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.
MIDWEST had a customer call and ask if it was okay to remove the cover from an old 400 amp circuit breaker, live. Our Infrared Scan indicated the load side connection was overheating. They wanted to repair it, but didn’t want to turn the power off to the whole panel board. They needed to remove the cover of the circuit breaker to make the repair and thought they could just remove the four screws holding the cover on and carefully remove the cover. We explained politely that they were crazy to try such a thing. This was an old molded case circuit breaker and the arc chutes for this breaker were not fastened in place as they are in some breakers. In addition, the arc dividers were metal and they were held together with an insulated band. On some of these breakers we have to tape the band to hold the arc dividers together or they just fall apart. So the danger would be that you remove the cover with the line side still hot and one of the arc chutes falls out and the metal arc dividers fall apart. It would be almost certain that one of the metal arc dividers would short a stationary contact to a moveable contact and cause a horrific arcing blast, arcing fault. Depending on the instantaneous, ie fault, setting of the main breaker, the fault might last for seconds and result in tremendous damage to the equipment and expose anyone nearby to serious injury or death from an arc blast. Because the fault is on the line side of the breaker, it wouldn’t take much to create a panel board bus fault. This is a good way to get someone seriously injured or killed and a good way to destroy a whole panel board. To remove the cover off any circuit breaker with the line side hot is a very bad idea. But to remove the cover of some of the older circuit breakers, with the line side hot, is just crazy because of the construction of the breaker. Inside molded case circuit breakers there are other devices that may fall out when you remove the cover. Besides all this, there may be something defective inside the breaker, just waiting there, for the first unfortunate person to take the cover off, and then it falls apart or breaks completely. You could get very unlucky. We call these things incipient failures and they can be some of the most nasty and dangerous defects in electrical equipment, because you are not expecting them. This is true whether it’s a Square D, Cutler Hammer, Westinghouse, GE General Electric, or Siemens circuit breaker or any other breaker manufacturer. Turn the main power off!
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.