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