Ground Loop (EMC, Signal Cabling)

An unwanted closed circuit that arises when the shield or reference conductor of a signal cable is connected to multiple, electrically non-identical earth points, allowing a circulating current to flow through the cable shield or signal reference due to small potential differences between those earth points (for example caused by voltage drop in the earthing system, power currents or induced currents from nearby power cables). In measurement and control systems, this circulating current typically manifests as hum interference (at mains frequency or its multiples) superimposed on the desired signal, measurement errors or, with digital communication, occasional communication errors. Unlike EMC earthing of a cable shield (where double-ended earthing is often actually preferred for optimal shielding against high-frequency interference), for sensitive low-frequency signal circuits single-point earthing of the signal reference is sometimes precisely the appropriate solution to avoid a ground loop — the correct choice depends on the frequency range of both the signal and the interference to be suppressed.