Electrical Troubleshooting Quiz

A 2,000A breaker tripped, bringing down several production lines. Another electrician responded to the trouble call. His DMM didn't find a fault between any of the phases or between any phase to ground. He looked at each of the large loads and didn't find anything obvious: nothing melted, no burning smell, no sign that anything happened. No branch circuit protection devices opened, either.

The production superintendant now insists it must have been a random nuisance trip, since maintenance didn't find a cause and that breaker needs to be closed so production can resume. Should you close the breaker or investigate further? If the latter, what should you look at?

For a small branch circuit, you might close it and see what happens. For a feeder handling this much energy, don't count on being lucky enough that the breaker will protect you one more time. You could close it only to create a massive fireball.

A DMM is an excellent instrument, but there's a lot it can't tell you. You need to solve for the cause of this event. Now is a good time to contact your electrical testing firm. If they can't get a tech on site immediately, they can at least get that ball rolling.

Meanwhile, you may find the problem by running these tests:

• Low ohms testing. Measure across all connections; replace with new hardware as needed.
• Visual inspection. Use a very bright light and look carefully for dirt, burn marks, etc.
• Insulation resistance testing. Check the load wiring to ground.
• Recommended PM for that breaker.