MIDWEST has its own internal blogosphere that especially hums away when something unusual happens, like when one of the Engineers or Engineering Technician discovers something important or rare or both with the electrical equipment they are working on. Here is an example of some internal buzz that also proved the
importance of removing the covers from electrical power circuit breakers, whether Square D circuit breakers, Cutler Hammer circuit breakers or GE General Electric circuit breakers. The need is not related to the manufacturer. In this example, the Engineering Technician was reconditioning and testing a 1000 amp Square D circuit breaker. When he removed the cover of the circuit breaker, he was amazed to find the center pole arc chute was gone. Completely missing. Fortunately the arcing and main contacts appeared undamaged. There was some arcing marks on the arcing contacts, but nothing serious. One could do high current tests, contact resistance tests, and insulation resistance tests and never catch this defect. But the first time this breaker tried to interrupt a large fault current, it could have failed, with the resulting arcing causing an eventual phase to phase arcing fault on the line side of a large circuit breaker. This arcing fault might not be large enough to trip the main breaker immediately. If for some reason the panel board was a MLO, main lugs only, the protective device might be the main breaker or fused switch on the line side of an upstream transformer. This is a worst case environment, but it does happen. It is especially dangerous because the arcing fault may last several minutes before the main protective device operates. And more scary is the possibility of someone being injured by the fault. This is an incipient failure. Also an insidious failure. It is another example of the reason to properly recondition replacement power circuit breakers. Testing alone is not enough.