Protection of internal electrical networks with voltage up to 1000 V. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Protection of internal electrical networks with voltage up to 1000 V. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electrician's Handbook

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Electric networks in houses must have protection against short circuits, and lighting networks that feed a group of houses must also have overload protection. It is allowed to protect different sections of the same network with fuses and circuit breakers. Circuit breakers that have only an electromagnetic instantaneous release (cut-off) should not, as a rule, be used in the internal networks of residential and public buildings, since they do not have overload protection (without thermal releases).

Rated currents thermal and combined releases of automatic switches, fuse links for protecting group lines and apartment inputs, including lines to electric stoves, regardless of their installation location, should be:

- 16 A for the lighting network and sockets for 6-10 A;

- 25 A for power lines of an electric stove with a rated power of up to 8 kW. In residential buildings with a cross section of phase wires up to 16 mm2 inclusive, the neutral wires of the supply lines and risers of apartments must have the same brand and cross section (electrical conductivity) as those of the phase wires, and for large cross sections of the phase wires - at least 50% of the phase wire.

As automatic switches used in electrical installations of residential buildings, single-pole automatic switches of the A-63 type are used. They protect the electrical network from short circuits and overloads.

A-63 switches are available in two modifications:

- A-63MP with an electromagnetic current release with a hydraulic delay;

- A-63M without hydraulic retarder. A-63 type circuit breakers have the following rated release currents: 0,63; 0,8; 1,1; 1,25; 1,6; 2; 2,5; 3,2; 4; 5; 6,3; 8; 10; 12,5; 16; 20; 25A. The tripping time of the release at 6 times the current of the rated current of the release is 3-20 seconds.

For internal wiring, circuit breakers of the AE1000 series with combined releases or the AE2000 series are currently used.

For example, a single-pole circuit breaker type AE1031 is designed for a rated current of 10 or 16 A. The circuit breakers have a toggle type control handle.

Instead of plug fuses, a threaded circuit breaker of the PAR type for 6,3, 10 A is often used. The circuit breaker, unlike a plug fuse, operates both during short circuits and during overloads in the network, which is very important. PAR is a reusable switch. After operation, it is enough to press a button of a larger diameter, and the network turns on. The circuit breaker is turned off by pressing the button. The circuit breaker is screwed into the cork fuse block (instead of the cork).

It should be borne in mind that neither fuse fuses, nor circuit breakers of the AE, A-63, PAR series provide human protection when directly touching live parts of an electrical installation that is energized. They may also fail to operate in case of an incomplete single-phase circuit with significant contact resistance at the fault location, and a person may be exposed to life-threatening voltage.

Currently, the most perfect and the only one with direct contact with uninsulated live parts under voltage is protection that responds to earth leakage currents.

Protection works and disconnects the power supply:

- when the phase wire is shorted to the neutral wire, to the zeroed body of the electrical installation, as well as to the ground;

- when touching an uninsulated phase wire of an electrical network that is energized.

The protection prevents fire from single-phase short circuits, since the network is turned off at insignificant leakage currents of the order of 0,002-1 A. The most sensitive protection has a ground leakage current setting of only 0,002 A (2 mA). If a person touches the phase wire of the network and a current of 2 or more flows through it to the ground, the protection will turn off the power supply in a time not exceeding 0,1 s. However, it should be remembered that this protection will not turn off the network in case of a phase-to-phase short circuit (with a two- or three-phase supply), as well as when a phase wire is shorted to a neutral working wire. Therefore, in the network, in addition to such protection, it is necessary to install circuit breakers or fuses that also operate in the event of a short circuit between the phases, the phase and the neutral working wire.

Circuit breakers must have thermal protection that disconnects the network during overloads, since leakage current protection devices do not react to overload in the network. The BZOU-2 type residual current device is designed like a PAR type circuit breaker. It is screwed into the threaded cartridge of the safety shoe.

Author: Korshevr N.G.

See other articles Section Electrician's Handbook.

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