Molded Case Circuit Breakers

[timturner]

Hello,

For a molded case circuit breaker, does the frame size only determine the interrupt rating? I can't find any good information on this, and I just wanted to clarify.

Thanks!

 

[DRWeig]

Tim,

Frame size only determines the physical size of the breaker (and thus limits its application range, lug size, etc...).

You can get different interrupting ratings within a given frame size.

Example: Square-D 250A frame class 520 comes in all standard trip ratings from 100A to 250A, all physically the same size, and each available with standard, intermediate, or high interrupting ratings.

Good on ya,

Goober Dave

 

[timturner]

I was thinking that the most important consideration when selecting frame size was the maximum interrupt rating.

For example:
Allen Bradley 250A H frame - Int. Rating: 60kA.

I'm guessing that you would want to take your inrush current and multiply by a factor to determine your max interrupt rating. In the past I have never considered this, but I think maybe I should.

Does this all sound right?

Thanks,

Tim

 

[DRWeig]

Tim,

If you'll look thru the catalog, you'll see that A-B has a number of different interrupting ratings in the 250A frame, as do most of the other manufacturers. Higher ratings cost lots more. Frame size is simply a physical size (dimensional) statistic.

It sounds like you may not have been around power engineering very long -- interrupting ratings don't have anything to do with inrush current (as in a motor inrush or transformer inrush). You have to compute fault currents to determine what interrupting rating you need.

I'd suggest you re-post this in the Electric Power Engineering forum and get some more feedback. Or find a local engineer with power distribution design experience.

Good on ya,

Goober Dave

 

[LionelHutz]

Well, a certain frame size will be capable of a certain interrupting rating. But, it may also be built with lower interrupt ratings by installing cheaper mechanisms inside.

You calculate the fault current that would occur if the wires that connect to the incoming side of the breaker were shorted together. That current is the interrupting rating.

Also, You have to look at the interrupt rating of the whole package or the breaker and the components after the breaker. For example, you put together a breaker and a full-voltage starter and size the breaker for the system 50kA fault rating. However, if a fault on the output side of the full-voltage starter occurs either the contactor or overload could explode because it can't handle the 50kA fault current.