Here is a pop quiz: What is the expected contact-resistance measurement for a 4000A 480V air circuit breaker in good condition?
This question might be confusing to some technicians because there has been very little written on the subject and contact resistance readings in the field are often all over the place.
Manufacturers will occasionally recommend values in their literature, but not often. The universal rule of thumb for contact resistance has often been to get it as low as possible and as even as possible between each phase. The higher the frame size, the lower the contact resistance readings should be.
While this may be good advice, it does not reveal any underlying problems with the breakerís lubrication or contact pressure.
Jim White has written about this in the Summer 2007 issue of NETA world. In his article titled ďCritical Maintenance for Circuit Breakers,Ē he says the expected contact resistance of a 4000 ampere 480 volt circuit breaker should have a reading of something less than 30 microhms. White says smaller breakers may have readings of 60 to 80 microhms, with molded case breakers having contact resistance readings in the hundreds of microhms.
This criteria is based on the ampere rating of the breaker rather than its voltage rating, White says. For example, a 2000 ampere medium-voltage circuit breaker will exhibit similar contact resistance to a 2000 ampere low-voltage breaker.
An experienced maintenance group knows the normal average contact resistance for a given size circuit breaker, according to White. A value somewhat higher than that average may indicate that the lubricant is beginning to dry out but hasnít reached the problem plateau yet. The important part of this is to perform the test annually and trend the changes.
So there you have it, I have attached the entire article if anyone is interested in learning more circuit breaker maintenance techniques. Itís a good read.