Ground loop

Ground loop: In an electrical system, an unwanted current that flows in a conductor connecting two points that are nominally at the same potential, i.e., ground, but are actually at different potentials.

Note 1: For example, the electrical potential at different points on the surface of the Earth can vary by hundreds of volts, primarily from the influence of the solar wind. Such an occurrence can be hazardous, e.g., to personnel working on long grounded conductors such as metallic telecommunications cable pairs.

Note 2: A ground loop can also exist in a floating ground system, i.e., one not connected to an Earth ground, if the conductors that constitute the ground system have a relatively high resistance, or have, flowing through them, high currents that produce a significant voltage ("I·R") drop.

Note 3: Ground loops can be detrimental to the operation of the electrical system. Contrast with ground current.

Source: from Federal Standard 1037C