The substances of earth's crust contain a moderate degree of electric conductivity. The water present in the soil contains salts in solution and thus forms an electrolyte which conducts electricity. This means that electric currents can pass through the soil if differences in potential (voltage) occur at various points of the earth's surface. Alternatively, any potential difference will be immediately measured, so that this surface in effect constitutes an equipotential surface i.e., with a potential which is the same at all points.

Since measurements of potential are relative, the earth’s potential is adopted as a reference value and is, taken as zero. Electrical equipment or conductors which are connected to the earth are said to be earthed or grounded (Figs 1 and 2) which means that no difference of potential in relation to the earth can occur in them. Earthing provides a safeguard against electric shocks to anyone who happens to touch the metal parts concerned. A person who touches a live metal part which is not earthed is liable to receive a shock.

The choice of earthing system has implications for the safety and electromagnetic compatibility of the power supply. The regulations for earthing (grounding) systems vary considerably between different countries. A protective earth (PE) connection ensures that all exposed conductive surfaces are at the same electrical potential as the surface of the earth, to avoid the risk of electrical shock if a person touches a device in which an insulation fault has occurred. It ensures that in the case of insulation fault i.e. a short circuit, a very high current flows, which will trigger an overcurrent protection device (fuse, circuit breaker) that disconnects the power supply.

Earthing can be used for obtaining a field-free or zero field space i.e., a space in which there is no electric field that might, for e.g. disturb delicate electrical measurements. For this reason the walls of the laboratory are lined with wire netting or metal plates which are electrically interconnected and earthed.

The room is then covered in equipotential surfaces which have the same potential as the earth, i.e., zero potential, so that no potential gradient i.e. differences in potential from one point to another and therefore no electric field can develop inside it. Faraday was the first to apply this principle, and for this reason a space screened against external fields is called a Faraday cage (Fig.3).

The pollution of the atmosphere with electromagnetic fields from a multitude of radio and television transmitters has made Faraday’s discovery a particularly important one for various present day scientific and technical purposes. The metal body of a modern motor car also forms a kind of Faraday cage and provides excellent protection for the occupants against external electrostatic influences. Besides, present-day motor tyres contain carbon black which makes the rubber conductive to electricity and thus ensures adequate earthing.

The prior condition for effective earthing is that the contact resistance between the earth wire and soil should be very low, i.e., very good contact should be provided. Various methods of establishing a good earth connection are illustrated in Figs 4a-4c: wide metal strips (4a) or metal rods (4c) well spread out and buried deep in the ground; connection to an underground metallic pipe system e.g. water supply pipes.