Molded Case Circuit Breaker PM strategies

[CKH17]

Good morning,
Please share with me your current practices/strategies on Molded Case Cicuit Breaker ownership.
What do you do for PMs? Do you PM those MCCCB or run to failure?
what mitigation plan? (putting a fuse in line with the breaker, etc..)
MCCB are not draw out breakers, they are bolted on to the bus, and it is not likely to take the bus down for breaker PM.
How certain can you be that your breaker will trip when required?
No PM on your breaker send the incident energy levels in the 3 digits numbers, where your 100cal/cm^2 flash suit can't insure protection.
I need some opinions/ suggestions on that topic.
Thank you for sharing your knowledge.

CKH17(electrical)

 

[davidbeach]

About all the PM you can do with an MCCB is to exercise it periodically and do IR scans.

 

[CKH17]

Thanks David,
The manufacturers usually say to exercise it twice a year, but there is applications where it is not really feasible due to the process downstream. There is also cases in the industry where the MCCB did not trip when it was required to, altough it was exercised. Also, do you think that current injection testing does more harm than good to the breaker?

 

[dpc]

Current injection testing should not really do much harm to the breaker, as long as you are not injecting something close to the interrupting rating.

I'm only a consultant, but I'm aware of a high-tech firm that does current-injection trip test of EVERY molded-case breaker at installation. The testing outfit told me that the infant mortality on small molded case breakers right out of the box has been about 2%. I'm not sure what they do for periodic maintenance testing.

There are plenty of cases where MCCBs fail to trip. I think exercising the breakers periodically is essential.

 

[slavag]

Hi.
It's not my field, but:
PM of every MCCB!!!! twice a year!!!!
IR scan, yes- must, but current injection for every MCCB????
May be critical MCCB or group MCCB.
Regards.
Slava

 

[CKH17]

more thoughts?..

 

[rcw retired EE]

In the 70's & '80's most telephone companies in the US followed a Bell System's Standard that required current injection testing of all circuit breakers 100Amps and larger. (Some locations even wanted 30A breakers tested.) Time frame was about 3-5 years between tests plus all new breakers.

This PM program was started by a MCB failure. A panelboard breaker failed to trip and the resulitng melt down and fire took down a Central Office in New York City around Wall Street. Big losses.

The MCB failure rate durign tests was less than 2-3%. Most problems were loose connections or, once in a while, a sticky trip unit. Cleaning the equipment and torquing the bus as we did the testing was probably more beneficial than the actual tests. Later, IR scans located most problems and the test program was stopped.

I would recommend an annual IR scan, tripping the breaker a couple of times with the handle.

 

[alehman]

Periodic injection testing is best if it is feasible.

NFPA 70B and NETA Maintenance Testing Standards recommend inspection and exercise every 12 to 24 months and injection trip/contact resistance every 3 to 5 years. Also any breaker that has interrupted a fault close to its interrupting rating should be inspected and tested before being returned to service.

 

[ScottyUK]

Can you inspect an MCCB without voiding the manufacturer's warranty? Some of the newer European designs have the interrupting mechanism in a sealed capsule which is only possible to open by destroying. We are forced to treat them as throwaway items following a major fault - much to the delight of the manufacturers.


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If we learn from our mistakes I'm getting a great education!

 

[alehman]

I think in the case of MCCB's inspection refers to external only. No, they typically can't be opened without voiding the warranty and listing.