Ground Loop

In an electrical system, a ground loop or earth loop occurs when two points of a circuit are intended to have the same ground reference potential but instead have a different potential between them.This can be caused, for example, in a signal circuit referenced to ground, if enough current is flowing in the ground to cause two points to be at different potentials.

Ground loops are a major cause of noise, hum, and interference in audio, video, and computer systems. Wiring practices that protect against ground loops include ensuring that all vulnerable signal circuits are referenced to one point as ground. The use of differential connections can provide rejections of ground-induced interference. Removal of safety ground connections to equipment in an effort to eliminate ground loops also eliminates the protection the safety ground connection is intended to provide.

A ground loop is caused by the interconnection of electrical equipment that results in there being multiple paths to ground, so a closed conductive loop is formed. A common example is two pieces of electrical equipment, A and B, each connected to a utility outlet by a 3 conductor cable and plug, containing a protective ground conductor, 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 of the data cable is typically connected to the grounded equipment chassis of both A and B, forming a closed loop with the ground conductors of the power cords, which are connected through the building utility ground wire. This is the ground loop.

In the vicinity of electric power wiring there will always be stray magnetic fields oscillating at the utility frequency, 50 or 60 hertz. These ambient magnetic fields passing through the ground loop will induce a current in the loop by electromagnetic induction. In effect, the ground loop acts as a single-turn secondary winding of a transformer, the primary being the summation of all current carrying conductors nearby. The amount of current induced will depend on the magnitude of nearby utility currents and their proximity. The presence of high power equipment such as industrial motors or transformers can increase the interference. Since the wire ground loop usually has very low resistance, often below one ohm, even weak magnetic fields can induce significant currents.

Since the ground conductor of the signal cable linking the two pieces of equipment A and B is part of the signal path of the cable, the alternating ground current flowing through the cable can introduce electrical interference in the signal. The induced alternating current flowing through the resistance of the cable ground conductor will cause a small AC voltage drop across the cable ground. This is added to the signal applied to the input of the next stage. In audio equipment such as sound systems, the 50 or 60 Hz interference may be heard as a hum in the speakers. In a video system it may cause onscreen “snow” noise, or syncing problems. In computer cables it can cause slowdowns or failures of data transfer.

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