Grounding and protective measures for electrical safety. General requirements

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Section 1 General Rules

Grounding and protective measures for electrical safety. General requirements

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Encyclopedia of radio electronics and electrical engineering / Rules for the installation of electrical installations (PUE)

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1.7.49. The current-carrying parts of the electrical installation should not be accessible for accidental contact, and the open and third-party conductive parts accessible to touch should not be energized, which poses a risk of electric shock both in the normal operation of the electrical installation and in case of damage to the insulation.

1.7.50. To protect against electric shock in normal operation, the following protective measures against direct contact must be applied individually or in combination:

basic insulation of current-carrying parts;
enclosures and shells;
setting up barriers;
placement out of reach;
the use of ultra-low (small) voltage.

For additional protection against direct contact in electrical installations with voltages up to 1 kV, if there are requirements of other chapters of the PUE, residual current devices (RCDs) with a rated differential breaking current of not more than 30 mA should be used.

1.7.51. In order to protect against electric shock in the event of insulation failure, the following protective measures against indirect contact must be applied individually or in combination:

protective grounding;
automatic power off;
equalization of potentials;
potential equalization;
double or reinforced insulation;
ultra-low (small) voltage;
protective electrical separation of circuits;
insulating (non-conductive) rooms, zones, sites.

1.7.52. Measures of protection against electric shock must be provided in the electrical installation or part of it, or applied to individual electrical receivers and can be implemented in the manufacture of electrical equipment, or during installation of the electrical installation, or in both cases.

The use of two or more protective measures in an electrical installation should not have a mutual influence that reduces the effectiveness of each of them.

1.7.53. Protection against indirect contact must be carried out in all cases if the voltage in the electrical installation exceeds 50 V AC* and 120 V DC*.

In rooms with increased danger, especially dangerous and in outdoor installations, protection against indirect contact may be required at lower voltages, for example, 25 V AC and 60 V DC or 12 V AC and 30 V DC, subject to the requirements of the relevant chapters of the PUE.

Protection against direct contact is not required if the electrical equipment is located in the area of the potential equalization system, and the highest operating voltage does not exceed 25 V AC or 60 V DC in rooms without increased danger and 6 V AC or 15 V DC - in all cases.

* Here and throughout the chapter, AC voltage refers to the rms value of the AC voltage; DC voltage - DC or rectified current voltage with a ripple content of not more than 10% of the rms value.

1.7.54. For grounding electrical installations, artificial and natural grounding conductors can be used. If, when using natural grounding conductors, the resistance of the grounding devices or the contact voltage has an acceptable value, and the normalized values of the voltage on the grounding device and the permissible current densities in natural grounding conductors are provided, the implementation of artificial grounding conductors in electrical installations up to 1 kV is not necessary. The use of natural grounding conductors as elements of grounding devices should not lead to their damage when short-circuit currents flow through them or to disruption of the operation of the devices with which they are connected.

1.7.55. For grounding in electrical installations of different purposes and voltages, geographically close, as a rule, one common grounding device should be used.

A grounding device used for grounding electrical installations of the same or different purposes and voltages must meet all the requirements for grounding these electrical installations: protecting people from electric shock if the insulation is damaged, operating conditions of networks, protecting electrical equipment from overvoltage, etc. in throughout the entire period of operation.

First of all, the requirements for protective earthing must be observed.

Grounding devices for protective grounding of electrical installations of buildings and structures and lightning protection of the 2nd and 3rd categories of these buildings and structures, as a rule, should be common.

When making a separate (independent) grounding conductor for working grounding, under the conditions of operation of information or other equipment sensitive to interference, special measures must be taken to protect against electric shock, excluding simultaneous contact with parts that may be under a dangerous potential difference if the insulation is damaged.

To combine grounding devices of different electrical installations into one common grounding device, natural and artificial grounding conductors can be used. Their number must be at least two.

1.7.56. The required values of contact voltage and resistance of grounding devices when ground fault currents and leakage currents flow from them must be provided under the most unfavorable conditions at any time of the year.

When determining the resistance of grounding devices, artificial and natural grounding conductors should be taken into account.

When determining the resistivity of the earth, its seasonal value corresponding to the most unfavorable conditions should be taken as the calculated one.

Grounding devices must be mechanically strong, thermally and dynamically resistant to earth fault currents.

1.7.57. Electrical installations up to 1 kV in residential, public and industrial buildings and outdoor installations should, as a rule, be powered from a source with a solidly grounded neutral using the TN system.

To protect against electric shock in case of indirect contact in such electrical installations, automatic power off must be performed in accordance with 1.7.78 - 1.7.79.

Requirements for the selection of TN-C, TN-S, TN-CS systems for specific electrical installations are given in the relevant chapters of the Rules.

1.7.58. The power supply of electrical installations with voltage up to 1 kV AC from a source with an isolated neutral using the IT system should be performed, as a rule, if a power interruption is unacceptable during the first ground fault or to open conductive parts associated with the potential equalization system. In such electrical installations, for protection against indirect contact during the first earth fault, protective grounding must be performed in combination with network insulation monitoring or RCDs with a rated differential breaking current of not more than 30 mA should be used. In the event of a double earth fault, automatic power off shall be performed in accordance with 1.7.81.

1.7.59. Power supply of electrical installations with voltage up to 1 kV from a source with a solidly grounded neutral and with grounding of open conductive parts using a ground electrode not connected to the neutral (TT system) is allowed only in cases where electrical safety conditions in the TN system cannot be ensured. For protection against indirect contact in such electrical installations, automatic power off must be performed with the mandatory use of RCDs. In this case, the following condition must be met:

where Ia is the operating current of the protective device;

Ra is the total resistance of the grounding conductor and the grounding conductor, when using RCD to protect several electrical receivers - the grounding conductor of the most distant electrical receiver.

1.7.60. When using a protective automatic power off, the main potential equalization system must be made in accordance with 1.7.82, and, if necessary, an additional potential equalization system in accordance with 1.7.83.

1.7.61. When using the TN system, it is recommended to re-ground the PE and PEN conductors at the input to the electrical installations of buildings, as well as in other accessible places. For re-grounding, natural grounding should be used first. The resistance of the re-grounding earth electrode is not standardized.

Inside large and multi-storey buildings, a similar function is performed by potential equalization by connecting a zero protective conductor to the main ground bus.

Re-grounding of electrical installations with voltage up to 1 kV, powered by overhead lines, must be carried out in accordance with 1.7.102 - 1.7.103.

1.7.62. If the automatic power off time does not satisfy the conditions of 1.7.78 - 1.7.79 for the TN system and 1.7.81 for the IT system, then protection against indirect contact for individual parts of the electrical installation or individual electrical receivers can be performed using double or reinforced insulation (class II electrical equipment ), extra-low voltage (class III electrical equipment), electrical separation of circuits, insulating (non-conductive) rooms, zones, sites.

1.7.63. An IT system with a voltage up to 1 kV, connected through a transformer to a network with a voltage above 1 kV, must be protected by a blowout fuse from the danger arising from damage to the insulation between the high and low voltage windings of the transformer. A blowout fuse must be installed in the neutral or phase on the low voltage side of each transformer.

1.7.64. In electrical installations with a voltage above 1 kV with an isolated neutral, to protect against electric shock, protective grounding of exposed conductive parts must be made.

In such electrical installations, it should be possible to quickly detect ground faults. Earth fault protection should be installed with a tripping action throughout the electrically connected network in cases where it is necessary for safety reasons (for lines supplying mobile substations and mechanisms, peat mines, etc.).

1.7.65. In electrical installations with voltages above 1 kV with an effectively grounded neutral, protective grounding of open conductive parts must be made to protect against electric shock.

1.7.66. Protective grounding in the TN system and protective grounding in the IT system of electrical equipment installed on overhead line supports (power and instrument transformers, disconnectors, fuses, capacitors and other devices) must be carried out in compliance with the requirements given in the relevant chapters of the PUE, as well as in this chapter.

The resistance of the grounding device of the overhead line support on which the electrical equipment is installed must comply with the requirements of Ch. 2.4 and 2.5.

See other articles Section Rules for the installation of electrical installations (PUE).

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