Fault Currant Ratings

[chetm]

If you have a old Mcc with a 22,000 fault currant rating and out of that Mcc
you have a starter with a breaker feeding a motor. Should the rating of the breaker be more or less than the 22,000 rating of the MCC?

Thanks for any help

 

[LarryFine]

If you have a old Mcc with a 22,000 fault currant rating and out of that Mcc
you have a starter with a breaker feeding a motor. Should the rating of the breaker be more or less than the 22,000 rating of the MCC?

It should be at least as high as the available current at the breaker's lugs.

 

[davidr43229]

Because you said "Old", What I would do is look at the transformer KVA rating and figure out what Fault Current you have available. Simply by putting in a rated breaker at the 22KA rating does not insure a safe application.
You asked!

 

[Mike01]

fault

fault

without looking at the complete system it is hard to say exactly, chances are the available fault current is less than 22,000 at the bus the easiest way to verify is (like stated above) using the xfrm ahead of the bus find the secondary fla and the %Z to do a quick fault study, this will tell you if you need to need to do a more throurgal study. Another thing is don?t forget if the motor feeds the fault. If your rating is right at 22,000 you might need to look at the motor too.

 

[don_resqcapt19]

There should be no need to install a breaker with a rating above 22kA in an MCC with a 22kA rating. If the available fault current exceeds 22kA the MCC needs to be replaced.
Don