ground loop
A (usually unwanted) current that flows between two points that are meant to be at the same potential, but are not.
ground loop: the hum you get from duplicate earth paths
A ground loop occurs when two or more grounding paths exist between equipment that should share a single reference point. Instead of current taking one route to ground, it finds multiple paths and circulates between them, inducing unwanted voltages and noise into audio, video, or measurement signals. The loop acts like a transformer secondary winding, with the mains frequency (50 Hz or 60 Hz) or its harmonics driving current through the unintended circuit.
The classic setup involves audio or video cables. Suppose a microphone is plugged into a mixer that is grounded through the mains cable, and the mixer is also connected via balanced XLR to a powered speaker that has its own ground through a separate wall outlet. Both the mixer and speaker now have a direct path to earth ground through their power supplies. The audio cable shield, which should carry only a reference level, becomes a secondary ground path. Current flows between the two ground points through the shield at mains frequency, inducing a 50 or 60 Hz hum into the audio. The amplitude depends on the impedance of the loop and the voltage difference between the two earth points, which may be only a few volts but enough to corrupt the signal severely.
Ground loops appear wherever there is shielding or a metallic connection between equipment and more than one ground reference. In instrumentation and data acquisition, they degrade measurement accuracy by injecting 50/60 Hz noise and sidebands into analog signals. In video systems, they cause hum bars or subtle roll in the picture. The problem intensifies in longer cable runs and in facilities with variable earth resistance or currents from heavy machinery flowing through the earth grid.
The standard remedies are to ensure that all equipment uses a single, low-impedance ground reference and to use balanced connections with proper shield grounding. In audio, this means grounding the shield at only one end (usually the source), allowing the twisted pair to carry signal differentially while the shield floats at the receiving end. Isolation transformers in power supplies, ground isolation amplifiers in measurement circuits, and optical isolation in digital connections all work by breaking the physical loop while preserving the signal path. In some cases, a ground lift switch on XLR connectors simply disconnects pin 1 at one end, defeating the parallel path without removing the shield itself.
Diagnosis requires a signal tracer or oscilloscope to confirm the 50/60 Hz and its harmonics, then systematic disconnection of cables and equipment to identify which path completes the loop. The term itself reflects the electrical reality: ground, which should be a reference potential, becomes a conductor carrying current in a closed loop, much as current loops in any other circuit when driven by an EMF.