Testing Molded Case Breakers with Current Injection 1

[mls1]

I have found several posts discussing testing of MCCBs with various opinions on size and required testing but not many have provided backing references. We have had a difficult time testing breakers smaller than 100A and have opted to perform primary injection acceptance testing only on breakers 100A or greater. I believe we based that on guidance but can not seem to relocate where we got the 100A break point at. It's not in NETA or NFPA 70B. If anyone knows a specific reference which provides a recommendation to primary inject MCCBs, thermal overloads, and/or MCPs for sizes 100A and greater it would be appreciated.

 

[TestMonkey]

It is possible that the current standards no longer recommend the sizes for testing. I have seen older NFPA 70B documents stating 400A and above, but that looks to have been removed from the current version. By not recommending a size to test, the standard writers distance themselves should a problem arise. Perhaps you should look into the recommendations of insurance companies. Or you may also look at what some government agencies recommend. The size of the breaker is best determined by a case by case basis. Testing small breakers is only extra work, by no means are they any more challenging to test than larger breakers (with the proper testing equipment). After all, the size of the breaker is an arbitrary number, no matter the size it is still a protective device.

 

[oldfieldguy]

One of my clients in my former life specified that EVERY 3-pole molded case circuit breaker be tested during his three-year maintenance. As a testing contractor, I thought this overkill, but he's the client and hours are hours. We tested them all - literally dozens from a major production unit shutdown, both thermal-magnetic and magnetic-only.

to test the thermal response, we connected the three poles in series and did one shot at 300% from a cold (ambient) breaker, comparing the time with the manufacturer's curve if available, or lacking the manufacturer's curve, the table found in the ANSI/NETA Maintenance Test Specification. Attempts to do single-phase testing of the thermal characteristic require a long cooling period as the heat from one phase 'bleeds' over to the next, pre-heating it and distorting results.

We tested the magnetic characteristic by starting test current at about 70% of nominal rating and increasing it until trip. These were single-phase tests.

Manufacturers will rapidly tell you that a +40 -30% tolerance is the best that can be expected for integral trip units. If the breaker has an electronic trip unit, greater accuracy can be expected.

It was common to find that 10% of the tested breakers failed these tests. These were usually the smaller ratings - <100A nominal.

My present concern, since I am no longer the testing agent, but the owner, is that removing bolted-in molded case breakers for testing introduces a greater exposure for failure upon re-installation due to having to re-install conductors and bolted connections, as well as which breakers should be 'tested' to meet NFPA 70E's requirement that overcurrent protective devices must be 'maintained' in order to validate the findings of an arc-flash study.

old field guy