There are times, as thermographers, we get what I call “tunnel vision.” At least I do, from time to time. Tunnel vision can be defined for our application as concentrating on the view screen and not looking at the target. This is especially true when we are pressed for time or “in the zone.” We scan the equipment looking for the faults to “jump out” on the screen or to break our alarm triggers. When I find myself doing this, I remember an inspection that nearly caused a great amount of trouble.
I recall a time I was inspecting a large motor control room. It was getting close to lunch time and I was trying to finish up a section. I was still waiting for anything to jump out in the last cell (bucket). All there was is maybe a 2 ° C delta T between A and B phases at the top of the fused disconnect. I was going to close the door when something caught my eye as the latches caught. I could have said “That’s it lets go to lunch.” Instead, I asked my escort to open the door again. Something in there didn’t look right before and the lighting in this corner of the room was not great.
Therefore, I got my little flash light out of my belt pouch and looked closer at the bucket. I noticed at the bottom there were some of the usual dead bugs. That wasn’t what caught my interest, but the charred bits of something or another among the bugs. Going up to the starter there were some more charred bits. None of the conductors to or from the starter had signs of damage. Going up to the fuses all looked good. Then I saw the conductors at the line side of the disconnect. There was no insulation left on the B phase conductor between the bus stab and the disconnect. The copper was about the same color as the conductors of A and C phases!
At a quick glance it would have not been noticed. My first reaction was to restart the infrared camera and look again to make sure I didn’t overlook the line side. Nope. The image on the screen looked OK. I had the escort take an Amp reading on each phase, each phase was pulling about 175 amps. That checked out with the thermal image and all looked balanced. However, why was the burnt up conductor showing different? I called the Electrical Engineer and the Operations Supervisor to shutdown the pump. They asked me why and I told them there was a conductor with no insulation in the bucket. They understandably did not believe it. How could it not be shorting out?
The conductor was coming through in insulated opening in the bus stab block and the connection to the line side of the disconnect switch had no grounded surfaces near it. The supervisor and the engineer came to the area and looked for themselves. They decided to run the pump until replacement parts could be brought to the MCC. 90 minutes later the pump was shut down and the parts replaced. The B phase connection was welded solid, so the disconnect was also replaced along with the bus stab.
The question on everyone’s mind: why was the connection appearing normal when in fact it failed. The high resistance connection at one point arced enough to repair itself, but would have quickly failed again.
My thought was how did I nearly miss finding this problem. I was in the zone. I had been rolling along with the knowledge that any fault will produce heat. Therefore, only be found looking at the screen. So, avoid tunnel vision and getting stuck “in the zone.” Slow down and be aware of your environment. If something doesn’t feel or look quite right, always take the time to go back in for a second look.