Ground Loops and Their Cures
The National Electrical Code (NEC) defines a ground as 'a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.' A ground loop can be defined as any objectionable current flowing in a circuit's ground or return path. Here is a short guide that will help identify possible sources of ground loops in your electrical systems and how to solve them.
The simplest ground loop involves connection between two different earth grounds as shown in the top figure. With earth ground #1 at one potential, and earth ground #2 at a different potential, a ground loop current will flow in the loop as indicated. While NEC requires grounding electrodes to be connected together and metal parts to be bonded together, there will still be differences in ground potentials in the system. The further apart the connections, the more likely there will be a significant potential difference.
One common cause of ac power ground loops is the double bonding of the neutral. The NEC requires the neutral to be bonded to ground at only one place, either the service entrance or source (for separately derived systems), or at the first disconnect or overcurrent device. Double bonding of the neutral usually occurs in downstream distribution panels. When the neutral is double grounded, returning neutral current will split per Ohm's law and will flow in the ground circuit. This current can cause varying voltage reference to equipment in the system. Remove the illegal neutral to ground bonds and the ground loop will be eliminated.
DC power systems used for instrument and loop power are subject to a number of possible ground loops. This type of dc power system has its return path or negative side grounded in only one place. One common ground loop occurs when a grounded thermocouple is used without isolated inputs or an isolated transducer. Since the grounded thermocouple is typically a long distance from the dc power system's reference ground, a substantial difference in ground potential can exist. Large currents can flow causing varying reference potentials in the system, which can sometimes cause strange effects. The solution to this type of ground loop is to use an ungrounded thermocouple or to isolate the thermocouple ground from the instrument system ground by using an isolator, an isolated transducer, or isolated inputs. Generally, it is good practice to isolate even when using ungrounded thermocouples.
The shield drain wire of an instrument signal cable is another place susceptible to ground loops. A shield wire is normally grounded only at the zero-signal reference point of the circuit, which is normally the dc instrument power system reference point. If any intermediate point on the shield becomes grounded, a ground loop will be formed. Not only will this ground loop corrupt the dc reference, current will flow in the shield which will generate noise in the signal wires. Care must taken to ensure that the field portions of the shields are terminated properly and that they are not exposed to environmental conditions that might cause a sneak path from the shield to ground.
Two systems which communicate digitally to each other and are referenced to ground at two physically different points within the same grounding electrode system are commonly prey to ground loops. This type of ground loop is solved by using isolated communication devices or preferably using a fiber-optic link.
The method to solving ground loop problems is generally twofold. Remove any extra grounds so that there is one ground in the system. If there must be more than one ground, make sure to isolate each from other(s).