Ground loop (electricity)

In an electrical system, a ground loop or earth loop is an equipment and wiring configuration in which there are multiple paths for electricity to flow to ground. The multiple paths form a loop which can pick up stray current through electromagnetic induction which results in unwanted current in a conductor connecting two points that are supposed to be at the same electric potential, often, but are actually at different potentials.

Ground loops are a major cause of noise, hum, and interference in audio, video, and computer systems. They do not in themselves create an electric shock hazard; however, the inappropriate connections that cause a ground loop often result in poor electrical bonding, which is explicitly required by safety regulations in certain circumstances. In any case the voltage difference between the ground terminals of each item of equipment is small. A severe risk of electric shock occurs when equipment grounds are improperly removed in an attempt to cure the problems thought to be caused by ground loops.

A ground loop is the result of careless or inappropriate design or interconnection of electrical equipment that results in there being multiple paths to ground where this is not required, so a complete loop is formed. In the simplest case, two items of equipment, A and B, both intended to be grounded for safety reasons, are each connected to a power source (wall socket etc) by a 3 conductor cable and plug, containing a protective ground conductor, usually green/yellow, in accordance with normal safety regulations and practice. This only becomes a problem when one or more signal cables are then connected between A and B, to pass data or audio signals from one to the other. The shield (screen) of the data cable is typically connected to the grounded equipment chassis of both A and B. There is now a ground loop.

The ground loop will be carrying some current at the local supply frequency, typically 50 Hz or 60 Hz, due to electromagnetic induction from current-carrying conductors nearby. There is an AC magnetic field everywhere in developed areas; however, its magnitude and direction depend strongly on the local environment and arrangement of current-carrying wiring. This influences the current induced in the ground loop, which is also dependent on the area enclosed by the ground loop and its orientation. The impedance of the loop, basically just its resistance at low frequencies, also influences the induced current that will flow. It is important to realise that for a ground loop to cause problems, there must be a local magnetic field produced by power frequency or other systems. It is the combination of radiated field and receptive ground loop that causes problems, and the situation can be improved by attention to either one, or both of these. In effect, the ground loop is the secondary of a very loosely coupled transformer, the primary being the summation of all current carrying conductors nearby. Transformer action limits the induced voltage in the loop, and the weak coupling limits the maximum current that will flow, even if the loop resistance is very low. The maximum induced energy is normally quite small, and except in situations very close to a radiating loop carrying high current, there is no prospect of inducing sufficient energy to cause a hazard to humans.

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