Circuit Breaker with Multiple Interrupt Ratings

[trchambe]

A low voltage molded case circuit breaker is often given several ratings of interrupt current. For example:

14kA at 600V
18kA at 480V

Let's say I'm using a breaker with a 600V frame on a 480V system. Is my interrupt rating 14kA or 18kA?

I was under the impression that the interrupt rating was related to both the energy that would melt/'blow up' the circuit breaker, and the magnetic forces that may not be overcome by the contacts. In either case, I would think that would be an issue related more to current and independant of voltage, as magnetic force is proportional to inductance and so current, while energy is proportional to the square of the current.

Is there something I'm not considering?

 

[alehman]

That circuit breaker would be rated 18kA when used on 480V circuits. I think the arc interruption capability is the main factor. The conducting parts also have to be braced to withstand the highest ampere rating for which it's labeled.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[trchambe]

That clears it up. I was discussing this problem with a colleague, and neither of us were certain. Thanks for the help

 

[jraef]

You said it yourself actually...

"I was under the impression that the interrupt rating was related to both the energy that would melt/'blow up' the circuit breaker...."

Current is not "energy", it is a component of energy. Voltage is another component of energy, as is time. Assuming the time is constant (mechanical clearing time), then the energy in the circuit is affected by both the current AND the voltage. Higher voltage = more energy, so lower current will do the same damage, i.e. 14kA vs 18kA.


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[Zogzog]

480*18,000=8,640,000VA
600*14,000=8,400,000VA

Close enough, I am sure they round off the AIC

 

[alehman]

Zog,
Be careful with such calculations on low-voltage breakers. Unless it's been tested, listed and labeled appropriately, you can't use it at the higher interrupting rating.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.