Section 1 General Rules

Grounding and protective measures for electrical safety. Protective measures for indirect contact

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

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1.7.76. Protection requirements for indirect contact apply to:

1) cases of electrical machines, transformers, devices, lamps, etc.;

2) drives of electrical apparatus;

3) frames of switchboards, control panels, shields and cabinets, as well as removable or opening parts, if the latter are equipped with electrical equipment with a voltage above 50 V AC or 120 V DC (in cases provided for by the relevant chapters of the PUE - above 25 V AC or 60 V DC);

4) metal structures of switchgears, cable structures, cable boxes, sheaths and armor of control and power cables, sheaths of wires, sleeves and pipes of electrical wiring, sheaths and supporting structures of bus ducts (bus ducts), trays, boxes, strings, cables and strips on which reinforced cables and wires (except for strings, cables and strips along which cables with grounded or grounded metal sheath or armor are laid), as well as other metal structures on which electrical equipment is installed;

5) metal sheaths and armor of control and power cables and wires for voltages not exceeding those specified in 1.7.53, laid on common metal structures, including common pipes, boxes, trays, etc., with cables and wires on higher voltages;

6) metal cases of mobile and portable power receivers;

7) electrical equipment installed on moving parts of machine tools, machines and mechanisms.

When used as a protective measure for automatic power off, these exposed conductive parts must be connected to the solid earthed neutral of the power supply in the TN system and earthed in the IT and TT systems.

1.7.77. It is not necessary to intentionally connect to the source neutral in a TN system and to earth in an IT and TT system:

1) cases of electrical equipment and apparatus installed on metal bases: structures, switchgear, switchboards, cabinets, machine beds, machines and mechanisms connected to the neutral of the power source or grounded, while ensuring reliable electrical contact of these cases with the bases;

2) the structures listed in 1.7.76, while ensuring reliable electrical contact between these structures and the electrical equipment installed on them, connected to the protective conductor;

3) removable or opening parts of the metal frames of switchgear chambers, cabinets, fences, etc., if no electrical equipment is installed on the removable (opening) parts or if the voltage of the installed electrical equipment does not exceed the values ​​specified in 1.7.53;

4) fittings of insulators of overhead power lines and fasteners attached to it;

5) open conductive parts of electrical equipment with double insulation;

6) metal brackets, fasteners, sections of pipes for mechanical protection of cables in places where they pass through walls and ceilings and other similar parts of electrical wiring up to 100 cm2, including pull-in and branch boxes of hidden electrical wiring.

1.7.78. When performing automatic power off in electrical installations with voltages up to 1 kV, all exposed conductive parts must be connected to a solidly grounded neutral of the power source if the TN system is used, and grounded if the IT or TT systems are used. At the same time, the characteristics of the protective devices and the parameters of the protective conductors must be coordinated in order to ensure the normalized time for disconnecting the damaged circuit by the protective switching device in accordance with the rated phase voltage of the supply network.

In electrical installations in which automatic power off is applied as a protective measure, potential equalization must be carried out.

For automatic power off, protective switching devices that respond to overcurrents or differential currents can be used.

1.7.79. In the TN system, the automatic power off time should not exceed the values ​​\u1.7.1b\uXNUMXbspecified in table. XNUMX.

The given disconnection times are considered sufficient to ensure electrical safety, including in group circuits supplying mobile and portable electrical receivers and class I hand-held power tools.

In circuits supplying distribution, group, floor and other boards and boards, the shutdown time should not exceed 5 s.

Off-time values ​​are allowed more than those indicated in Table. 1.7.1, but not more than 5 s in circuits supplying only stationary electrical receivers from switchboards or shields when one of the following conditions is met:

1) the total resistance of the protective conductor between the main ground bus and the switchboard or shield does not exceed the value, Ohm:

where Zц is the total resistance of the "phase-zero" circuit, Ohm;

U0 - rated phase voltage of the circuit, V;

50 - voltage drop in the section of the protective conductor between the main ground bus and the switchboard or shield, V;

2) an additional potential equalization system is connected to the PE bus of the switchboard or shield, covering the same third-party conductive parts as the main potential equalization system.

It is allowed to use RCDs that respond to differential current.

Table 1.7.1. Longest allowable protective shutdown time for a TN system

1.7.80. It is not allowed to use RCDs that respond to differential current in four-wire, three-phase circuits (TN-C system). If it is necessary to use an RCD to protect individual electrical receivers powered by the TN-C system, the protective PE conductor of the electrical receiver must be connected to the PEN conductor of the circuit supplying the electrical receiver to the protective switching device.

1.7.81. In the IT system, the time for automatic power off in case of a double short circuit to exposed conductive parts must comply with Table. 1.7.2.

Table 1.7.2. The longest allowable safety shutdown time for an IT system

1.7.82. The main potential equalization system in electrical installations up to 1 kV must interconnect the following conductive parts (Fig. 1.7.7):

1) zero protective PE or PEN conductor of the supply line in the TN system;

2) ground conductor connected to the grounding device of the electrical installation, in IT and TT systems;

3) a grounding conductor connected to the re-grounding conductor at the entrance to the building (if there is a grounding conductor);

4) metal pipes of communications included in the building: hot and cold water supply, sewerage, heating, gas supply, etc.

If the gas supply pipeline has an insulating insert at the entrance to the building, only that part of the pipeline that is relative to the insulating insert from the side of the building is connected to the main potential equalization system;

5) metal parts of the building frame;

6) metal parts of centralized ventilation and air conditioning systems. In the presence of decentralized ventilation and air conditioning systems, metal air ducts should be connected to the PE bus of power panels for fans and air conditioners;

7) grounding device of the lightning protection system of the 2nd and 3rd categories;

8) a grounding conductor of functional (working) grounding, if there is one and there are no restrictions on connecting the working grounding network to a protective grounding grounding device;

9) metal sheaths of telecommunication cables.

Conductive parts entering the building from the outside should be connected as close as possible to their point of entry into the building.

To connect to the main potential equalization system, all of these parts must be connected to the main ground bus (1.7.119-1.7.120) using the conductors of the potential equalization system.

(click to enlarge)

M - open conductive part;

C1 - metal water pipes entering the building;

C2 - metal sewer pipes entering the building;

C3 - metal gas supply pipes with an insulating insert at the inlet, entering the building;

C4 - ventilation and air conditioning ducts;

C5 - heating system;

C6 - metal water pipes in the bathroom;

C7 - metal bath;

C8 - third-party conductive part within reach of open conductive parts;

C9 - reinforcement of reinforced concrete structures;

GZSH - main ground bus;

T1 - natural ground electrode;

T2 - lightning protection ground electrode (if any);

1 - zero protective conductor;

2 - conductor of the main potential equalization system;

3 - conductor of an additional potential equalization system;

4 - down conductor of the lightning protection system;

5 - contour (main) of working grounding in the room of information computing equipment;

6 - conductor of working (functional) grounding;

7 - potential equalization conductor in the working (functional) grounding system;

8 - ground conductor

Rice. 1.7.7. Potential equalization system in the building:

1.7.83. The additional potential equalization system must interconnect all open conductive parts of stationary electrical equipment that are simultaneously accessible to touch and third-party conductive parts, including metal parts of building structures accessible to touch, as well as zero protective conductors in the TN system and protective grounding conductors in IT and TT systems, including protective conductors of socket outlets.

For potential equalization, specially provided conductors or open and third-party conductive parts can be used if they meet the requirements of 1.7.122 for protective conductors with respect to conductivity and continuity of the electrical circuit.

1.7.84. Protection by means of double or reinforced insulation may be provided by the use of class II electrical equipment or by enclosing electrical equipment having only basic insulation of live parts in an insulating sheath.

Conductive parts of equipment with double insulation must not be connected to the protective conductor and to the potential equalization system.

1.7.85. Protective electrical separation of circuits should be used, as a rule, for one circuit.

The maximum operating voltage of the separated circuit must not exceed 500 V.

The circuit to be separated must be powered from an isolating transformer complying with GOST 30030 "Isolating transformers and safety isolating transformers", or from another source that provides an equivalent degree of safety.

Current-carrying parts of a circuit powered by an isolating transformer must not be connected to grounded parts and protective conductors of other circuits.

Conductors of circuits powered by an isolating transformer are recommended to be laid separately from other circuits. If this is not possible, then for such circuits it is necessary to use cables without a metal sheath, armor, screen or insulated wires laid in insulating pipes, boxes and channels, provided that the rated voltage of these cables and wires corresponds to the highest voltage of the jointly laid circuits, and each circuit protected from overcurrents.

If only one electrical receiver is supplied from an isolating transformer, then its exposed conductive parts must not be connected either to the protective conductor or to the open conductive parts of other circuits.

It is allowed to supply several electrical receivers from one isolation transformer, provided that the following conditions are met simultaneously:

1) exposed conductive parts of the circuit to be separated must not have electrical connection with the metal case of the power source;

2) the open conductive parts of the circuit to be separated must be interconnected by insulated ungrounded conductors of the local potential equalization system that does not have connections with protective conductors and open conductive parts of other circuits;

3) all socket outlets must have a protective contact connected to a local ungrounded potential equalization system;

4) all flexible cables, with the exception of those supplying class II equipment, must have a protective conductor used as a potential equalization conductor;

5) the shutdown time of the protective device in case of a two-phase short circuit to open conductive parts should not exceed the time specified in Table. 1.7.2.

1.7.86. Insulating (non-conductive) rooms, zones and sites can be used in electrical installations with voltage up to 1 kV, when the requirements for automatic power off cannot be met, and the use of other protective measures is impossible or impractical.

The resistance relative to the local ground of the insulating floor and walls of such premises, zones and sites at any point must be at least:

50 kOhm at a rated voltage of the electrical installation up to 500 V inclusive, measured with a megohmmeter for a voltage of 500 V;

100 kOhm at a rated voltage of the electrical installation of more than 500 V, measured with a megaohmmeter for a voltage of 1000 V.

If the resistance at any point is less than specified, such rooms, areas, areas should not be considered as a measure of protection against electric shock.

For insulating (non-conductive) rooms, zones, sites, it is allowed to use electrical equipment of class 0, subject to at least one of the following three conditions:

1) open conductive parts are removed from one another and from third-party conductive parts by at least 2 m. It is allowed to reduce this distance out of reach to 1,25 m;

2) exposed conductive parts are separated from external conductive parts by barriers of insulating material. At the same time, distances not less than those specified in paragraphs. 1, must be secured on one side of the barrier;

3) third-party conductive parts are covered with insulation that can withstand a test voltage of at least 2 kV for 1 min.

No protective conductor shall be provided in insulating rooms (zones).

Measures must be taken to prevent potential drift to third-party conductive parts of the room from the outside.

The floor and walls of such rooms should not be exposed to moisture.

1.7.87. When performing protection measures in electrical installations with voltages up to 1 kV, the classes of electrical equipment used according to the method of protecting a person from electric shock in accordance with GOST 12.2.007.0 "SSBT. Electrical products. General safety requirements" should be taken in accordance with Table. 1.7.3.

Table 1.7.3. The use of electrical equipment in electrical installations with voltage up to 1 kV

Class according to GOST 12.2.007.0 R IEC536	Marking	Purpose of protection	Conditions for the use of electrical equipment in an electrical installation
Class 0	-	On indirect contact	1. Application in non-conductive rooms. 2. Power supply from the secondary winding of an isolation transformer of only one electrical receiver
Class I	Safety clip - sign or the letters PE, or yellow-green stripes	On indirect contact	Connecting the grounding clamp of electrical equipment to the protective conductor of the electrical installation
Class II	Sign	On indirect contact	Regardless of the protective measures taken in the electrical installation
Class III	Sign	From direct and indirect contact	Powered by a safety isolating transformer

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