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Posts Tagged ‘operating mechanism’

Circuit Breakers in Switchboard Buried in Sand

December 6th, 2010 Comments off

Sometimes MIDWEST runs into switchgear and circuit breakers in such harsh environments that you would wonder how they don’t blow up, much less work properly.  An example is some switchgear and old circuit breakers found in foundry environments. The condition of electrical equipment in foundries is 100 times better than 25 years ago. But there is still one thing that has not changed for some foundries and that is sand in electrical switchgear. Some foundries still have their main panel boards and some switchgear in open foundry areas, rather than in clean positive pressure rooms.

 

We recently were called in to repair a 2000 amp circuit breaker used in an open foundry environment. It turned out the old circuit breaker was not a breaker at all, but rather was a 2000 amp bolted pressure switch. The electrical switchboard had over 6 inches of sand in the bottom and 3 or 4 inches on top. The main horizontal bus feeding the risers for the circuit breakers, was partially buried in foundry sand. The service technicians said they actually scooped the sand out before even trying to use vacuum equipment. Fortunately the sand didn’t carry anything with it that acted as a conductor. This isn’t always true. In this case, the sand was just more insulation.

 

Maintaining the bolt lock switch and the circuit breakers was a nasty job. The covers had to be removed from every breaker to clean the operating mechanism and to get the sand out of the contact and arc chute area. And all our efforts were only temporary since the environment was unchanged. More serious was the fact that foundry dust would be inside the over current trip devices of the circuit breakers. Therefore the operation of the trip devices was unreliable, even unsafe. It wouldn’t make any difference whether these old circuit breakers were Square D, Westinghouse, GE General Electric or Cutler Hammer. Foundry dust and sand doesn’t care who the manufacture is. Even a brand new circuit breaker would be a victim to the sand.

The illusion was the circuit breakers were okay because they didn’t trip. It was only when the owner tried and failed to operate the main switch did they realize that maybe the panel board and breakers needed some attention. This was not the first, nor will it be the last, switchgear, panel board, or circuit breakers that we find basically buried in sand.

Old or New Circuit Breaker Hearing Test

October 13th, 2010 Comments off

  

PAL36200 Square D Molded Case Circuit Breaker

PAL36200 Square D Molded Case Circuit Breaker

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 breakersFirst 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.