Protective grounding of electrical installations. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Protective grounding of electrical installations. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Grounding and grounding

If a person touches live parts of an electrical installation that are energized, or metal parts that are energized due to insulation failure, an electric shock may occur to a person (in the form of electric shock or electrical injury (burns)). As a result of an electric shock, a person may lose consciousness, have convulsions, stop breathing and blood circulation. Electrical shock can be fatal. Lethal electric shock to a person is possible at voltages of 12 V and above.

To prevent a person from accidentally touching bare current-carrying parts, they are placed at a height or fences are installed. To ensure the safety of people working at installations with voltages up to 1000 V and above, they construct grounding or zeroing devices and ground, or zero, metal parts of electrical equipment and electrical installations. Grounding (nulling) devices must meet the requirements determined by the mode of operation of networks and surge protection.

When calculating and arranging grounding and grounding in electrical installations, the following basic terms are used.

Earthing switch - a metal conductor or a group of conductors (electrodes) in contact with the ground.

Grounding conductors - metal conductors connecting the grounded parts of the electrical installation with the ground electrode.

Grounding any part of the installation - intentional electrical connection of it with the ground electrode.

Grounding device - a set of grounding conductors and grounding conductors.

Resistance of the grounding device - the sum of the resistances of the grounding conductor (relative to the ground) and the grounding conductors.

Spreading resistance - the resistance that the ground electrode provides in the area of current spreading:

Rz \uXNUMXd Uz / Iz,

where Uz - voltage on the ground electrode, V; Iz - current flowing through the ground electrode to the ground, A.

Earth fault - accidental electrical connection of energized parts of an electrical installation with parts that are not insulated from earth, or directly with earth.

Short circuit to the body - electrical connection of individual parts of machines, devices, lines with grounded structural parts of the electrical installation.

Earth fault current - the current passing through the ground at the fault.

Electrical installations with high earth fault currents - electrical installations with voltages above 1000 V, in which the single-phase earth fault current is more than 500 A.

Electrical installations with low earth fault currents - electrical installations with voltages above 1000 V, in which the single-phase earth fault current is equal to or less than 500 A.

Solidly grounded neutral - the neutral of the transformer or generator, connected to the grounding device directly or through low resistance (current transformers, etc.).

Isolated neutral - a neutral not connected to a grounding device or connected through devices that compensate for the capacitive current in the network, voltage transformers and other devices with high resistance.

Zero working conductor of electrical installations up to 1000 V - a conductor used to power electrical receivers, connected to a solidly grounded neutral of a generator or transformer in three-phase current networks, with a solidly grounded output of a single-phase current source, a solidly grounded midpoint of a direct current source.

Zero protective conductor of electrical installations up to 1000 V - a conductor connecting parts to be nulled with a solidly grounded neutral of a generator or transformer in three-phase current networks.

Switching off electrical installations in case of single-phase ground faults can be carried out using a protective shutdown, which is performed in addition to grounding (zeroing). If it is impossible to perform grounding (zeroing) and provide a protective shutdown of the electrical installation or it is difficult to perform for technological reasons, maintenance of electrical equipment from insulating platforms is allowed. In this case, the possibility of simultaneous contact with ungrounded parts of electrical equipment and parts of buildings or equipment that are connected to the ground should be excluded.

Potential equalization must be carried out to ground in order to ensure safety in electrical installations with high fault currents.

Working ground - connection to the grounding device of any point of the electrical circuit, necessary to ensure the proper operation of the installation in normal or emergency conditions, which is carried out directly or through special devices (breakdown fuses, arresters and resistors) is called.

At the ends of overhead lines and branches longer than 200 m, as well as near the inputs of cable or overhead lines into the premises, the neutral wire must be re-grounded. Inside the premises, the neutral wire, which has re-grounding, is connected to the ground network at all switchboards, distribution points and shields. The resistance of the grounding devices of all repeated grounding of the neutral wire should be no more than 5, 10, 20 ohms for voltages of 660, 380, 220 V.

The resistance of the grounding device should be no more than 0,5 Ohm in electrical installations with a voltage above 1000 V with a solidly grounded neutral with high ground fault currents.

Resistance in electrical installations with voltages above 1000 V with an isolated neutral with low earth fault currents must satisfy the condition:

Rz ≤ Iz,

where U3 = 250 V if the earthing device is only used for installations above 1000 V; U3 = 125 V if the earthing device is also used for installations up to 1000 V; I3 - rated earth fault current, A.

If the grounding device is common for switchgears of electrical installations of various voltages, then the smallest of the required values \uXNUMXb\uXNUMXbis taken as the calculated value of the grounding resistance.

Capacitive earth fault current is determined by the approximate formula:

Iz \u35d U (350Ikab + Iv) / XNUMX,

where U is the line voltage of the network, kV; Ikab and Iv - the total length of electrically interconnected cable and overhead lines, km.

According to experimental data, in electrical installations with low ground fault currents in the operation of power supply systems, the current of operation of the relay protection against phase-to-phase faults or the melting current of the fuses is taken as the rated capacitive current, if this protection ensures the shutdown of ground faults. In this case, the ground fault current must be at least one and a half times the current of the relay protection or three times the current of the fuses.

Author: Bannikov E.A.

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