Protective earthing: safety of electrical appliances

Protective earthing is a specially made 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 non-conductive metal parts of the electrical installation with current-carrying elements, voltage may also occur, for example, with a lightning bolt.

With the help of protective earthing, they eliminate the danger of electric shock when touched by metal live parts or the housing of equipment, which, due to the short-circuit to the housing, were energized.

It is necessary to distinguish the protective ground from the worker, which is intended for the correct operation of electrical installations, as well as grounding for protection against lightning. Worker can include the grounding of neutral currents winding generator, power transformer and other devices.

For the earthing device, it is necessary to use natural tools and materials: metal and water pipelines that do not have anti-corrosion coating, excluding pipelines with explosive and combustible gases or liquids. For lead-in, cable lead covers, fittings and metal elements of reinforced concrete buildings or structures will suit. Only it is necessary to take into account that such a ground connection must have a connection to the ground. In addition, metal piles or casing pipes are suitable. But bare aluminum conductors and sheaths of cables from this material are prohibited.

If there are no natural earthing switches or if they have an increased resistance, artificial ones can be used to create protective earthing. 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, 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 to a depth 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 soil's specific resistance, electrode dimensions, freezing depth and other factors.

The protective earthing of electrical installations with voltages up to thousands of volts can be arranged 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 forbidden to use metal and lead thin casings 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, zeroing does not always guarantee the necessary safety for a person in contact with an 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 not allowed to be used to reset electrical appliances that are powered by other lines, as they can be turned off for repair.

To devices of protective switching-off carry the automatic switches reacting to current leakage. The use of these devices in apartments can increase electrical safety.