Archive for October, 2010

Circuit Breaker Infrared Scanning Disagreement – Lugs

October 27th, 2010 No comments

Infrared Scan of a Circuit BreakerWe read about a disagreement between two bloggers over whether or not infrared scanning, or thermography, was needed if you torque tightened the wire connections to power circuit breaker terminals during routine maintenance.  What occurred to MIDWEST were all the possible deficiencies we find in old, new, and replacement circuit breakers, using infrared scanning, that have nothing to do with whether or not the load terminals were tight.  One of the nasty deficiencies is when the cable lug in an old circuit breaker is very tight, but the lug is overheating because the screw, holding the lug to the breaker output tab, is loose. We’ve seen brand new circuit breakers and replacement circuit breakers fry the load side tab of the breaker so bad that the breaker had to be replaced. This is true for new or old Square D, GE General Electric, Westinghouse, Siemens, Cutler Hammer, ABB, any manufacturer. It has nothing to do with a specific circuit breaker manufacturer.


Sometimes the lug is welded to the tab from the arcing between the lug and tab. There is a very sophisticated test one can perform during a maintenance outage to check for this defect. First, check for voltage at the load and line side of the de-energized circuit breakers. Don’t care that the main breaker is off and all the feeder breakers are open.  Check voltage anyway.  You are checking for something that shouldn’t be, not for something you know should be.  We, rather I, have personal experience with getting my hand blasted because a breaker was back fed. Very bizarre set up, unbelievable, just waiting to injure someone.


After checking for voltage, carefully and gently try to move the conductor coming out of each phase of each circuit breaker.  You are trying to see if the cable is loose in the lug and you are trying to see if the lug is loose, moves or turns, in the circuit breaker. You are not trying to force it to move. Just use enough force to see if it is loose in the circuit breaker. If the lug itself is loose, the cable or cables will need to be removed from the lug; The mounting screw for the lug properly tightened; The cables properly reinstalled; And the cables tightened in the lug. Again, don’t be too forceful. On small breakers, you can always make the lug move. Repeating, you just want to use enough force to see if the lugs for that old obsolete circuit breaker are loose.


If the conductive interface, between the lug and the circuit breaker, is damaged from severe overheating or arcing, the defective circuit breaker may need to be replaced. Sometimes the damaged area can be repaired.  MIDWEST does not recommend replacing power circuit breakers while the switchboard is energized. Be safe. Turn things off. Check for voltage everywhere.

Old or New Circuit Breaker Hearing Test

October 13th, 2010 No comments


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.