Schematic diagram of a typical ground loop created by interconnection of two AC powered audio or video components.  A ground loop is an inadvertent closed loop or circuit created in the ground wiring of electronic components.  Ground loops are a major cause of hum, noise and interference in audio and video systems.  The image shows two components C1 and C2, which could be the tuner and amplifier of an audio system, or a DVD player and a television for example,  connected by a shielded cable S.  Both components are also connected to the building's utility ground wiring G through the ground wire ("third wire") P in their power cords.  The cable shield S with the ground wiring P and G form a closed conductive loop.  This can act as a huge shorted "transformer winding".   Stray magnetic fields (B, green)   from nearby AC electrical equipment or wiring can induce 50 or 60 Hz circulating currents I in the loop by electromagnetic induction.  Since the resistance of the loop is very low, very large currents can be induced.  The AC voltage drop across the signal cable shield S caused by this current will add to the signal, causing hum in audio equipment, or interference in video equipment.  Information and circuit from Larry Robinson, About Ground Loops, MidiMagic website
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