Providing protection against electric shock from indirect contact. Occupational Safety and Health

OCCUPATIONAL SAFETY AND HEALTH
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Providing protection against electric shock from indirect contact. Occupational Safety and Health

Occupational Safety and Health

Occupational Safety and Health / Legislative basis for labor protection

Protective earth - intentional electrical connection to earth or its equivalent of metallic non-current-carrying parts that may be energized.

The purpose of protective earthing is to reduce to a safe value the voltage relative to earth on the metal parts of the equipment that are not normally energized. As a result of a short circuit to the case of grounded equipment, the contact voltage and, as a result, the current passing through a person when touching the cases are reduced.

Protective grounding can only be effective if the ground fault current does not increase with a decrease in grounding resistance to current flow in the ground. This is only possible in networks with isolated neutral (systems IT), where during a short circuit to the case, the short circuit current I_З almost independent of ground resistance R_З, but is determined mainly by the insulation resistance of the wires.

The grounding device can be remote and contour. An external grounding device is used for low earth fault currents, and a contour grounding device for high ones.

According to the PUE, the grounding of installations must be performed:

at voltages above 50 V AC and above 120 V DC - in all electrical installations;
at voltages above 25 V AC and above 60 V DC - in rooms with increased danger, especially dangerous and in outdoor installations;
in hazardous areas at all voltages.

For grounding electrical installations, artificial and natural ground electrodes can be used, and in order to reduce the cost of grounding, natural ground electrodes are used in the first place.

As the natural grounding can be used:

water pipes laid in the ground;
metal structures of buildings and structures that have a reliable connection to the ground;
metal sheaths of armored cables laid in the ground (except for aluminum ones);
casing pipes of boreholes.

It is forbidden to use pipelines with flammable liquids and gases, pipes of heating mains and sewerage as ground electrodes.

Natural grounding conductors must be connected to the grounding line at least in two different places.

To determine the required number of vertical grounding conductors and their dimensions, the spatial arrangement of grounding conductors, the length of the connecting horizontal conductors and their cross section, protective grounding is calculated. The calculation of grounding can be carried out both according to the permissible grounding resistance, and according to the permissible touch and step voltages.

Currently, the calculation of ground electrodes is carried out in most cases according to the permissible ground resistance. In this case, the use factor method is mainly used (when the earth is considered homogeneous) and less often - the method of induced potentials (when the earth is assumed to be two-layer).

Protective auto power off

In a network with a solidly grounded neutral with a voltage of up to 1000 V (in systems TN) protective earthing of exposed conductive parts is ineffective, since the earth fault current depends on the earth resistance.

Therefore, in systems TN to protect against electric shock in case of indirect contact, an automatic power off must be performed, providing protection against both overcurrents (protective neutralization) and leakage currents (RCD).

Protective auto power off - automatic opening of the circuit of one or more phase conductors (and, if required, the neutral working conductor), performed for electrical safety purposes.

Insulation failure automatic power off is designed to prevent touch voltages that can be dangerous for long periods of time.

The term "protective automatic power off" adopted in the seventh edition of the PUE because it fully reflects not only the physical essence of the protection measure, but also the fact that this protection is a set of measures (see 1.7.78 PUE) and includes:

connection of open conductive parts to a solidly grounded neutral of a power source using a neutral protective conductor (protective grounding) in electrical installations with voltage up to 1 kV with a solidly grounded neutral (system ТN);
connection of exposed conductive parts using a protective earth conductor to an earth electrode not connected to the power supply earth electrode in systems IT и TT;
coordination of the parameters of the protective device and the protected circuit to ensure a safe combination of the disconnection time and the acting touch voltage;
potential equalization, which provides a voltage drop between open and third-party conductive parts that are simultaneously accessible to touch.

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

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

Automatic power off using protective communication devices that respond to overcurrents (protective zeroing)

Protective nulling - deliberate electrical connection of open conductive parts that may be energized with a solidly grounded neutral point of the current source winding in three-phase networks, with a solidly grounded output of the current source winding in single-phase networks and with a solidly grounded midpoint of the energy source winding in DC networks, performed for the purpose of electrical safety.

Protective grounding is used in networks with voltage up to 1000 V with a grounded neutral. In the event of a phase breakdown, a single-phase short circuit occurs on the metal case of the electrical equipment, which leads to a quick protection operation and, thereby, automatic disconnection of the damaged installation from the mains. Such protection are: fuses, automatic fuses, automatic switches.

Zeroing is calculated on the breaking capacity; on the safety of touching the case when the phase is shorted to earth (calculation of neutral grounding); on the safety of touching the case when the phase is shorted to the case (calculation of re-grounding of the neutral protective conductor).

The calculation for the breaking capacity is carried out for the most electrically remote points of the network.

Potential equalization

Potential equalization (protective potential equalization) - electrical connection of conductive parts to achieve equality of their potentials, performed for the purpose of electrical safety.

If in an installation or in a part of it the requirements for the application of protective measures against electric shock in the event of insulation failure cannot be met by switching off, then potential equalization must be provided. The potential equalization system may cover the entire installation or any part of it.

Authors: Fainburg G.Z., Ovsyankin A.D., Potemkin V.I.

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