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In an electrical system, a ground loop is an equipment and wiring configuration which results in excessive unwanted current in a conductor connecting two points that are supposed to be at the same electric potential, often ground, 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. == Description == 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 magnetic 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 H-field (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 EMF (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. Some places where high fields may exist, with the potential for hazards to arise in a ground loop close by, are close to AC electrified railways with simple return through the rails, where the overhead wiring may be carrying several hundred amps at 50 or 60 Hz (the voltage is not important), or close to a power distribution network carrying high currents with widely spaced wires. Again, the voltage is not relevant. Inside a building, the wiring may include radiating loops, e.g. careless layout may result in the live feed to a lighting circuit, via the switch, take a different route than the neutral (return) conductor, creating a radiating loop, but this will not be able to couple sufficient energy to create a hazard. It has been alleged that a significant cause of current in the ground loop is voltage drop along the equipment protective ground conductors. This is rarely significant, because safety regulations place strict limits on ground leakage current, and ground conductor impedance. For modest sized pieces of equipment such as most audio equipment the ground leakage will be typically less than 2 mA, and the protective ground impedance less than 1 ohm, so this effect will only cause a few milliamps of current to flow in the loop. The current arising due to magnetic induction is liable to be at least one or two orders of magnitude greater. If A and B are simple electrical apparatus, the signals between them being of a simple nature, such as 24V DC to control relays, this ground loop is harmless and merely reinforces the grounding system, such that, if for example, B develops a fault causing its metalwork to become live, in addition to its own protective ground conductor there is an additional path to ground via A, so the momentary rise in potential before the fuse or circuit breaker operates to remove power will be reduced somewhat, improving safety. However, if A and B are electronic apparatus such as parts of an audio system, or a computer system, the signal(s), typically in the region of millivolts to a few volts, passing between them will be vulnerable to corruption. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ground loop (electricity)」の詳細全文を読む スポンサード リンク
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