When something from the past comes to the forefront several times in a row, one might think of it as a sign. I think of it as an indication that there must be even more occurrences out there. The concept of a ground loop is the latest to rise to that level of awareness, though the circumstances of each were different.
I was reminded of this a few times recently. The first time was during a Code-Making Panel session meeting for the 2008 revision of the National Electrical- Code (NEC, also known as NFPA 70). In this case, one of the attendees was denying the existence of ground loops, which is a somewhat bold position, given the countless examples of them documented through the trade journals, textbooks and the Internet. The second time was from a digital monitoring system for breaker operations in a substation. And the third reminder came from someone connecting the audio portion of a new entertainment unit and getting the classic “60 hertz hum.”
Electrically speaking, the “loop” portion references to a complete circuit, path or loop where current is flowing through one or more conductors that are referenced to two or more ground points in the circuit. The problem results from the condition where the ground potential at the different points in the circuit is at a different voltage level relative to other parts of the circuit. Where there is a voltage potential and an impedance (wire still has an impedance no matter how large the gauge), there will be current flow.
you will find many definitions of such on the Internet, some of which have significant inaccuracies. One of the more accurate and complete definitions comes from NFPA 70B: “Electrical Equipment Maintenance: 3.3.30 Ground Loop. Multiple intentional or unintentional connections from a conductive path to ground or the conductive body that serves in place of earth. Current will flow in the ground loop if there is voltage difference between the connection nodes. Re-grounding of the grounded circuit conductor (neutral) beyond the service point will result in ground loops. This might or might not be harmful depending on the application.”
In some cases, the multiple grounded connections are intentional and may even be required by code for safety as well as proper equipment operation. Other times, they are made through oversight or lack of understanding of ground loops by the installers and result in equipment misoperation. The situation at the substation was the latter. The signal wires for the status of the relays and breakers were carried through shielded cable, to prevent the high electromagnetic environment of a substation from being coupled into the signaling wires and resulting in misrepresentations of the system’s status. However, the shield in the cable at both ends was connected to “ground,” out at the breaker in the yard as well as at the recording instrument, which had an earth ground connection on its power supply, as well.
When a fault of one or more phase conductors to ground occurs in the system, there can be significant ground current as well as high fault current levels with associated electromagnetic fields generated even though a substation has a large ground grid designed to form an equipotential plane. The result was that the shield became a current-carrying conductor as well as an injector of noise into the signal wires, rather than keeping such out as was its original intent, due to the current flowing through the shield.
Troubleshooting ground loops is just like most power quality problems. It must be done through measurements and methodical steps, or the situation can be made worse and even a lethal hazard. According to ePanorama.net, an electronics information Web site, “The ultimate purpose of good grounding scheme is the preservation and adherence to the safety aspects while obtaining the maximum noise reduction possible. That is not usually an easy task to do.”
Isolating the ground on one end of the electrical system from another normally would result in a violation of the NEC. Use of a “cheater plug,” which takes three-prong plugs and eliminates the grounding prong, are dangerous ways to eliminate ground loops. Isolation transformers for separately derived systems are one proposed solution.
But since many times it is the communication or signal circuits that are experiencing problems with the ground loops (since they are normally much lower voltage circuits than the power circuits) and where the attention should be paid. For digital signaling circuits, use of opto-isolators can be a solution, whereas for video or audio, it can be a cable isolator or balun, respectively, that provides the solution. Communication systems using fiber optics inherently avoid ground loops since light is used instead of electricity.
In general, trying to keep all equipment electrically connected with as low impedance as possible and as close as possible to a central grounding point can help keep the ground potential the same throughout the system, and keep the system from getting thrown for a loop-da-loop.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.