Often times as thermographers we have it drilled into our head to look for “hot” spots. In our search for these hot spots, however, is it possible that we’ve been overlooking potentially important “cold” spots?
Let us consider these images below and ask ourselves what is, or can, be causing this? Also ask yourself, would I report it?
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At first look, the image to the left shows us a hot spot on phase B load side of the contactor, and some may stop there and report that, but this image should lead to the question. Do I have seven other connection issues (A and B phase line side and load side) or is there a problem with C phase? As thermographers we keep a hands-off or "no contact policy as a means of safety so it can be difficult to verify what the cause could be. If you have a qualified escort, and that escort has a meter with him (as I did), he can check for voltage and amperage. In this case, voltage was present in C phase but the current was at .05A the other phases were at 21.5A.
Not knowing the equipment, I had to ask the escort if there were single phase loads present on this system,(yet another question the image brings up!). The system in question was a heat treating process that they were having some issues with. The problem being, the liquid in the vat was not coming up to temperature very quickly. All of the elements should be “on” at the same time, the cold C phase indicated that there was an open in that phase or set of elements.
The image to the right brings up the same questions, but with different results. In this case the fuse group to the left would be normal at this point of the heating cycle (maintain current temperature). In this device, elements are turned on and off as needed to maintain a given temp. I’m not sure of the wiring method used, but when we came back later, both fuse groups looked the same when the vessel called for more heat.
So, what does all this mean to us?
The image on the left was reported as two faults, the B phase connection and the loss of C phase loading. The image to the right was not reported as there is no fault. Single phase loads on a three phase system can be tricky especially if they cycle in and out.
Things that look cold can indeed be a problem in an electrical system, but the process and the control methods must be understood before a “fault” can be declared. Don’t be afraid to ask questions!