PUE in Q&A. Earthing and electrical safety precautions
Grounding devices for electrical installations with a voltage of up to 1 kV in networks with a firmly grounded neutral
Where should the ground wire be connected, if in the PEN wire connecting the neutral part of the transformer or generator to the PEN busbar RU to I kV, TT installed?
Answer... It should not be connected to the neutral of the transformer or generator directly and to the PEN-conductor, if possible immediately to the CT. In this case, the separation of the PEN conductor into RE- and N-conductors in the TN-S system must also be done for TT. The TT should be placed as close as possible to the terminal. neutral of a transformer or generator.
What should be the resistance of the earthing device to which the neutrals of a generator or transformer or sources of single-phase current are connected?
Answer… It should be at any time of the year no more than 2, 4 and 8 ohms respectively at 660, 380 and 220 V three phase current source or 380, 220 and 127 V single phase current source.This resistance must be ensured by taking into account the use of natural grounding electrodes, as well as grounding electrodes of multiple grounding of PEN- or PE-conductor of overhead lines up to 1 kV with the number of outgoing lines at least two.
What should be the resistance of the earthing switch in close proximity to the neutral of a generator or transformer, or the output of a single-phase current source?
Answer. It must be no more than 15, 30 and 60 ohms, respectively, with line voltages of 660, 380 and 220 V three-phase current source or 380, 220 and 127 V single-phase current source. With a specific earth resistance ρ> 100 Ohm × m, it is allowed to increase the specified norms by 0.01 ρ times, but not more than tenfold.
At which points in the PEN network should a conductor be re-grounded?
Answer... It must be performed at the ends of overhead lines or their branches with a length of more than 200 m, as well as at the entrances of overhead lines to electrical installations in which, as a protective measure in case of indirect contact, an automatic power cut-off is applied .
What should be the total spread resistance of grounded electrodes (including number of natural) of all repeated PEN conductor grounds of each overhead line in each season?
Answer… There should be no more than 5, 10 and 20 ohms respectively with line voltages of 660, 380 and 220 V three phase current source or 380, 220 and 127 V single phase current source. In this case, the spread resistance of the grounding electrode of each of the repeated groundings must be no more than 15, 30 and 60 ohms, respectively, at the same voltages.With a specific earth resistance ρ> 100 Ohm × m, it is allowed to increase the specified norms by 0.01ρ times, but not more than ten times.
ZGrounding devices in electrical installations with a voltage of up to 1 kV with an isolated neutral
What condition must have the resistance of the earthing device used for protective earthing of the HRE (open conductor part) in the IT system?
Answer… It must meet the condition:
R ≤ U NS/ Me
where R is the resistance of the grounding device, Ohm;
U NS- contact voltage, the value of which is assumed to be 50 V; I — total earth fault current, A.
What are the requirements for the resistance values of grounding devices?
Answer... As a rule, it is not necessary to take the value of this resistance. less than 4 ohms The resistance of the earthing device is allowed up to 10 Ohms if the condition is met
R ≤ UNS/ I,
and the power of generators or transformers does not exceed 100 kVA, including the total power of generators or transformers operating in parallel.
Earthing switches
What can be used as natural grounding electrodes?
Answer… Can be used:
o metal and reinforced concrete structures of buildings and facilities located in contact with the ground, including reinforced concrete foundations of buildings and structures with protective waterproofing coatings in non-aggressive, slightly aggressive and medium aggressive environments;
o metal water pipes laid in the ground;
o casing of wells;
o piles of metal sheets of hydraulic structures, water pipes, built-in parts of covers, etc.;
o railway rails of main non-electrified railway lines and access rails in the presence of a deliberate arrangement of jumpers between the rails;
o other metal structures and structures located in the ground;
o metal sheaths of armored cables laid in the ground. Aluminum cable sheaths cannot be used as grounding conductors.
Is it permissible to use combustible piping as grounding conductors? liquids, flammable or explosive gases and mixtures and sewage pipes and central heating?
Answer… It is not allowed to be used. These limitations do not preclude the need to connect such pipelines to an earthing device for the purpose of equipotential bonding.
Ground wires
What is the cross-section of the ground wire working (functional) earther to the main ground bus in electrical installations up to 1 kV?
Answer... It should have a cross-section of at least: copper — 10 mm>2, aluminum — 16 mm2, steel — 75 mm?.
Main ground bus
What should be used as the main ground bus inside an input device? Answer… Use a PE busbar.
What are the requirements for the basic ground bus?
Answer... Its cross-section must be at least the cross-section of a PE (PEN)-conductor power line. As a rule, it should be honey. It is allowed to apply for it made of steel. The use of aluminum rails is not permitted.
What are the requirements for installing the main ground bus?
Answer... Places accessible only to qualified personnel, such as distribution rooms of residential buildings, should be installed outdoors.In places accessible to unauthorized persons, for example, entrances and basements of houses, it should have a protective cover - a cabinet or drawer with a door that can be locked with a key. There should be a sign on the door or on the wall above the tire.
How should the main grounding be made in case the building has several isolated inputs?
Answer… It should be done for each input device.
Protective wires (PE wires)
Which wires can be used as PE wires in electrical installations up to 1 kV?
Answer… Can be used:
— specially designed conductors, strands of multi-conductor cables, insulated or bare conductors in a common sheath with phase conductors, fixed insulated or bare conductors;
— HRS of electrical installations: aluminum cable sheaths, steel tubes electrical conductors, metal sheaths and support structures of busbars and complete prefabricated devices;
— some conductive parts of third parties: metal building structures buildings and structures (trusses, columns, etc.), reinforcement of reinforced concrete construction structures of buildings, subject to the requirements given in the answer to question 300, metal structures for industrial use (rails for cranes, galleries, platforms, elevator shafts, elevators, elevators, channel framing, etc.).
Can third parties be used as PE conductors? conductive parts?
Answer... They can be used if they meet the requirements of this chapter for conductivity and, in addition, meet the following requirements simultaneously: the continuity of the electric circuit is ensured either by their design or by suitable connections protected from mechanical, chemical and other damage; their dismantling is not possible if there are no measures to preserve the continuity of the circuit and its conductivity.
What is not allowed to be used as PE conductors?
Answer... It is not allowed to use: insulating metal sheaths of pipes and pipe wires, carrying cables for cable wiring, metal hoses, as well as lead sheaths of wires and cables; gas supply pipelines and other pipelines of combustible and explosive substances and mixtures, sewage and central heating pipes; water pipes, if any, have insulating inserts.
In what cases is it not allowed to use neutral protective conductors as protective conductors?
Answer… It is not allowed to use them as protective conductors neutral protective conductors of equipment powered by other circuits, as well as use HRE electrical equipment as neutral protective conductors for other electrical equipment, except for enclosures and supporting structures busbars and complete prefabricated devices that provide the possibility of connecting protective conductors to them in another place.
What should be the smallest cross-sectional areas of the protective conductors?
Answer… It should match the data in Table 1
table 1
Cross-section of phase conductors, mm 2 The smallest cross-section of protective conductors, mm S≤16 С 16 16 S> 35 S / 2
It is allowed, if necessary, to take the cross-section of the protective conductors less than required, if it is calculated by the formula (only for tripping times ≤ 5 s):
S ≥ I √ t / k
where S is the cross-sectional area of the protective conductor, mm 2;
I — short-circuit current providing the time to disconnect the damaged circuit from the protective device or for a time not exceeding 5 s, A;
t is the reaction time of the protective device, s;
k — coefficient, the value of which depends on the material of the conductor, its insulation, initial and final temperature. K-values for protective conductors under different conditions are given in table. 1.7.6-1.7.9 Chapter 1.7 of the Rules for Electrical Installation Devices (Seventh Edition).
Combined neutral protective and neutral working conductors (PEN conductors)
Which circuits can be combined in one conductor (PEN-conductor) functions of protective zero (PE) and neutral working (N) conductors?
Answer... It can be combined in multi-phase circuits in a TN system for permanently laid cables, the conductors of which have a cross-sectional area of not less than 10 mm2 on copper or 16 mm2 on aluminum.
In which circuits is it not allowed to combine the functions of zero protection and zero working wires?
Answer… It is not allowed in single-phase and direct current circuits. As the neutral protective conductor in such circuits must be provided with a separate third conductor.This requirement does not apply to branches from overhead lines up to 1 kV to single-phase electricity consumers.
Are third party conductive parts allowed to be used as a single PEN wire?
Answer… Such usage is not permitted. This requirement does not preclude the use of third-party exposed and conductive parts as an additional PEN conductor when connected to the equipotential bonding system.
When the neutral working and neutral protective conductor are separated, starting from any point of the electrical installation, is it allowed to combine them behind this point on the power distribution?
Answer… Such a merge is not allowed.
Earthing connections and connections, protective conductors and conductors control systems and potential equalization
How should the earth and neutral conductors and equipotential bonding conductors to the HRE be protected?
Answer… They should be bolted or welded.
How should the connection of each HRE electrical installation to a neutral protective or protective earth conductor be carried out?
Answer… It should be done with a separate branch. Series connection to the HRE protective conductor is not permitted.
Is it possible to include switching devices in PE and PEN wires?
Answer. Such switching is not permitted, except in the case of powering electrical receivers using plugs.
What are the requirements for contacts and plug connections, if there are protective wires and / or equipotential bonding wires can it be disconnected with the same plug connection?
Answer… They must have special protective contacts for connecting protective conductors or equipotential bonding conductors to them. Portable electrical receivers
What measures can be taken to protect against indirect contact in circuits supplying portable energy consumers?
Answer... Depending on the category of the room according to the degree of danger of damage to people with electric shock, automatic power cut-off, protective electrical separation of circuits, extra low voltage, double insulation can be applied.
What are the requirements for connection to the neutral protective conductor in the TN system or to the ground in the IT system of portable metal-encased electrical receivers when applying automatic disconnection?
Answer… Special Protection (PE) should be provided for this. a wire located in the same sheath with the phase wires (the third core cables or wires — for single-phase and constant current consumers, the fourth or fifth core — for three-phase energy consumers), can be attached to the housing of the electrical receiver and to the protective contact of the plug connector. Use for these purposes of a zero worker (N) conductor, including one located in a common sheath with phase conductors, is not allowed.
How should the contacts with a rated current of not more than 20 A external installation, as well as internal installation, but to which portable energy consumers, used outside buildings or in rooms with increased danger, be connected?
Answer… An RCD with a rated breaking point must be protected. differential current not more than 30 mA. Manual use is allowed.power tools equipped with RCD plugs.
Mobile electrical installations
What should be applied for automatic shutdown?
Answer. A combination overcurrent protection device with a residual current-sensitive RCD or a continuous-flow insulation-monitoring device operating on tripping or a body-to-earth potential-responsive RCD should be applied.