Four compressors, each powered by a 400-hp motor, provide plant air. The breaker supplying the branch circuit for one compressor has tripped several times in the past week. Yet, the motor overloads have not blown.
The technician normally working in that area didn’t find any reason for this trip, but he found an anomaly when taking a reading on the phase B conductor. When disconnected at the breaker, it read 17,800 ohms to ground. He stored this reading on his DMM to show you. Using Ohm’s Law, he also showed you this can’t be a breaker-tripping ground fault.
How can you determine what’s going on?
The 9V battery in that DMM doesn’t present the same challenge to conductor insulation that the 480V supply to the breaker presents. But an insulation resistance (IR) tester can. To find out what’s going on, use the right instrument for the job.
Even if you test the insulation at a low (for an IR tester) setting such as 1kV, you’ll probably read almost 0 ohms instead of 17,800 ohms.
The conductor was probably damaged during installation. A nick in the insulation can cause a weak spot where leakage occurs. It’s not a dead short, but at some point it will become one. Moisture in the raceway can exacerbate the leakage to the point where the overcurrent protective device trips — as is happening here.
After pulling conductors to critical equipment, perform baseline insulation resistance tests. Perform these as part of regular PM and trend them against the baseline data so you can replace conductors before they fail.