Thermal Magnetic Breaker Trip cycle limit

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rlundsrud

Senior Member
Location
chicago, il, USA
So I recently read that some breaker manufacturers (Eaton, Square D) say that after 2 trips at maximum current they should be replaced. I was a bit surprised to hear this, can anyone else comment on this. Did everyone know this and I came late to the party??
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
What’s true is that although a molded case circuit breaker (MCCB) built to NEMA AB-1 standards and listed under UL489 must be capable of (I think) 2000 mechanical operations, it is only REQUIRED to trip twice at full rated fault capacity. In other words it will trip, can be reset, then must trip again, but it is not REQUIRED to reset after the second time. That doesn’t mean it won’t, it just is not required in the testing / listing of a breaker and if it does reset, you’re on your own because it is not technically required to trip again after that. The reality is that MOST breakers are not really operating at their full interrupt capacity, it’s just that they must be tested at that in order for the rating to mean something. But regardless, nobody can give you a number of trips at less than rated capacity, so the only safe bet is to just assume the worst.

This is not new, it’s been this way from the beginning and it’s not specific to any brand, it’s the same for everyone.

POWER breakers are the same, but are DESIGNED to be rebuilt. MCCBs are not, they are throw-away.
 

kwired

Electron manager
Location
NE Nebraska
To what is the OP referring when he says "maximum current?"
I would presume for a 10kAIC rated breaker that means it can interrupt 10kA, once and still is supposed to work, second time no guarantees.

When I was still in college our class got to tour plant where QO breakers are made. Would appreciate some of what I saw even more today, but still was a good tour. One very interesting thing was a test lab, one test they demonstrated for us was to take a three pole breaker, don't know the amp rating, but it had maybe #6 or #8 sized conductors tied to load terminals and bolted together, and presumably connected to a source that delivers applicable rated fault current. Then they closed a big heavy door and energized it. It made a pretty good bang. They opened the door, there was still some smoke lingering around in there and reset the breaker - it did reset. It also had a carbon deposits on it and looked like you probably wouldn't want to use it again, but standards state it should still be usable.
 

sameguy

Senior Member
Location
New York
Occupation
Master Elec./JW retired
The 10k is the explosion level, if hit with that I doubt not only the breaker but the panel would need replacement.
Full load, thermal/magnetic, could change the trip curve. Remember everything is the lowest bidder.
 

kwired

Electron manager
Location
NE Nebraska
The 10k is the explosion level, if hit with that I doubt not only the breaker but the panel would need replacement.
Full load, thermal/magnetic, could change the trip curve. Remember everything is the lowest bidder.
From what I saw ~30 years ago in that Square D lab, yes it certainly didn't look like you would want to use it again. The test standards however did say it must be able to survive that test and for no more than we got to see - the breaker at least did reset.

I was only 19 or 20 years old, if I could only have known what questions I would like to ask them now back then.
 

sameguy

Senior Member
Location
New York
Occupation
Master Elec./JW retired
I was in my mid 20s and went to Furnus for training and they had the same stuff only motor control, very interesting.
 
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