Electrical Testing
Tip of the Week: Lessons Learned from a Test and Measurement Case Study

Tip of the Week: Lessons Learned from a Test and Measurement Case Study

Unless workers are properly trained on your measurement equipment, you may misdiagnose problems.

Test equipment doesn’t tell you everything. In fact, to get usable information from it, you have to know what you’re looking for, how to measure it correctly, and how to interpret what you’re measuring.

Consider this example.

Jim works in a manufacturing plant as a Level II thermographer. He’s the only thermographer in the plant. The day before he’s due to arrive back from vacation, an 800A breaker undergoes two nuisance trips.

Mike, a plant electrician, dutifully suits up in his protective gear and does some checking with his digital multimeter (DMM). He doesn’t find anything that would indicate a problem. His clamp-on ammeter shows just over 300A on that breaker, so it’s not tripping from overload.

Mike reports his findings to the plant engineer and says they need to either lease a power analyzer or hire a qualified testing firm to come in and hook theirs up. “We may have some sort of short-term condition that is causing this.”

The plant engineer wants to use existing resources before spending any money. So he tells Mike to get out that thermographic camera and take a look. “Really, Mike, it can’t be that hard to use.”

Photo credit: maxkabakov/iStock/Thinkstock

Mike does figure out how to get some pretty good-looking images. He goes back to the plant engineer, and hot dog! The culprit is that breaker. Look how hot it is. No excess current, and a hot breaker. The problem must be a defective breaker.

The plant engineer orders a replacement, and it’ll be there in two days. Maybe, he thinks to himself, they don’t need Jim after all. “I can lay him off and save money in the budget. All that training and this thermography is a snap; Mike figured it out in less than half an hour.”

Jim comes back from vacation and gets the full story (except the part about possibly being laid off). He takes the camera out, and sure enough with Mike’s settings it looks like one hot breaker. But switchgear is full of metal, and getting an accurate image with all those reflective surfaces is tricky. Jim, having been trained, knows how to do it.

Jim’s image shows the breaker is not standing out as particularly hot. So it’s probably not defective and maybe not the culprit in this nuisance tripping mystery. And in any case, breaker testing should have been performed before spending the money to buy another one. Jim decided to do further troubleshooting, so went back to the shop for the drawings.

Jim’s troubleshooting efforts revealed that Mike has been looking in the wrong place (maybe the light was better there). Nobody had looked at the loads that breaker supplies. (Good thing Jim hasn’t been laid off; who else is going to do proper troubleshooting?).

One of the loads on that breaker is an air compressor that supplements other units. Normally, it doesn’t run. But when low air pressure signals that compressor to kick on, that compressor’s motor starts across the line.

Insulation resistance testing produced readings that were suspiciously low. It turned out that a new lubrication technician had pumped the motor full of grease. Trying to start that motor meant a far greater current draw than normal, and it was this that caused the breaker to trip.

The breaker was doing its job. Apparently, the same cannot be said of the plant engineer; he should have ensured the lubrication technician was trained and qualified. And there is the fiasco arising from the false conclusions reached by his ersatz thermographer.

Using test equipment correctly can quickly lead you to correct conclusions. But if you don’t have the training, you may be “troubleshooting” for a long time and unnecessarily replacing perfectly good components.

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