Earthing
Earth is a reference for a voltage potential. This reference is obtained by making a good conducting connection between an electrical device with good conductivity and the earth. In this way it is possible to avoid undesirable electrical voltage reaching devices that have a conductive housing.
Sound earthing has a minimum electrical resistance to earth (e.g. the groundwater). Domestic electrical installations used to be typically earthed on the water mains, but with the arrival of non- or low-conductive material for water mains pipes and water meters made of synthetic materials, these are now usually earthed in the ground with a copper pen which must be driven in to a certain depth, depending on the local conductivity of the earth. The earth resistance is a measure for the quality of the earth connection. The earth connection serves to conduct the electricity from lightning strikes to the earth or as a voltage reference in grid connections. Electricity grids usually have one principal earth connection, usually at the power station.
Earthing different connected devices to the same point prevents voltage differences between these devices. This is extremely important with audio installation in order to avoid a so-called ground loop which can cause a hum that is extremely difficult to trace and eliminate.
By applying earth connections, it is possible to ensure - outside of the circuit of the current-carrying conductors such as the phase and the zero - that metal housings of e.g. washing machines do not become electrically charged and cause an undesirable electrical current or static electricity which could injure people or damage sensitive electronic equipment.
An earth leakage circuit breaker measures whether the supplied current (via the phase wire) and the returned current (via the zero) are equal. If not, part of the power is undoubtedly being conducted via the earth. This is an undesirable situation and so the supply of current is interrupted. If too much power were conducted through the earth, two kinds of problems could arise:
- A dangerous voltage may arise if the current is too much greater than the earth resistance, leading to the risk of an electrical shock.
- If consequently too much power is conducted through the earth itself (stray current) then metal compounds may be electrochemically dissolved.
