Protective earthing is considered specialmade electrical connection to the ground of non-conductive metal parts, which, due to the short circuit to the housing of the device, may be energized. This occurs when an accidental electrical connection of the non-conductive metal parts of the electrical installation with current-carrying elements, voltage may also occur, for example, in the case of a lightning strike.
With the help of protective earthing eliminatethe risk of electric shock when touching metal live parts or the housing of equipment, which, due to the short-circuit to the housing, were energized.
It is necessary to distinguish protective grounding fromworking, which is intended for the correct operation of electrical installations, as well as grounding for protection against lightning. To the worker it is possible to carry a grounding of neutral currents of a winding of the generator, the power transformer and other devices.
For the earthing device it is necessary to usenatural appliances and materials: metal and water pipelines that do not have anti-corrosion coating, excluding pipelines with explosive and flammable gases or liquids. For lead-in, cable lead covers, fittings and metal elements of reinforced concrete buildings or structures are suitable. It is only necessary to take into account that this grounding must have a connection to the ground. In addition, metal piles or casing pipes are suitable. But bare aluminum conductors and cable sheaths of this material are prohibited.
If there are no natural earthing switches orthey have an increased resistance can be used artificial, with the help of which create a protective grounding. They are made from pieces of angular steel 5x5x0.5 cm long 2-3 meters, the same size of steel pipes with a diameter of 5 cm, the lengths of steel round (length up to 5 meters, diameter 12.0 - 14.0 mm). The used parts (electrodes) must be immersed in the ground at a distance from each other in 3-5 meters, all the elements should be joined together by a steel strip, the cross section of which is 40x4 mm.
The connecting strip is laid in a trench ondepth of 0.70 meters, at the same distance from the surface are the upper ends of the electrodes. All elements are connected by welding. The number of electrodes is calculated based on the specific resistance of the soil, the dimensions of the electrodes, the depth of freezing, and other factors.
Protective earthing of electrical installations by voltageup to a thousand V can be organized using metal structures of the building (if there is a reliable connection of all circuit elements), steel pipes for wiring, aluminum cable sheaths, metal pipelines (excluding central heating, gas and sewerage). Also used metal trays, ducts, busbar housings and zero wires of electric circuits (protective zeroing).
As protective zero conductors it is forbiddenuse metal and lead thin shells of wires and cables. Air networks of alternating current are nullified by a zero wire, which is laid on the same supports as the phase wires.
However, nullification does not always guaranteenecessary safety for the person in contact with the electrical installation. In some cases, in rooms with increased danger, together with zeroing, a re-protective grounding is used or a protective switch-off is used. Protective zeroth wires are forbidden to be used for zeroing electrical appliances that feed on other lines, since they can be turned off for repair.
For circuit breakers, automatic switches that react to current leakage are included. The use of these devices in apartments can increase electrical safety.
