Your plant has had a high rate of equipment failure lately. Without consulting the maintenance department, your "ever-helpful" plant manager hired a consultant. The plant manager now wants you to install isolated grounds on all of the major equipment. When you asked on what basis this recommendation was made, he said, "I saw on the ground meter with my own eyes the ground resistance was only 2Ω. So the harmonics flow right into the system!"

What was the plant manager actually seeing and what data would you collect to show the correct measurement? More importantly, how can you identify the cause of the high failure rates?

Your plant manager was seeing the resistance between the neutral and ground bond. Clamp-on ground testers are very useful, but to get a valid reading you must know a great deal about the system and where you're taking the measurement. This instrument can help an expert; it cannot substitute for expertise. An expert would also be skilled in performing ground testing using soil probes.

A high rate of equipment failure typically will be exacerbated, not solved, by installing isolated grounds.

To solve this problem, first characterize the failures and failure modes. You want to look for a common thread. For example, are these mostly premature motor failures? If so, is the failure mode mostly overheating? The root cause could be voltage imbalance, which is a load distribution issue rather than a grounding issue. On the other hand, if you have high rates of transformer failures, voltage imbalance is a less likely culprit while power quality issues are more likely.