Electrical Troubleshooting Quiz - April 4, 2017

The plant has a 400-hp motor that drives a big compressor for the plant air system. A few months ago, it was tripping its feeder overcurrent protection device (OCPD) on a regular basis. The plant engineer suspected a problem with the motor windings, so he brought in a thermographer to check things out.

The thermographer noticed discoloration on the feeder cables. He recommended shutting down the compressor long enough for him to perform some tests on those cables, including leakage testing and insulation resistance testing. The plant engineer agreed, and the cables were found to be faulty. They were replaced, and the nuisance tripping stopped. But now it’s back. What could be the problem this time?

This could be complicated if no baseline testing was performed. If you do have baseline testing data, then you just need to compare those to a new set of readings.

It’s good that the thermographer found the cable was “faulty,” but he didn’t specify what kind of fault. Knowing the failure mode (e.g., bolted fault vs. pinhole fault) makes it much easier to determine the cause. A bolted fault and a pinhole fault have different causes.

If the new cables are already failing, the cause can be anything from installation errors to transient events. Take new test data and find out exactly what is wrong.