Ground loops have the potential for disastrous consequences. They can compromise your shielding, rendering your instrumentation and control system useless. They can cause electrolysis, wreaking havoc on submerged equipment. And they can wipe out your defenses against power surges, which can fry your electrical equipment.
When a difference of electrical potential exists between two ground points that are tied together, a ground loop is born. This causes current to flow between the two ground points.
A ground loop can have devastating effects on any electrical circuit. The immediate effect is the compromise of your shielding. In order for a shielded cable to function properly, the actual shield must be tied to ground. Introducing a current into the shield is adding electrical noise, thus voiding the shield.
A less immediate effect, though potentially more devastating, is electrolysis. A ground loop can pass a current through liquids. The minerals in the liquid carry the current, and electrolysis takes place. This often eats away at metal equipment submerged in the liquid.
Finally, the most dangerous possible effect is the loss of surge protection. While immediate, it would go unnoticed until a power surge event. Most surge protectors work by using the ground as a reference point for measuring voltage. When voltage is high enough, it diverts the surge to the ground wire. When voltage is flowing through the ground wire due to a ground loop, the reference point has changed, and the surge protector will not divert the power surge at the right time.
This loss of surge protection may not fry all your circuits immediately. However, heightened voltage can put undue stress on electrical components, and wear them down over time. Whether or not a surge results in catastrophic failure, it will certainly shorten the life of your electrical equipment.
Checking for ground loops can and should be done with an ammeter. To do this, disconnect one of the ground points and check for a current. If a ground loop is present, simply leave one of the ground points disconnected. If there is no current flow, then connect the ground point.
As always, give us a call if you have an questions about proper sensor wiring or ground loops.