Fused Current Limitng Power Circuit Breaekrs
Fused Current Limitng Power Circuit Breaekrs
(OP)
For power circuit breakers that have internal current limiters, are these current limiters intended to extend the interrupting rating of the breaker to higher interrupting values or are these current limiters intended to limit let-through current to downstream devices short circuit rating or Arc Flash purposes?
In terms of interrupting rating I've seen newer power circuit breakers that have interrupting ratings up to 100kA without any limiters. With limiters these ratings appear to be extended as high as 200kA. So are these limiters only applicable in cases where the available fault current at the breaker exceeds the maximum interrupting rating of the breaker without limiters?
If intended for current limiting or reducing peak let-through at downstream devices then I know this is a complicated subject matter with these ratings often being mis-applied.
In terms of interrupting rating I've seen newer power circuit breakers that have interrupting ratings up to 100kA without any limiters. With limiters these ratings appear to be extended as high as 200kA. So are these limiters only applicable in cases where the available fault current at the breaker exceeds the maximum interrupting rating of the breaker without limiters?
If intended for current limiting or reducing peak let-through at downstream devices then I know this is a complicated subject matter with these ratings often being mis-applied.






RE: Fused Current Limitng Power Circuit Breaekrs
RE: Fused Current Limitng Power Circuit Breaekrs
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Fused Current Limitng Power Circuit Breaekrs
I'd definitively be interested in the long story with historical context.
RE: Fused Current Limitng Power Circuit Breaekrs
RE: Fused Current Limitng Power Circuit Breaekrs
I'm guessing perhaps they are used to limit available fault current downstream? From what I understand you simply cannot "blindly" apply current limiting breakers to achieve reduced fault levels downstream for device evaluation unless they are "tested" combinations. The NEC allows engineering judgment however due to dynamic impedances and other factors it becomes very difficult to apply these for fault limiting applications without tested and listed combinations?
RE: Fused Current Limitng Power Circuit Breaekrs
RE: Fused Current Limitng Power Circuit Breaekrs
I would think that for the parallel transformer application you mentioned it would be strictly to increase the interrupting rating of the breakers as a result of the increased fault current from both breakers operating in parallel.
RE: Fused Current Limitng Power Circuit Breaekrs
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Fused Current Limitng Power Circuit Breaekrs
Due to the way UL tests, a 35kA rated contactor is not capable of surviving 35kA of fault current going through it. So, you can't just say that the breaker and limiters only allows 35kA of fault current to pass so it co-ordinates with this contactor.
RE: Fused Current Limitng Power Circuit Breaekrs
I know Westinghouse/Cutter Hammer has power circuit breakers with interrupting ratings up to 100kA without limiters but haven't looked much at what other manufacturers offer.
RE: Fused Current Limitng Power Circuit Breaekrs
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Fused Current Limitng Power Circuit Breaekrs
I always thought that the KA rating was a "this component will not become a bomb with that much current running thru it". I agree with jref on that statement.
current limit rating
The fault current limit rating was a rating to where you could use this info, so that you could judge what calorie rated arc flash equipment you needed.
Beyond that i am still a bit unclear on other factual statements about both ratings.
How else would you use these two ratings?
But I thought that the use of combination of components fell under the UL umbrella rating combination of components?
I am still confused based on these two questions.
RE: Fused Current Limitng Power Circuit Breaekrs
No, not always.
If a breaker is rated for 50kA then you can apply 50kA to it's incoming terminals.
A contactor rated for 50kA was not tested with 50kA flowing through it. It was tested in a circuit with a thermal magnetic breaker, some connecting wire and some wire shorting the output. The rated fault current (50kA) is the current that flowed when the cables connected to the incoming side of the breaker were shorted. By the time you add the breaker and interconnecting wire and the contactor and the wire used to short the 3 phases together (maybe 18" or 24"??), the fault current that actually flows through the contactor will will be less than 50kA.
If you want to do higher current co-ordination, then you need the actual current that flowed during the above tests. Say it was 35kA. You can then co-ordinate with a 100kA breaker without testing if you can prove the breaker current limits the 100kA input to <35kA on the output.