MCCB Primary Injection Testing

[RonShap]

For circuit breakers in the 600-1200A frame size, does anyone know a published example of a failure rate for breakers showing up on site?
I know that manufacturers perform a small sample of circuit breaker trip unit tests, but I'm curious if anyone has anecdotal or even better published failure rates after they leave the factory and prior to energizing on site?
Try to place a conceptual price on primary injection testing of MCCB's on site prior to energizing in a critical facility.
The client is being convinced by an on site electrician tat the primary injection testing is a waste, and I think it is worth it. BTW, I think the electrical contractor didn't carry any money for the testing which is carefully detailed in the contract.

 

[dpc]

I've been told that overall failure rate for smaller new MCCBs during acceptance testing is about 2%. I would hope that reliability would be better for 600-1200 A size.

For 600 A and above, the current test seems like a good idea to me. We have had clients who test EVERY MCCB, down to 15A.

 

[oldfieldguy]

Ron--

I did contract work for a client who tested every three-pole 480-volt MCCB and MCP. This was a large refinery and during the course of maintenance on a production unit these sometimes numbered over a hundred units. It was common to find as many as 10% failures based on direct current injection tests.


old field guy

 

[Zogzog]

The hardest part of a primary injection test for a MCCB is getting it out of the gear, you might as well do acceptance testing on arrival. Should be tested per NETA ATS.

 

[dpmac]

When you discuss "failure" of the MCCB what does that mean to you?

Failure to mechanicaly open or close?
Failure to trip at all?
Failure to trip within mfg spec?
Failure to trip within NEMA spec?
Contact resistance out of (what)spec?
What else?

Thank you in advance.

 

[oldfieldguy]

dpmac--

Any of these will do.

"Failure to open or close" on manual operation means that the device cannot be depended on to provide the isolation and control as designed.

"Failure to trip" means that the device can't provide the selectivity for coordination with other devices, meaning a branch circuit fault brings down much more equipment than intended, or much more energy is released into a fault causing more damage and safety issues.

"Contact resistance out of spec." means heat buildup and eventual (and often catastrophic) failures.



old field guy

 

[peebee]

The IEEE Gold Book, Table 10, indicates Circuit Breakers, Fixed (Including Molded Case) -- All, Above 600A, as having a failure rate of 0.0096 failures per unit-year, an industry average downtime of 9.6 hours per failure, and a median plant averate downtime of 9.0 hours per year. The >600A breakers actually have worse numbers than the 0-600A breakers, which only have 0.0035 failures per unit-year and lower downtime numbers.

Based on these numbers, MTBF = 1/0.0096 = 104 years; MTTR = 9.6 hours; availability = 100*(1-(9.6/104yr*365*24))=99.9989%

I don't see any info in the Gold Book on defect rates on new breakers. Anecdotally, I've generally heard numbers more in the 2-5% range on new breaker failure rates, and I've seen trip units delivered with an 80% failure rate, they just would not trip at all at any current level (which really sucks when the replacements need to be shipped to the US from Europe).

Hope this helps.

 

[dpc]

...trip units delivered with an 80% failure rate..

Makes me wonder how many of these were put in service and never tested. Pretty scary.

 

[Zogzog]

Thats what you get from "batch testing", which is why acceptance testing is so important.

 

[INEOSReliability]

We just finised current injection and contact resistance on our MCCB (mag and thermal mag) on old 480 gear and have a 61% failure.

Field testing should be done according to NETA standards and not to manufacture's - especally with thermal mag units. Too hard to create a bench enviroment same as they use.

We require all of our new MCCB to be tested by supplier before they are put into service.

 

[Zogzog]

That is not suprising for breakers that have been in service for awhile and not maintained. Testing before installation and then at some interval you can live with after that is vital to the proper operation of your system and safety of personel, thats why it was added to the safety related maintenence requirements of the 2009 NFPA 70E (And they use the NETA/ANSI MTS as a reference).

Dont trust a supplier that they tested it, they all say they do, ask for a detailed test report that meets or exceeds NETA specs. (Our specs greatly exceed NETA's for every breaker that ships)