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Overheating 1200 Amp Square D Circuit Breaker – Bus Bolts Bottomed Out

March 31st, 2011 Comments off

Square D 1200 Amp Circuit Breaker - Cat. No. MAL361200 For Sale by MIDWEST

MIDWEST found a seriously overheating 1200 amp Square D circuit breaker during the annual Infrared Scan of a Foundry.  The upper left corner of the newly installed circuit breaker lit up like a light bulb when viewed with Infrared. The foundry third shift electricians tightened all the bus connections to the back of the circuit breaker. They said a couple bolts turned a quarter turn and they exercised the breaker. Rescanning showed no change. They didn’t want us working on the circuit breaker in place, so MIDWEST provided a temporary rental circuit breaker so they could remove the defective Square D circuit breaker for us to repair, if possible. Before testing the circuit breaker in our shop, we inspected it. The technician immediately noticed there were different bolts holding the line and load side breaker bus details. In our famous words, “We’ve seen this before.” Got to love experience. As soon as he removed the bolts he determined the bolts, holding the bus detail in the overheating area of the breaker, were bottoming out before the bus detail was tight. This was a quick fix because no overheating damage had occurred. This was an old Square D molded case circuit breaker. But we have seen the same problem on Westinghouse 2000 amp circuit breakers and less frequently on GE circuit breakers. We have never seen it when the circuit breakers have been installed by the manufacturer. It happens when a breaker has been replaced for some reason.