A schematic diagram showing a ground loop between two electronic components C1 and C2, such as audio or video components and how leakage currents (red) flowing through the building's utility ground wire system  can flow through the ground loop, causing interference in the electronic components.  Ground loops are one of the most common causes of hum and interference in audio, video, and computer systems.  Leakage currents from power appliances like motors (A) can flow through the "third wire" ground system (G) in home utility wiring, causing voltage drops of up to 5 - 10 volts between ground terminals on different outlets. In this diagram there is a "ground loop" consisting of the shield conductor of the signal cable S connecting the two components, plus the "third wire" ground wires P in their power cords, that connects the ground terminals of the two outlets, providing a second path for the leakage current.   The leakage current from the appliance divides, and some flows up the power cord ground wire P of C1, through the shield conductor of S to C2, and down the ground wire P of the C2 power cord, and back into the utility ground wire G on its way to the utility ground.  The 50/60 Hz AC current through cable S shield conductor causes a voltage drop that is added to the signal at the destination end (C2), and may manifest itself as "hum" in audio speakers, "snow" or interference on video, or erratic operation and crashes in computers.
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