Expected contact resistance measurement based on breaker frame size

[SecondGen]

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 breaker’s lubrication or contact pressure.

Jim White has written about this in the Summer 2007 issue of NETA world. In his article titled “Critical Maintenance for Circuit Breakers,” 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 hasn’t 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. It’s a good read.

Update:

You can usually find recommended values in circuit breaker instruction books, and if you compare them, you will find that they are all pretty similar. Here is an example from Square D:



NW08 = 800A, NW12 = 1200A, etc.

Contact Resistance Test

Circuit breaker pole resistance tests are not reliable indicators of circuit breaker performance because the resistance values are influenced by a number of transient factors including contact surface oxidation, foreign material between the contacts, and testing methods. NEMA AB 4 paragraph 6.4.1 states: “The millivolt drop of a circuit breaker pole can vary significantly due to inherent variability in the extreme low resistance of the electrical contacts and connectors. Such variations do not necessarily predict unacceptable performance and shall not be used as the sole criteria for determination of acceptability.”

High pole resistance may also be caused by eroded contacts, low contact force, and loose termination. The only one of these factors likely to be present on a new circuit breaker is a loose termination, since the contacts are new and there has been no opportunity for contact pressure to have drifted from the factory setting. A loose termination can be corrected in the field.

If a contact resistance test is done, it is important to do it after the contacts have been conditioned by instantaneous primary injection testing to ensure the contacts are clear of resistive films, oxidation and foreign material. If the circuit breaker has been in service with no performance issues, (overheating or nuisance tripping), contact resistance measurements are redundant and of little value.

Square D™ recommends that a Digital Low Resistance Ohmmeter (DLRO) be used, using a 10 A dc test current for circuit breaker ratings below 100 A, and using 100 A dc for circuit breakers rated 100 A and above. the median (middle) value of three readings (toggling the circuit breaker between each reading) should be recorded for each pole tested.

If this value is equal to or less than the value listed in Table 26, the pole is acceptable. If the reading is higher, the cause should be investigated and corrected if possible. Contact your local field office for more information.

http://www2.schneider-electric.com/r...202%202015.pdf

[thomas81874]

Read the article, it was very informative. Some points where known but others were new.

[slts1991]

good article. thank you

[rgrabis]

agreed, good info.

[stuebes]

Good article. I'm aware from experience where most breakers should come in, but wasn't aware that Jim White had an article out on this.

Thanks.

[krunal]

very informative. Thanks

[ElectricalTestTech]

This looks like it could be a great article!