of carrying the maximum probable earth fault current.
The trolley frame and the bridge frame of overhead gantry cranes shall be bonded by means of a separate bonding conductor.
In electric hoists and in vehicles equipped with a hoist (such as mobile cranes), where an external power source is used, the metal conduits carrying the cables connected to the hoist shall be bonded to the metal parts of that vehicle, and the metal parts shall in turn be bonded to the equipment earthing conductor. Likewise, tools and lighting fixtures shall be connected to earth by means of an equipment earthing conductor.
In non-electric hoists that have electrical conductors connected to the vehicle, the metal frame of the vehicle, which is normally accessible to persons, shall be bonded to the earthing system.
Hoisting equipment and equipment with moving parts at height, such as concrete pump trucks, shall, owing to the risk of capacitive induction, electrical arcing and inadvertent contact with overhead power transmission lines, have a reliable earthing connection while operating in the vicinity of such lines.
Chapter 17 - Earthing of Information Technology Systems, Sensitive Electrical and Electronic Equipment, Telecommunications, Fibre-Optic Installations, and Radio and Television Systems
In the design of the electrical installations of industrial facilities with highly sensitive equipment, the requirements of standard IEC 61000 shall be observed to avoid electromagnetic interference.
In workplaces that make extensive use of computer networks and information technology systems, the earthing system shall be of the TN-S type, and the use of a TN-C system is not permitted.
In information technology systems, all accessible metal parts that do not normally carry electric current shall be bonded to the equipment earthing conductor.
In information technology (IT) equipment rooms, the signal reference network shall be bonded to the earthing conductor of the IT equipment. [Footnote 83: "Signal reference structure"]
To prevent a potential difference and induced voltage from arising between the protective earthing system and the instrumentation earth (clean earth), these two systems shall be connected to each other. [Footnote 84: "Clean earth"]
Note: Bonding the protective earth to the cathodic protection system causes interference with the cathodic protection system; therefore, the two systems shall not be connected directly to each other. Instead, a surge arrester with an air gap (surge gap) shall be installed between the two systems. [Footnote 85: "Surge Gap"]
In locations with telecommunication and industrial automation installations, where a lightning rod (air termination) is required, a separate earth electrode shall be provided for it, which shall be connected to the protective earth electrode and the operational earth electrode at the bonding busbar.
The start and end points of metal conduit, trays, and metal ladders (cable ladders) containing incoming telecommunication wire or cable shall be connected to an earthing electrode by a bonding conductor or an earth electrode conductor.
Where a telecommunication cable with a metallic sheath is used, the metallic components of the cable sheath shall be connected to earth.
Metal masts and structures supporting the antenna, and the antenna discharge unit, shall have proper earthing.
Chapter 18 - Earthing of Industrial X-Ray Equipment (Non-Medical, Non-Dental) and Induction and Dielectric Heating Equipment
The metal enclosure of the capacitor guards of industrial X-ray equipment, the metal enclosures protecting all high-voltage parts of this equipment, and likewise all metal parts of the X-ray equipment and its peripheral equipment (including controls, tables, the X-ray tube support, the transformer tank, shielded cables, X-ray tube heads, and the like) that do not normally carry electric current, shall have proper earthing.
Mobile and portable X-ray equipment shall be fitted with a suitable earthed plug.
In induction and dielectric heating equipment, for the operation of the circuit and to limit radio-frequency voltages to a safe level, bonding by means of an equipment earthing conductor shall be carried out between all accessible parts of the equipment that do not normally carry electric current, between all parts of the equipment and the objects surrounding them, and between those objects and earth.
Chapter 19 - Earthing of Swimming Pools, Fountains, and Similar Installations
For the supplementary protective equipotential bonding of swimming pools, fountains, and similar installations, compliance with all sub-clauses of Clause 702-415, "Supplementary Protection," of Iranian National Standard No. 702-7-1937 shall be observed.
Supplementary (additional) equipotential bonding shall be carried out throughout the area surrounding swimming pools. For this purpose, all exposed live conductors and extraneous conductive parts, including metal ladders, metal diving-board supports, and poolside railings located in Zones 0, 1, and 2, shall be connected to one another.
The supplementary (additional) bonding connection for equipotential bonding in the pool area shall be made using an insulated conductor of suitable cross-sectional area.
Earth and bonding terminals used in swimming pools, fountains, and similar installations shall be of a type specifically intended for use in wet and corrosive environments.
Bonding and earthing connections installed in wet, damp, or corrosive environments shall be made of copper, copper alloy, or stainless steel.
For wet-niche luminaires supplied by flexible wire or cable, all exposed metal parts that do not normally carry electric current shall be connected to earth by means of an insulated copper equipment earthing conductor contained within the wire or cable. This earthing conductor shall be connected to the earth terminal in the supply junction box and in the transformer enclosure or other enclosures.
In swimming pools and similar environments, all types of wall-mounted luminaires, wet-niche luminaires, dry-niche luminaires, and no-niche luminaires shall be connected to an insulated copper equipment earthing conductor installed together with the circuit conductors (wires).
In swimming pools and similar installations, junction boxes and the enclosures of transformers, power supplies, and ground-fault circuit interrupters (GFCIs) that are connected to a cable conduit, where that conduit runs directly to a supporting stake or to the enclosure of a no-niche luminaire, shall be fitted with a sufficient number of earth terminals.
In swimming pools and similar installations, the equipment earthing conductor terminals of a junction box and the enclosure of a transformer or other enclosures in the supply circuit to a no-niche or wet-niche luminaire, and the wiring compartment of a dry-niche luminaire, shall be connected to the equipment earth terminal of the electrical panel. This terminal shall be connected directly to the enclosure of the electrical panel.
For the purpose of equipotential bonding, the following items in swimming pools, fountains, and similar installations shall be connected to one another: 1. Conductive pool shells (poured concrete, shotcrete/pneumatically applied concrete, and painted or coated concrete block are considered conductive materials, while fibreglass composite shells and vinyl-lined shells are considered non-conductive materials); 2. Perimeter surfaces extending horizontally 1 metre from the inner walls of the pool; 3. Metal components of the pool structure; 4. All metal enclosures and supporting stakes of underwater no-niche luminaires; 5. Metal parts of electrical equipment connected to the pool water circulation system, including pump motors, and metal parts of equipment connected to pool covers; 6. All fixed metal parts, including metal-sheathed cables, metal piping, metal fences, and metal door and window frames; 7. Metal ladders and diving-platform supports; 8. Floor drains and drainage lines; 9. Water inlet and outlet pipes.
The pool filter pump shall be equipped with a means of earthing only the internal, inaccessible metal parts that do not normally carry current. This earthing means shall be an equipment earthing conductor contained, together with the supply conductors, within a flexible wire (cable), and shall be connected to an earthed receptacle having a fixed earthing member.
In swimming pools and similar installations, all of the following shall be protected by ground-fault circuit interrupters (GFCIs): 1. All electrical equipment, including supply wiring, used in storable pools; 2. Receptacles supplying power to spas (hot mineral springs) or hot tubs; 3. Floating luminaires (lights), floating pumps, and other floating equipment of water features and fountains, unless limited to low voltage; 4. Electrical equipment of a water feature that is connected to the power supply by a cord and plug; 5. Receptacles supplying power to therapeutic (hydrotherapy) tubs and their reservoirs that are not readily movable; 6. All receptacles located within 1.8 metres of the tub; 7. Hydromassage tubs and electrical components connected to them; 8. All single-phase receptacles rated below 30 amperes located within a horizontal distance of 1.8 metres from the side walls of the hydromassage tub.
In spas (hot mineral springs), hot tubs, therapeutic tubs and reservoirs, and hydromassage tubs, the following items shall be bonded to one another: 1. Fixed metal parts within the tub structure or connected to it; 2. Metal parts of electrical equipment connected to the tub's water circulation system, including pump and blower motors; 3. Metal-sheathed cables and wiring conduits, and metal piping, located within 1.5 metres of the inner walls of the tub, where no permanent barrier separates them from the tub; 4. All metal surfaces within 1.5 metres of the inner walls of the tub, where no permanent barrier separates them from the tub; 5. Electrical devices and controls that are not mounted on the tub but are located less than 1.5 metres from these units.
All metal piping systems connected to a water feature or fountain shall be bonded to the equipment earthing conductor of the circuit supplying it.
For electrical equipment of a water feature or fountain that is supplied by a flexible cable, all exposed metal parts that do not normally carry electric current shall be connected to earth by means of an insulated copper equipment earthing conductor that is part of this flexible cable. The equipment earthing conductor shall be connected to an equipment earth terminal in the junction box, the transformer enclosure, the power-supply enclosure, or other enclosures.
Pool lifts used for persons with disabilities or limited mobility that operate at a voltage higher than the low-voltage touch-contact range shall be equipped with GFCI protection.
Chapter 20 - Earthing of Silos
To provide the main earthing network of a silo, reinforcing bars of reinforced concrete in footings and foundations that are not insulated from earth and whose depth below ground level is at least 1 metre may be used.
Steel structures mounted on the concrete silo footings that form the main earthing network shall be bonded to the concrete reinforcing bars by means of the structure's holding-down bolts or by using a cable.
In concrete silos, the metal enclosures of all medium-voltage and low-voltage switchboards and transformers shall be connected to one another by copper straps or stranded copper wires of suitable cross-sectional area, and then connected to the main earthing network.
All silos shall be equipped with a reliable lightning protection system.
On the roof of the silo, the lightning rods (air terminals) shall be interconnected by a copper strap with minimum dimensions of 3 × 20 millimetres.
All metal parts present on the roof of the silo, such as a pitched (sloped) roof or a metal canopy, shall be equipotentially bonded to the lightning-rod (air-termination) network.
Chapter 21 - Earthing of New and Renewable Energy Equipment and Installations
DC photovoltaic (PV) arrays of solar energy installations shall be equipped with suitable protection against DC earth faults.
For solidly earthed photovoltaic (PV) systems, the DC circuit earthing conductor shall be installed at a single point on the PV output circuit, and this earthed conductor shall be connected to the earth electrode system by means of an earthing electrode conductor.
Exposed metal parts of PV module frames, electrical equipment, and conductive enclosures of PV systems that do not normally carry current shall have suitable earthing.
Metal PV support structures shall have designated bonding jumpers that connect separate metal parts together and are connected to the equipment earthing conductor.
The equipment earthing conductors for the PV array and its support structure shall be run inside a single raceway or cable.
The building or support structure of the PV array shall be equipped with an earth electrode system.
The equipment earthing conductors of the PV array shall be connected to the earth electrode system of the building or support structure of the PV array.
In a fuel cell power system, the DC earthing system shall be bonded to the AC earthing system.
In a fuel cell power system, a separate equipment earthing conductor shall be installed.
In wind turbines, exposed metal parts of towers, turbine and other equipment bases, and conductive bases that do not normally carry current shall be earthed and bonded to the site earthing and bonding system.
The wind turbine tower shall be connected to the earth electrode system.
In close proximity to the galvanized foundation or anchor components of the wind turbine tower, galvanized earthing electrodes shall be used.
Equipment earthing conductors or supply-side bonding jumpers shall be provided between the wind turbines and towers and the site earthing system.
Chapter 22 - Earthing of Lighting Installations and Street Equipment
In a TT earthing system, lighting columns whose circuits are protected by a common RCD (residual current device) shall not be earthed by means of independent earth electrodes.
Where the TN-S system is used for street-installed equipment, a supply cable with separate phase, neutral, and earthing conductors shall be used.
For supplying and protecting street equipment using the TN-C-S system, a cable with a combined neutral-earthing conductor (PE) shall be used.
Accessible conductive parts of street equipment shall be connected to the equipment earthing terminal as well as to the earthing terminal of the supply circuit.
In the TN-C-S method for new installations, accessible conductive enclosures shall be connected to the neutral terminal by means of a copper conductor, and the cross-sectional area of this conductor shall be at least 10 mm² (wire No. 10) or equal to the cross-sectional area of the neutral conductor of the supply circuit.
Note – Small, separate metal components with a low probability of contact with accessible conductive parts, extraneous conductive parts, or the earthing conductor (such as small metal doors and door frames) shall not be connected to the earthing system in the manner described above.
Where a circuit supplies more than one street device (for example, in a loop configuration), an earthing electrode shall be installed at the last unit or the one preceding it, and the earthing resistance at any point, before connecting any bonding conductor or earthing conductor to the neutral terminal, shall be less than 20 ohms; and if this electrode resistance exceeds 20 ohms, additional earthing electrodes shall be installed along the circuit at equal intervals from one another.
Where the supply system is TN-C, but the company supervising public lighting wishes to use cables with separate earthing and neutral conductors, and also in cases where the electricity company has provided the earthing terminal but does not make the earth pit available for use by the lighting company, the lighting supervisory company shall install its own protective earth electrode, and in this case the earthing system shall be of the TT type.
The neutral earth electrode of the supply transformer (TN-C) is an important part of the connection loop, but its resistance relative to the earthing electrode is not under the control of the street lighting company, and in such circumstances residual current protective devices shall be used to ensure disconnection of equipment that has developed a fault.
The use of metal lamp posts or the metal frames of control units and the like as protective earthing electrodes is prohibited.
Chapter 23 - Earthing of Temporary Scaffolding and Other Similar Metal Structures
In structures erected using bolted connections or screw clamps, the multiple parallel paths formed across the body of the structure provide a low-resistance path for the flow of electric current; however, low path resistance across the overall structure does not mean electrical continuity throughout all parts of the metal structure, and this assumption shall not be adopted in designs. Power cables for equipment mounted on the structure shall have a separate earthing conductor.
The adequacy of an effective earthing connection for a structure cannot be inferred from the nature of the structure's connections, the manner of contact between the structure's legs and the earth (soil), or the connection of a temporary metal structure to a permanent structure, and reliance on these factors to presume the earthing to be adequate is prohibited.
In a temporary structure where electricity or electrical equipment is not used, or where the working voltage of the electrical circuit used is less than 50 volts alternating current (AC) and there is no risk of electric shock, a protective earthing bonding connection is not required.
Where temporary structures carry lighting circuits or small loads, the structure shall be bonded with a protective conductor.
A temporary metal structure is regarded as an extraneous conductive part and is connected to a protective conductor for safety against the risk of electric shock. This conductor is made of copper wire with a corrosion-protective sheath and shall be installed along a route where the operation of equipment and adjacent activities will not damage it. The distance between the structure's bonding connection points to the protective earth shall not exceed 20 metres.
If a temporary structure is erected next to an overhead high-voltage power line and there is a possible risk of contact between equipment or personnel and the power lines, a durable metal barrier or metal mesh (fence) with a proper connection to the protective earth shall be installed between the metal structure and the conductors of the overhead lines. Red-and-white banding, signage, and a danger warning notice shall be installed on the barrier and metal mesh.
The need to provide a lightning protection system for an elevated temporary structure shall be assessed on the basis of the risk assessment criteria of Iran National Standard (INSO) IEC 62305-2.
For a building equipped with a system for protection against lightning strike or a lightning rod, metal scaffolding attached to its external wall shall be bonded, at the bottom, to the dedicated earth electrode terminal of the lightning protection system, and at the top, to the air termination of the lightning rod.
The employer and all persons involved in the execution of work shall comply with the provisions of this Regulation, other laws currently in force in the country, and other approved regulations.
This Regulation, comprising twenty-three (23) chapters, 374 articles, (28) notes, and its annexes, was drafted pursuant to Articles 85, 86, and 91 of the Labour Law of the Islamic Republic of Iran at the 180th session of the Supreme Council for Technical Protection on 23/08/1401, and was ratified by the Minister of Cooperatives, Labour and Social Welfare on 30/9/1401 (2022).
This Regulation supersedes the Earthing System Regulation approved by the Supreme Council for Technical Protection on 21/03/1385 (2006).
Minister of Cooperatives, Labour and Social Welfare – Solat Mortazavi
Annex A - Types of earthing systems
Identification of conductors in alternating-current systems: - Letter L [footnote 86]: denotes the phase (line) conductor. - Letter N [footnote 87]: denotes the neutral conductor. - Letter PE [footnote 88]: denotes the protective conductor. - Letters PEN [footnote 89]: denote the combined protective/neutral conductor.
Accordingly, single-phase systems are as follows: 1. Two-wire systems: L1+L2; L1+N; L1+PEN 2. Three-wire system: L1+N+PE
Three-phase systems are as follows: 1. Three-wire system: L1+L2+L3 2. Four-wire systems: L1+L2+L3 plus (PEN, or PE, or N) 3. Five-wire system: L1+L2+L3+N+PE
Classification of low-voltage protective earthing systems: The types of low-voltage earthing systems are: 1. TN, comprising TN-C, TN-S, and TN-C-S 2. TT 3. IT
The naming of these electrical systems is as follows. Of the two main identification letters, the first (left-hand) letter denotes the relationship of the system to earth: - First letter T [footnote 90]: denotes that one point of the system is connected to earth. - First letter I [footnote 91]: denotes that the system is isolated from earth, or is connected to it through a large resistance.
Of the two main identification letters, the second (left-hand) letter denotes the relationship of the exposed conductive parts of the equipment to earth: - Second letter N [footnote 92]: denotes that the exposed conductive parts are connected to the earthed neutral conductor (neutral). - Second letter T: denotes that the exposed conductive parts are connected to earth independently of the system's earthing (neutral).
Auxiliary letters denote subsystems (C and S): - Third letter S [footnote 93]: denotes that the exposed conductive parts are connected, through a dedicated protective conductor (PE), to the neutral point of the system at the source (the TN-S system). - Third letter C [footnote 94]: denotes that the exposed conductive parts are connected to earth through a dedicated combined protective-and-neutral conductor (PEN) (the TN-C system).
Footnotes: 86. Derived from the word "Live". 87. Derived from the word "Neutral". 88. Derived from the words "Protective Earthing". 89. Derived from the words "Protective Earthing Neutral". 90. Derived from the Latin word "Terra", meaning earth. 91. Derived from the word "Isolated". 92. Derived from the word "Neutral". 93. Derived from the word "Seperatec" [sic - intended: "Separate"]. 94. Derived from the word "Common".
A-1. The TN system: In this system, the power source (substation transformer or generator) is earthed (connected to earth) at one or more points, and the exposed conductive parts and the extraneous conductive parts of the installation are connected, solely through an earthing conductor, to the earthed point or points of the source. In other words, a conductive path exists for the earth-fault current of the installation to flow to the earthed point or points of the source. This system is divided into the following types:
A-1-1. The TN-C system (Figure P-A-1): In this system, the earthing conductor and the neutral conductor are combined. In other words, the neutral wire that runs from the neutral busbar of the main switchboard to the loads is used both as the neutral and as the earthing conductor; that is, a branch is taken from the neutral wire to the exposed conductive parts of the load equipment and connected as the earthing conductor. Earthed concentric cables, or earthed metal-sheathed cables that provide a return path for the earth-fault current, are examples of this system. (Figure P-A-1)
A-1-2. The TN-S system (Figure P-A-2): In this system, the neutral and earthing conductors are separate from each other; that is, at the main electrical switchboard, in addition to the neutral busbar there is another busbar, called the earthing busbar, to which the main earthing conductor is connected from the earth electrodes. From there it runs, parallel to the neutral and phase conductors (as a five-wire arrangement for the three-phase case and a three-wire arrangement for the single-phase case), to the load equipment and is connected to their exposed conductive parts. (Figure P-A-2 - The TN-S earthing system)
A-1-3. The TN-C-S system (Figure P-A-3): Only in part of this system (usually at its beginning) are the neutral and earthing conductors combined; from that point onward, a separate wire branches off from the neutral and is connected separately to the exposed conductive parts of the load equipment. (Figure P-A-3 - The TN-C-S earthing system)
A-2. The TT system (Figure P-A-4): In this system, the power source (substation transformer or generator) is earthed at one or more points, and the exposed conductive parts and extraneous conductive parts are connected to a local earth electrode, or to electrodes that are electrically independent of the system's operational earthing. That is, the protective earthing has no connection whatsoever with the operational earthing. (Figure P-A-4 - The TN earthing system [sic, per source])
A-3. The IT system (Figure P-A-5): In this system, the power source (substation transformer or generator) is either entirely unearthed, or is earthed through a large impedance, and the exposed conductive parts of the installation are likewise connected to an earth electrode that is electrically independent. In this system too, the protective earthing and the operational earthing have no connection with each other. In the IT earthing system, the secondary side of the supply transformer, as the power source, shall be isolated from the earthing system, and the transformer body (transformer enclosure) shall be connected to earth. (Figure P-A-5)
Annex B - Soil resistivity
The resistance of an earth electrode depends on the electrical resistivity of the soil in which the electrode is installed. For this reason, this factor can be important for decision-making in the selection of protective systems.
Soil resistivity depends on the soil's moisture content, the chemical compounds and dissolved salts present in the soil, and the size and distribution of the soil grains and their closeness to one another. The resistivity of some soil types, in ohm-metres, is given in Table P-B-1.
Table P-B-1: Resistivity of some soil types, in ohm-metres
Columns: Soil type | Normal to heavy rainfall, more than 500 mm/year - probable value (ohm-metre) | Normal to heavy rainfall, more than 500 mm/year - range of actual values (ohm-metre) | Desert conditions and low rainfall, less than 250 mm/year - range of actual values (ohm-metre) | Groundwater (saline spring) - range of actual values (ohm-metre)
| - Alluvial clay soil: 5 | * | * | 1 to 5 |
|---|---|---|---|
| - Clay soil: 10 | 5 to 20 | 10 to 100 | 1 to 5 |
| - Calcareous soils: 20 | 10 to 30 | 50 to 300 | - |
| - Porous limestone (such as gypsum): 50 | 30 to 100 | - | - |
| - Porous black rock (clayey rocks and Keuper black rock): 100 | 30 to 100 | - | - |
| - Quartz, dense and crystalline limestone (such as marble): 300 | 100 to 100 | - | - |
| - Clay slate and clayey rocks: 1000 | 300 to 3000 | more than 1000 | 30 to 100 |
| - Granite: 1000 | - | - | - |
| - Schist and igneous rock: 2000 | more than 1000 | - | - |
* Depends on the local water table.
Annex P — Tables
Table P-P-1 — Minimum size of steel earth electrode types
| Finish | Electrode type | Diameter (mm) | Cross-sectional area (mm²) | Thickness (mm) |
|---|---|---|---|---|
| Strip | — | 90 | 3 | |
| Hot-dip galvanised or stainless | Round rods for deep earth electrodes | 16 | — | — |
| Hot-dip galvanised or stainless | Round wire for surface earth electrodes | 10 | — | — |
| Hot-dip galvanised or stainless | Pipe | 25 | — | 2 |
Table P-P-2 — Minimum size of copper earth electrode types
| Electrode type | Diameter (mm) | Cross-sectional area (mm²) | Thickness (mm) |
|---|---|---|---|
| Bare strip | — | 50 | 2 |
| Bare stranded rope | 1.8 per strand | 25 | — |
| Bare round wire for surface earth electrode | — | 25 | — |
| Bare pipe | 20 | — | 2 |
| Tin-coated stranded rope | 1.8 per strand | 25 | — |
| Zinc-coated strip | — | 50 | 2 |
Table P-P-3 — Minimum cross-sectional area of the protective conductor
| Minimum cross-sectional area of the phase conductor, S | Corresponding minimum cross-sectional area of the protective conductor (PE) |
|---|---|
| S <= 16 | S |
| 16 < S <= 35 | 16 |
| S > 35 | S/2 |
Table P-P-4 — Minimum cross-sectional area of the supplementary bonding conductor to the earthing system
| Type of supplementary bonding conductor | Minimum cross-sectional area of the supplementary bonding conductor |
|---|---|
| Conductor with mechanical protection | 2.5 mm² |
| Conductor without mechanical protection | 4 mm² |
| Conductor used in bathrooms and damp or wet locations | 4 mm² |
Table P-P-5 — Minimum cross-sectional area of supplementary bonding conductors
| Material | Minimum cross-sectional area (mm²) — protected against mechanical damage | Minimum cross-sectional area (mm²) — not protected against mechanical damage |
|---|---|---|
| Copper | 2.5 | 4 |
| Aluminium | 16 | 16 |
Table P-P-6 — Minimum thickness of metal sheaths or metal pipes in air-termination systems
| LPS class | Material | Thickness (t), mm | Thickness (t'), mm |
|---|---|---|---|
| I to IV | Lead | — | 2.0 |
| I to IV | Steel (stainless and galvanised) | 4 | 0.5 |
| I to IV | Titanium | 4 | 0.5 |
| I to IV | Copper | 5 | 0.5 |
| I to IV | Aluminium | 7 | 0.65 |
| I to IV | Zinc | — | 0.7 |
Note: t prevents puncture. t' applies only to metal sheaths where protection against puncture, hot spots, or sparking hazards is not important.
Table P-P-7 — Earth electrodes and their minimum size with respect to corrosion, rusting and mechanical strength
| Electrode material | Shape | Diameter (mm) | Cross-sectional area (mm²) | Thickness (mm) | Coating thickness (micron) |
|---|---|---|---|---|---|
| Steel embedded in concrete (bare, hot-dip galvanised, or stainless steel type) | Bare wire or round rod | 10 | — | — | — |
| Steel embedded in concrete (bare, hot-dip galvanised, or stainless steel type) | Strip | — | 75 | 3 | — |
| Hot-dip galvanised steel | Strip (with rounded edges) | — | 90 | 3 | — |
| Hot-dip galvanised steel | Round-section rod, installed vertically | 16 | — | — | 45 |
| Hot-dip galvanised steel | Bare wire, installed horizontally | 10 | — | — | 45 |
| Hot-dip galvanised steel | Pipe (double-walled, galvanised) | 25 | — | 2 | 45 |
| Hot-dip galvanised steel | Bare multi-strand wire, embedded in concrete | — | 70 | — | — |
| Copper-clad steel | Round-section rod, installed vertically | 15 | — | — | 2000 |
| Copper-clad steel, fused | Round-section rod, installed vertically | 14 | — | — | 250 |
| Copper-clad steel, fused | Strip, installed horizontally | — | 90 | 3 | 70 |
| Stainless steel | Strip | — | 90 | 3 | — |
| Stainless steel | Round-section rod, installed vertically | 16 | — | — | — |
| Stainless steel | Bare wire, installed horizontally | 10 | — | — | — |
| Stainless steel | Pipe | 25 | — | 2 | — |
Table P-P-7 (continued) — Earth electrodes and their minimum size with respect to corrosion, rusting and mechanical strength
| Electrode material | Shape | Diameter (mm) | Cross-sectional area (mm²) | Thickness (mm) | Coating thickness (micron) |
|---|---|---|---|---|---|
| Copper | Strip | — | 50 | 2 | — |
| Copper | Bare wire, installed horizontally | — | 25 | — | — |
| Copper | Round-section rod, installed vertically | 12 | — | — | — |
| Copper | Bare multi-strand wire (minimum diameter of each strand 1.7) | — | 25 | — | — |
| Copper | Copper plate | — | — | 2 | — |
| Copper | Pipe | 20 | — | 2 | — |
Annex D - Methods for Measuring the Earthing System
D-1 Two-Point Method for Measuring Earth Impedance
In this method, the resistance of an earth electrode is measured in series with an auxiliary earth electrode. It is assumed that the resistance of the auxiliary earth electrode is negligible compared with the resistance of the earth electrode under test. The measured value then represents the resistance of the earth electrode under test.
One application of this method is measuring the resistance of an earth rod relative to a nearby residential building. A residential building typically has a low-impedance earthing system, because it is connected to the neutral conductor of the supply system. Using such an earthing system as the auxiliary earth can yield a test result with acceptable accuracy.
It is clear that this method produces large errors when measuring low-resistance earths. If the earth under test and the auxiliary earth are very close to each other, the mutual resistance between the earths can also be a source of error.
D-2 Three-Point Method for Measuring Earthing
This method uses two auxiliary electrodes with known resistances r2 and r3.
The resistance of the electrode under test is denoted r1. The resistance between each pair of electrodes is measured and denoted r12, r13 and r23, such that:
r12 = r1 + r2 r13 = r1 + r3 r23 = r2 + r3
Solving the three equations simultaneously gives the following relationship:
[Formula]
By measuring the series electrical resistance of each pair of earth electrodes and substituting the values into the equation, the value of r1 can be obtained. If the two auxiliary electrodes are made of materials with higher resistance than the electrode under test, the errors of the independent measurements become severely magnified in the final results. For an accurate measurement, the electrodes must be spaced far enough apart from one another (in accordance with the instrument manufacturer's instructions) that the mutual resistance between them is minimised. If the spacing between the electrodes is insufficient, meaningless results such as zero or negative resistances may be observed. When measuring the resistance of an earth rod, the three electrodes must be spaced at least three times the depth of the rod under test from one another. It is also assumed that the auxiliary electrodes are placed in the soil at the same depth as, or a shallower depth than, the earth rod under test. As the earth electrode system becomes larger and more complex, this method becomes harder to carry out, and other methods - particularly where higher accuracy is required - are preferred.
D-3 Fall-of-Potential Method [Footnote 95]
In the fall-of-potential (FOP) method, a current is passed between an earth electrode (G) and a current probe (CP), and the voltage between G and a voltage probe (PP) is then measured.
(Figure D-1)
Figure D-1 - Fall-of-Potential Method
To minimise the internal-electrode effects caused by mutual resistances, the current probe is usually placed at a considerable distance from the earth electrode under test. This distance is usually at least 5 times the largest dimension of the earth electrode under test. The voltage probe is also placed in the same direction as the current probe. However, it can also be placed in the opposite direction, as shown in Figure D-1. In practice, the distance X for the voltage probe is often taken as 62% of the current-probe distance (when the voltage and current probes are aligned in the same direction). This distance is theoretically selected based on the correct position for accurately measuring electrode impedance in soil of uniform resistivity, on the assumption that there is sufficient distance between the earth electrode under test and the test probes for the test probes to be treated as a hemisphere, and also assuming that the earth electrode has no external interconnections.
When the conditions and constraints for the current probe are satisfactory, the position of the voltage probe is critical for accurately measuring the resistance of the earth electrode. This position must be free of any influence from the earth electrode under test and from the current probe. A practical method for determining whether other electrodes are influencing the voltage probe is to take several resistance readings while moving the voltage probe between the earth grid and the current probe. Two or three consecutive constant readings can be taken as the resistance value (the smooth-slope method). Figure D-2 shows typical graphs of impedance versus the distance of PP from the earth. The solid line represents the auxiliary electrodes aligned in the same direction, starting near the earth electrode (zero) and ending near the current probe. The graph corresponding to PP and CP placed in opposite directions behaves differently, as shown by the dashed line. In an ideal situation, a clear inflection (knee) point can be identified, which gives the impedance of the electrode under test. The graphs shown in Figure D-2 correspond to soils of uniform resistivity. For non-uniform soils, these graphs will not show the zero-slope sections that normally indicate influence-free regions.
(Graph)
Figure D-2 - Impedance versus voltage-probe distance for the fall-of-potential method. Fall-of-potential theory states that, for soil of uniform resistivity, the dashed line shown in Figure D-2 will always approach the solid line from below, but the required separation distance is greater than when the probes are used in different directions. Also, deviations due to soil non-uniformity are larger when PP and CP are placed in opposite directions.
Additional limitations of the FOP method prevent it from achieving a correct impedance value. An accurate impedance measurement is achievable only when the earthing system in question is properly taken into account. By definition, an effective electrical centre is a point on an earthing system through which the largest test current flows. Most isolated earth grids with simple geometric shapes can be represented by an equivalent hemisphere. For complex earthing systems, such as a large substation earth grid (or even a small substation earth grid with internal interconnections to shield and neutral systems), achieving such a representation is difficult.
Another method exists, called the "slope method". In this method, the assumptions of uniform soil resistivity and representation of the earth electrode system as an equivalent hemispherical electrode remain valid. However, this method allows the probe distances to be measured from a convenient point, such as the edge of an earth electrode system, and the resulting error distances are accounted for in the fall-of-potential equation. This method can be summarized as follows:
a) Select a convenient starting point for the linear measurements and choose a suitable distance for CP. b) Measure resistances R1, R2 and R3 by placing the PPs at distances of 0.2 CP, 0.4 CP and 0.6 CP, respectively. c) Calculate the slope-deviation coefficient.
mu = (R3 - R2) / (R2 - R1)
d) Obtain the value of PPT/CP corresponding to the value of mu from Table D-1. e) Measure the resistance with the voltage probe placed at distance PPT.
Interpretation of Results
In the fall-of-potential method, for a given current-probe location, there is a voltage-probe distance that gives the correct impedance of the earth under test. However, determining the correct distance can be very difficult, especially if the earth grid has a complex shape. Also, the correct distance is a function of the soil profile, as shown in Figure D-3, which applies to small earthing systems. As shown in Figure D-3, the required voltage-probe distance X (when the probe is located between C and E and the soil is uniform) is such that the following ratio holds:
x/d = 0.618
(for small hemispherical electrodes)
The preceding material shows that for the 62% rule to apply, the following conditions must be met:
1. Reasonably uniform soil. 2. Large distances between the earth grid under test and the reference electrodes, such that all electrodes can be treated as hemispheres. 3. The electrode under test has no external earth connections.
Footnote 95 - Fall of Potential
Table D-1 - Slope-Method Coefficients
| Ppr/cp | mu | Ppr/cp | mu | Ppr/cp | mu |
|---|---|---|---|---|---|
| 0.494 | 1.20 | 0.580 | 0.80 | 0.643 | 0.40 |
| 0.491 | 1.21 | 0.579 | 0.81 | 0.642 | 0.41 |
| 0.488 | 1.22 | 0.577 | 0.82 | 0.640 | 0.42 |
| 0.486 | 1.23 | 0.575 | 0.83 | 0.639 | 0.43 |
| 0.483 | 1.24 | 0.573 | 0.84 | 0.637 | 0.44 |
| 0.480 | 1.25 | 0.571 | 0.85 | 0.636 | 0.45 |
| 0.477 | 1.26 | 0.569 | 0.86 | 0.635 | 0.46 |
| 0.474 | 1.27 | 0.567 | 0.87 | 0.633 | 0.47 |
| 0.471 | 1.28 | 0.566 | 0.88 | 0.632 | 0.48 |
| 0.468 | 1.29 | 0.564 | 0.89 | 0.630 | 0.49 |
| 0.465 | 1.30 | 0.562 | 0.90 | 0.629 | 0.50 |
| 0.462 | 1.31 | 0.560 | 0.91 | 0.627 | 0.51 |
| 0.458 | 1.32 | 0.588 | 0.92 | 0.626 | 0.52 |
| 0.455 | 1.33 | 0.556 | 0.93 | 0.624 | 0.53 |
| 0.452 | 1.34 | 0.554 | 0.94 | 0.623 | 0.54 |
| 0.448 | 1.35 | 0.552 | 0.95 | 0.621 | 0.55 |
| 0.445 | 1.36 | 0.550 | 0.96 | 0.620 | 0.56 |
| 0.441 | 1.37 | 0.548 | 0.97 | 0.618 | 0.57 |
| 0.438 | 1.38 | 0.546 | 0.98 | 0.617 | 0.58 |
| 0.434 | 1.39 | 0.544 | 0.99 | 0.615 | 0.59 |
| 0.431 | 1.40 | 0.542 | 1.00 | 0.614 | 0.60 |
| 0.427 | 1.41 | 0.539 | 1.01 | 0.612 | 0.61 |
| 0.423 | 1.42 | 0.537 | 1.02 | 0.610 | 0.62 |
| 0.418 | 1.43 | 0.535 | 1.03 | 0.609 | 0.63 |
| 0.414 | 1.44 | 0.533 | 1.04 | 0.607 | 0.64 |
| 0.410 | 1.45 | 0.531 | 1.05 | 0.606 | 0.65 |
| 0.406 | 1.46 | 0.528 | 1.06 | 0.604 | 0.66 |
| 0.401 | 1.47 | 0.526 | 1.07 | 0.602 | 0.67 |
| 0.397 | 1.48 | 0.524 | 1.08 | 0.601 | 0.68 |
| 0.393 | 1.49 | 0.522 | 1.09 | 0.599 | 0.69 |
| 0.389 | 1.50 | 0.519 | 1.10 | 0.597 | 0.70 |
| 0.384 | 1.51 | 0.517 | 1.11 | 0.596 | 0.71 |
| 0.379 | 1.52 | 0.514 | 1.12 | 0.594 | 0.72 |
| 0.374 | 1.53 | 0.512 | 1.13 | 0.592 | 0.73 |
| 0.369 | 1.54 | 0.509 | 1.14 | 0.591 | 0.74 |
| 0.364 | 1.55 | 0.507 | 1.15 | 0.589 | 0.75 |
| 0.358 | 1.56 | 0.504 | 1.16 | 0.587 | 0.76 |
| 0.352 | 1.57 | 0.502 | 1.17 | 0.585 | 0.77 |
| 0.347 | 1.58 | 0.499 | 1.18 | 0.584 | 0.78 |
| 0.341 | 1.59 | 0.497 | 1.19 | 0.582 | 0.79 |
Figure D-3 - Required Position of the Voltage Electrode in a Two-Layer Soil
In addition, a reference area is needed for measuring the distances of the auxiliary electrodes (current and voltage probes). For hemispherical earths, the reference point is the centre of the earth electrode. For large earthing systems, some specialists introduce the concept of the electrical centre and describe a method for determining the earth impedance of extensive systems buried in uniform soil.
The user should note that, for a large and complex earth-grid system, the electrical centre will probably not coincide with the geometric centre of the earth grid. Unlike the geometric centre, the electrical earth location depends heavily on the current-density profile within and around the earth-grid conductors. The fall-of-potential test measures the resistance between a point on the earth grid and remote earth. If a value is much larger than expected, the test leads may be attached to a conductor that is improperly connected to the earth grid. If a poor connection is suspected, the measurements can be repeated at other points on the earth grid. If the original and repeated values differ significantly from one another, the earth grid is probably damaged, and an earth continuity test should be performed on the earth grid. Alternatively, continuity tests can be performed before carrying out the fall-of-potential test.
In general, the best way to obtain a satisfactory measurement is to achieve a distance between the earth grid and the current probe (Figure D-1) such that all mutual resistances are sufficiently small and the fall-of-potential curve flattens out (Figure D-2). The main advantage of the fall-of-potential method is that the voltage and current electrodes can have much higher resistance than the earth electrode under test without having any significant effect on measurement accuracy.
Reminder - When measuring the earthing system by the fall-of-potential method, the auxiliary current electrode must be kept out of reach of the personnel performing the test.
D-4 Clamp Method (Stakeless)
The clamp measures the resistance value of an earth electrode by clamping onto the down-conductor, as shown in Figure D-4. When the instrument is switched on, a voltage of a specific frequency - usually between 1 kHz and 3.4 kHz - is induced into the integrated multi-grounded system that includes the earth electrode being measured. The induced voltage causes a current (Itest) to flow within the multi-earthed system, and this current value is measured by the instrument. The ratio of voltage to current (impedance) is then determined and displayed digitally on the instrument's screen. This method relies on the assumption that the impedance of the multi-grounded neutral (or shield) system, excluding the earth electrode under test, is small enough compared with the impedance of the earth electrode that it can be treated as zero (Zeq is approximately 0). Under this assumption, the displayed reading estimates the resistance of the earth electrode, provided the method is used correctly. The accuracy of the clamp instrument depends on proper engagement of the jaws. To ensure proper operation, the instrument must be calibrated periodically.
Although this is a practical method that is widely used for transmission and distribution lines, its underlying theory is subject to certain application limitations, as listed below:
1. Its application is limited to an earth electrode that is connected to an earthing system with relatively low impedance. 2. If the inductive reactance of the multi-earthed shield or the neutral system under test is significant compared with the resistance being measured, a large error appears in the measurement results. It is true, to some extent, that the clamp instrument requires high test frequencies (1 kHz to 3.4 kHz) to keep its dimensions compact, on the assumption that no attempt is needed to calculate the reactance in the test circuit. The high frequency injected into the test circuit increases the circuit's reactive impedance and, where the inductance is significant, can produce a large disturbance in the test results.
Figure D-4 - Resistance Measurement by the Clamp Method
3. Corrosion effects at connection points on the neutral system (or shield wire) can affect the test results. In general, however, an open shield or neutral wire is usually indicated by the instrument. 4. This method is not applicable to an earth electrode system with multiple connections, such as a substation earth grid or structure earths. Structure earths must be disconnected everywhere except at the point being measured. Ensure that every earth wire is disconnected in a safe manner. 5. High-frequency noise in the system can affect the measurement results. When measuring a high-resistance earth, a high noise-to-signal ratio may occur.
D-5 Combined Fall-of-Potential and Clamp Method
The independent resistance of an earth electrode can also be measured by combining the fall-of-potential and clamp methods (Figure D-5). The voltage and current probes are placed in one direction, as required by the fall-of-potential (FOP) method. In addition to injecting the current, the current flowing through the earthing system is measured [by the clamp]. The ratio of the measured voltage to the measured earth current then indicates the independent resistance of the earthing system.
Figure D-5 - Tower Earth Resistance Measurement System Using the Fall-of-Potential Method and Leakage-Current Measurements
The combined fall-of-potential/clamp method is often used to measure the resistances of multi-leg towers or wire transmission-line structures that do not have dedicated earth electrode systems. To measure the earth current, a large split-core current transformer [Footnote 96] is used
(Figure D-5). In the case of a four-leg tower, the resistance of each leg is measured before they are combined, to determine the total resistance of the structure. An instrument has also been developed that allows all four resistances to be measured simultaneously.
As with the clamp method, high-frequency noise in the system can affect the reading. Likewise, when measuring a high-resistance earth, a large noise-to-signal ratio may occur.
D-6 Method Using a Computerized Earth Multimeter
A computerized earth multimeter instrument has been developed that is capable of determining the impedance characteristics of an isolated or integrated (interconnected) earthing system. The test involves installing one current-return electrode and 6 voltage-sensing electrodes (via a pair of tricoaxial cables [Footnote 97], each connected to 3 electrodes).
(Figure D-6). The current-return electrode is placed at a distance of at least 2 times the longest dimension of the substation. The first voltage probe is automatically fixed relative to the first probe. For better accuracy, the voltage probes are placed as far as possible from other earthed structures (such as pipes and pole earths).
In general, this type of multimeter provides the user with the following measurement capabilities:
1. Earth impedance (isolated earthing system, or interconnected system of a tower, pole, or substation); 2. Soil resistivity; 3. Tower earth impedance (independent earth impedance of a tower/pole without disconnecting the shield or neutral wires); 4. Touch voltage; 5. Step voltage; 6. Independent substation earth impedance without disconnecting the shield or neutral wires; 7. Transferred voltage; 8. Low impedance/continuity (integrity test of the substation earth-grid conductors).
Figure D-6 - Earth Impedance Measurement Using a Computerized Earth Multimeter
Depending on the measurement selected, the user enters several parameters, including the type and size of the earthing system (earth rods, counterpoise wires, or an earth grid) and the approximate coordinates for the current and voltage electrodes. During the test, the power-supply unit injects continuous pulses (white noise) between the earth electrode under test and the current-return electrode. The current pulses are injected for a short duration, typically 0.5 seconds. In the earth-impedance mode, the ground potential differences (GPD) are measured by 6 voltage electrodes. The computer software then processes the measured current and ground potential differences (GPD) and performs the following operations:
1. Noise filtering; 2. Correction of voltage- and current-transducer errors; 3. Estimation of the earth-electrode impedance and the soil coefficient by solving a 6x2 matrix equation using the weighted least-squares method [Footnote 98]; 4. Display of an earth-impedance curve (magnitude and phase angle) versus frequency on the screen.
In other respects, such as the items below, the computerized method has limitations similar to the fall-of-potential method, which can affect data accuracy:
1. The current- and voltage-probe distances are measured from an assumed electrical centre, and consequently the impedance of an earthing system with internal interconnections is not determined accurately; 2. The measured impedance value changes as the voltage and/or current probe locations are changed. Nevertheless, this method provides an error range together with the impedance value.
Reminder - When measuring the earthing system at workplaces where the earthing system is in the form of a grid, if well-extended buried metallic objects are present in the ground and their electrical resistance is very small and negligible, these metallic objects can be used as both the auxiliary current electrode and the auxiliary voltage electrode. Under these conditions, the low-voltage neutral of a neighbouring substation can also be used as both the auxiliary current electrode and the auxiliary voltage electrode. In this case, the neutral must be tested, and it must be confirmed that it has been discharged of electrical energy.
Footnote 96 - Split-Core CT Footnote 97 - Tricoaxial Footnote 98 - Weighted Least Square