Who Really Cares About Ground Loops?

What is a ground loop?

To understand what a ground loop is one must first understand what ground is and what ground is not.

1. Without a lot of detail basically in both Ac and Dc systems ground is a safety conductor that is designed to protect personnel from shock hazards.
2. A grounding conductor has a low resistance path to earth.
3. A grounding conductor is not intended to carry steady state current.
4. When a phase fault (ac) or a return fault (dc) (Ground Fault) occurs to the framework of the device, the grounding conductor’s job is to return the high level of current to its source and trip the breaker as fast as possible to prevent a shock hazard.

A ground loop is very similar to a ground fault except that the current level of the ground loop under normal conditions never reaches the magnitude of current that would trip the breaker. This current becomes a steady state circulating current that continues to flow at all times throughout the grounding system. In most cases, as long as this ground loop current does not increase the system appears to have no problems.

1. Many electronic equipment devices have a designed high resistance fault between the power source and the metallic chassis.
2. In two wire circuits that use a coaxial cable (Radio, CATV, CCTV etc.) there almost always is a designed ground loop.

What standards say about ground loops?

Almost every bonding and grounding standard or code that has been written permits some amount of current on a grounding conductor before mitigation on that conductor takes place.

Why worry about ground loops?

When any type of power disturbance occurs that changes the voltage/current level of the facility (i.e., lightning strike, power surge or a ground potential rise) the voltage/current on ground loops will also rise. When this voltage/current level rises high enough equipment can be damage as well as system damage. It is also possible that the circuit breakers could trip and then the equipment or system would lose its power source.

1. A power surge of some magnitude occurs every time an electrical device is turned off or turned on.
2. In the USA, each state has an average number of lightning days in a year the range from fewer than 10 to 90 or more. Most of the states average 30-70 lightning days in a year.

Ground loops are actually warning signs for your facility.

By evaluating the current that flows on grounding conductors a facility manager will actually realize that a potential exists to cause catastrophic equipment and system failures.

Because bonding and grounding standards as well as the National Electrical Code permits objectionable current flow most facility managers do not concern themselves with small currents on grounding conductors.

1. To ensure better equipment reliability and business continuity all ground loops should be identified and corrected.
2. In many cases a high ohm resistance exists between the phase (Ac) or return (Dc) and the metallic frame of the device and additional efforts must be made to eliminate the current flow on the grounding conductor. In this case a subject matter expert should be used to correct the problem.
3. The damage from ground loops does not have to be a cost of doing business, they can be prevented.

Ground loops will, at some point in time, cause equipment and system outages the only question is when and how much damage and downtime will it cause.