What is a ground loop and how is it prevented?
A ground loop is the closed ring formed when the same circuit is grounded at two separate points. Even a potential difference of a few millivolts between the two ground points drives a current around this ring and induces noise into the signal conductors. The countermeasure is single-point grounding and, on screened cable, connecting the shield to the chassis at one end only.
On a vehicle or construction machine, two different chassis points are never at exactly the same potential; the starter, engine or crane current passing between them creates differences at the millivolt level on the metal. If a signal cable is grounded at both ends, this difference forms a closed ring and the current circulating in the ring rides directly on your measurement signal.
- Grounding at one end: connect the shield of an analogue signal cable at one end only. The source or control side is usually chosen; the choice must be consistent throughout the installation.
- Bond to the chassis, not to circuit ground: the shield goes to the chassis with the lowest possible impedance.
- No pigtails: twisting the shield into a thin tail and screwing it down makes the shield useless at high frequency. Use a gland or connector providing 360-degree circumferential contact.
- Exception: if the signal frequency is above 100 kHz, or the cable length is longer than a twentieth of the wavelength, the shield is grounded at both ends or at multiple points.
Do not run power and signal cables along the same route; in practice leave at least 15 cm of separation, and if they have to cross, cross them at right angles.
Full coverage of the topic: Installation, redundancy and field continuity