Protective grounding, (zeroing). Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Protective grounding, (zeroing). Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Grounding and grounding

Protective grounding, (zeroing), is the main measure of protection against electric shock in the event of a short circuit of the phase wire to zero or grounded metal structures. The main purpose of this measure is to protect the user of the device from a possible electric shock in case of a short circuit to the case, for example, when the insulation is broken. In other words, grounding is a substitute for the protective functions of fuses.

There is no need to ground all electrical appliances in the house: most of them have a reliable plastic case, which itself protects against electric shock. But the electric stove that consumes a three-phase high power current, electrical equipment (machines) in the home workshop must be grounded, it is advisable to ground the refrigerator.

When the phase wire is closed to the zeroed body of the electrical installation, a large current occurs in the circuit (phase - neutral wire), called the short circuit current. When a short-circuit current passes through the wires of the network, the electrical installation will turn off due to the blown fuses of the fuses or the operation of the circuit breaker. If it is required to arrange grounding only for electrical installations, then this type of grounding can be reproduced using two types of ground electrodes: natural or artificial.

As natural grounding conductors (that is, those that already exist), you can use metal structures of buildings that have a reliable connection to the ground, steel pipes for electrical wiring, lead and aluminum cable sheaths, metal pipelines for all purposes laid openly (except for pipelines for combustible and explosive mixtures ).

The electrical installation is connected to natural grounding conductors - two conductors of the grounding lines of the installation itself. They are welded or screwed with clamps; the contact surface of both the conductors and the ground electrode must be carefully cleaned with a sandpaper from dirt, rust (and if there is a paint layer, then remove it too) to a metallic sheen and tinned. Artificial ground electrodes (electrodes) are all the same pipes, angle steel, steel strips, round steel, etc. They are buried in the ground, interconnected by welding (the welds should be covered with molten bitumen to protect against corrosion). The grounding line from steel tires is diverted from the grounding conductor to the location of the grounded electrical installations and two wires of the grounding line of the installation itself are connected to them in the same way as to natural grounding conductors. In order for the grounding resistance to be no more than 10 ohms, the number of electrodes should be from 2 to 20 pieces, depending on the quality of the soil, the length and location of the electrodes themselves in the ground.

The parts of ground electrodes located in the ground should not be painted. If the soil is too wet and there is a risk of increased corrosion, copper and galvanized materials can be used for the manufacture of electrodes. Protective grounding (zeroing) in electrical installations of residential and public buildings must comply with the requirements of the "Electrical Installation Rules (PUE) and SNiP 3.05.06-85".

Premises with an increased risk of electric shock in residential buildings include: basements, utility rooms in basements with conductive floors, undergrounds, attics, boiler rooms.

In accordance with the requirements of the PUE, grounding (zeroing) of electrical installations should be carried out:

- at a voltage of 380 V and above AC and 440 V and above DC - for all electrical installations;

- at a voltage of 42 V, but less than 380 V AC and from 110 V, but less than 440 V DC - for electrical installations located in rooms with increased danger, for especially dangerous and outdoor installations.

In networks with a voltage of 380/220 V with a solidly grounded neutral of the power source, the grounding of electrical installations is carried out by connecting them to the neutral protective wire of the network.

There is no need to ground (neutralize) electrical installations:

- at a voltage of less than 42 V AC and less than 110 V DC - in all cases, with the exception of electrical installations located in explosive areas of the premises;

- if the body of the electrical installation has double insulation (for example, an electric drill in a plastic case);

- if the electrical installation is located in a place inaccessible to humans and animals, including inside other products.

In living rooms, kitchens, in the presence of openly laid metal pipes of the heating and water supply system, radiators of the heating system and other metal structures that have connections to the ground, grounding of the metal cases of portable electrical receivers (electric irons, electric kettles, electric stoves, room refrigerators, electric vacuum cleaners, washing machines, sewing machines) should be provided. machines and desktop office equipment).

It is not required to ground the housings of portable electrical receivers in the case when, with non-conductive floors in the premises, there are no open, touchable metal pipelines, radiators of the heating system and other metal structures. It is not required to zero the cases of portable electrical receivers if pipelines, heating radiators and other metal structures are closed with insulating casings.

It is allowed temporarily, until the industry masters the production of electrical receivers with a grounded metal case (with a three-wire connecting cord), in rooms with non-conductive floors and in the presence of open metal pipelines and heating radiators, do not neutralize electrical installations.

In residential buildings are subject to grounding (zeroing):

- household electrical machines and appliances with a unit power of more than 1,3 kW;

- all stationary and portable electrical receivers of class I (not having double or reinforced insulation) located in rooms with increased danger;

- steel pipes and boxes of electrical installations, metal cases of electrical panels, electrical cabinets. Plug sockets installed in a 380/220 V network for connecting portable and mobile electrical receivers must have a contact connected to the grounding (zeroing) network;

- metal cases of bathtubs and shower trays. They should be connected with metal conductors to water pipes (to equalize electrical potentials when voltage appears on metal structures);

- metal housings of luminaires built-in or installed in false ceilings, made using metal.

In rooms where it is not required to carry out grounding of metal cases of lamps (dry heated and unheated), the hook for hanging lamps must be insulated.

Segments of pipes of metal protection of wires in places of their passage through walls and ceilings, wire leads from the floor to process equipment should not be grounded (nulled).

In electrical installations of various purposes and voltages, one common grounding device should be used.

For grounding (grounding) of metal cases of stationary and portable household appliances of class I, household electrical appliances with a power of over 1,3 kW, cases of three-phase and single-phase electric stoves, cooking boilers and other thermal equipment for grounding contacts of plug sockets, a separate conductor should be used (laid from the electrical panel supply network) with a cross section equal to the cross section and conductivity of the phase wire. This conductor should be connected to the neutral wire of the supply network in front of the meter (on the side of the wire entry into the building, in front of the disconnecting device), since there should not be separating and disconnecting devices in its circuit. Zero protective wire without a break is laid from the shield to the body of the electrical installation to be zeroed. It is forbidden to use a working neutral wire to ground electrical installations.

It is unacceptable to use metal sheaths of insulating pipes, pipes made of thin sheet metal with a seam (for example, TPRF brand wire, metal hoses, armor and lead sheaths of cables, pipelines of combustible and explosive substances, central heating, domestic water supply) as grounding (zeroing) conductors.

It is forbidden to use soil in electrical installations as phase or neutral wires.

In conjunction with zeroing in residential buildings, residual current devices (RCDs) should be used. Such devices are installed at the entrances to houses, and also built into household machines, appliances and adapter sockets. In this case, the danger of electric shock during installation and dismantling of the electric meter is eliminated.

For non-residential premises located in residential buildings or attached to them, settlement meters should be installed at the inputs of each of the premises, regardless of the power source. For each garden house on the site of a gardening partnership, 1 single-phase meter should be installed. In necessary cases, it is allowed to install a three-phase meter.

If the load current exceeds 5 A, but does not exceed 10 A, then a three-phase 10 A meter should be installed. It is unacceptable to use a 5 A electric meter connected through current transformers.

A break in the neutral wire is a great danger. In this case, electrical receivers can fail, and a person, touching the zeroed body of the electrical receiver, will be under voltage that is life-threatening (Fig. 72).

Protective grounding, (zeroing)
Rice. 72. The electrical circuit for connecting a person and electrical consumers to the network when the neutral wire breaks: 1 - the place of the neutral wire break; 2 - electric iron; 3 - TV; 4 - electric consumer with a zeroed body; 5 - metal case of the electric consumer; R3 - grounding resistance at consumer distance; QF - circuit breaker; XS1, XS2 - sockets; XS3 - socket outlet with neutral contact; I h - current flowing through a person.

For example, if the electric iron has a power of 1000 W, and the TV has 160 W, then the voltage on the TV can be more than 300 V, the current will increase by 60%, the TV will fail if you do not disconnect it from the network. Protection will not work, and the network will not automatically turn off.

To turn off the network and ensure electrical safety, you should use a device for automatically monitoring the health of the neutral circuit and leakage current protection devices (Fig. 73).

Protective grounding, (zeroing)
Rice. 73. Device for automatic monitoring of the health of the grounding circuit: TV1 - transformer at the consumer station; TV2 - voltage transformer supplying the SC relay; US - three-phase rectifier; V - zener diode; Rc - resistance limiting the stabilization current; Rn, S1 - resistance and button of the circuit for monitoring the operation of the relay KA; S2 - relay reset button; K1, K2 - relay contacts KA in the alarm and protection circuit; QF - circuit breaker; R3 - grounding resistance at a consumer substation; P - consumer of electricity.

The principle of operation of the device is that if the zeroing circuit of the KA relay is connected to the network, its contacts K2 in the circuit of the tripping coil of automatic shutdown QF will be closed (when the network is turned on through a magnetic starter, the contacts K2 will be included in the circuit of the tripping coil of the magnetic starter).

In the event of a break in the neutral wire or an unacceptable increase in the resistance of the circuit (phase - zero), the KA relay will turn off, contacts K2 will open, and K1 will close, and the network will be turned off by the circuit breaker (magnetic starter). At the same time, through contacts K1, the circuit for signaling a break in the neutral wire will automatically turn on.

After all these events, you can be calm - everything has been done to protect yourself, your family from electric shock.

Author: Korshevr N.G.

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