If you want to see a good picture of a large circuit breaker that has been damaged by an arc blast, look at the pictures with this blog. The breaker was a Westinghouse PC32000 molded case circuit breaker. It was a 2000 amp circuit breaker that failed to interrupt a fault and blasted the inside of the breaker until it actually blew a hole through the side of the circuit breaker frame. If you look closely, you will see where a MIDWESTswitchgear service technician actually stuck a screwdriver through the hole. The close up picture shows the hole and shows the extensive arcing damage to the moveable arcing and main contacts. The phase barriers, arc chutes, pretty much the whole Westinghouse PC32000 circuit breaker, was arc blasted beyond repair. One picture shows the destroyed circuit breaker next to a MIDWEST inventory stock PC32000 circuit breaker. This is a tough Westinghouse molded case circuit breaker. It wouldn’t make any difference whether the breaker was Square D or GE General Electric. If the breaker contacts begin to open, but fail to interrupt the current, arc damage will quickly occur. If the breaker is trying to interrupt fault current, extensive arc damage occurs very fast and will quickly expand into a phase to phase fault with catastrophic destruction. The arcing fault may explode outside the case of the circuit breaker. This is one of the reasons for wearing protective clothing and following safe work practices when operation circuit breakers. Bad things can happen very fast. Too fast for you to get out of the way. We suggest being paranoid about safety when working around electrical power equipment.
How would you feel if you were an Engineering Technician and you had just spent over an hour maintaining and testing a Square D PAL362000 circuit breaker and the Engineer walked up, operated the PAL362000 one time and said “It’s junk, throw it out?” You might think the Engineer should be thrown out. But actually, the Engineer was just confirming what the technician already knew. In this case the circuit breaker had been inspected for any deficiency. The cover had been removed, yes, carefully, and the contacts, arc chutes, operating mechanism were all checked and maintained. The line and load side terminals of the old Square D PAL362000 were clean and in good condition. There was no sign of rust, worn main contacts or arc damaged arcing contacts. The operating mechanism visually looked in good condition. There was discoloration to the movable contact fingers of each pole piece.
Tests were performed on the PAL362000 over current devices. The test results were all good.
The contact resistance test results and the insulation resistance test results were all good. The reset tests were all good. So what was wrong with this expensive PAL362000 Square D circuit breaker? There were two things wrong with the breaker. One deficiency was suspected based on the inspection and test procedure. The other was determined based upon our experience servicing Square D PAL362000 and PAF362000 circuit breakers. First of all the movable contact fingers, ie pole pieces, were discolored. We have seen this before and it usually means the circuit was heavily loaded. In this case the circuit breaker was on a feeder that routinely hit 1800 amps and occasionally the breaker had tripped due to the load. The other thing that told us the breaker was defective was also based on experience. The experience of operating Square D PAL361000, PAL361200, PAL361600, PAL362000, PAF361000, PAF361200, PAF361600 and PAF362000 circuit breakers has taught us to listen carefully to the closing and opening of the three pole pieces, the moveable contact assemblies. Circuit Breakers that have been in very harsh conditions or operated under continuous heavy load, have a tendency to not open and close all three pole pieces simultaneously. When the breaker is defective, you can hear two or more poles close or open at different times. You will hear two separate distinct contact closings or openings. We know, if the difference is very obvious, repair attempts tend to be very temporary. With proper cleaning, lubrication, and exercising, the breaker may seem to operate properly. But we know from experience, the following year, or even in a few months, the breaker will again not close or open properly. In these days of real concern for arc flash hazard protection, this defect can not be ignored.
In the example discussed here, the Engineer just confirmed what the technician already knew. The Square D circuit breaker failed the hearing test. In this case experience rules. And it applies to Westinghouse, Cutler Hammer, GE General Electric circuit breakers also.