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Posts Tagged ‘insulation resistance’

Circuit Breaker – POJ with perfect test results

May 30th, 2012 Comments off

Sometimes MIDWEST’s Engineering Technicians in our Switchgear Shop like to do what the Engineers call “Play.”  The technicians call it serious research.  Sometimes their adventures are quite worthwhile, even enlightening.  Recently an Engineering Technician dropped a circuit breaker test form on an Engineer’s desk and said, “Would you take a look at that and tell me if you think it’s okay.” The test results were excellent.  The contact resistance, insulation resistance, over current test results, reset tests, all the test results were excellent.  So why in the world was the Engineering Technician dropping it on the Engineer’s desk when the test results were so good?  The results looked like those for a new circuit breaker.  What they had done was test an old Westinghouse circuit breaker that had been practically crushed.  The case was broken and the arc chutes damaged.  It looked like it had been dropped from 50 feet and hit on one corner.  But, oddly enough, against all reason, it operated mechanically and the test results were all good.  But, of course, it failed the visual inspection.  When the Technician dragged the Engineer to the Switchgear Shop to evaluate the breaker, everyone had a good laugh.  This was good fun and the technical evaluation was unanimous, POJ, Piece of Junk.  One of our favorite highly technical terms.  In this case it was very obvious.  But frequently we find defects in used circuit breakers and in brand new circuit breakers, and other electrical equipment, that would never be revealed by testing alone.  That’s why the experience, training and knowledge of the Technicians and Engineers are extremely important.  There is no perfect test standard for every POJ.

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.