Breaking capacity for soft starter


Thread Starter


Dear All,

I am bit confused about the breaking capacity of a breaker. Recently I have seen Siemens soft starter catalogue in which the KA rating of a recommended breaker for a soft starter is 100KA but the soft starter itself is only 32KA. Now my doubt is that if there is a short circuit and the current is less than 100KA the breaker will not trip so since the current is more than 32KA the soft starter will not damage or my understanding about KA rating is wrong?

Other doubt is that in a network usually KA rating of an upstream breaker would be higher (say 50KA) than the downstream breaker (say 32KA). If we keep both 50KA then what would happen?

Please advice.

Thanks in advance.
I think you are confusing breaking capacity with TRIP current. You should rate breakers which can handle the short circuit current of your system, at the breaker outgoing terminals. This is normally self-limited by the supply transformer and length/impedance of the connecting cables.
Dear Jones,

Thanks for the reply. You mean to say that the breaking capacity is only to protect the breaker itself and no relation to the instantaneous tripping of the breaker?

Please advice.


thiagarajan sriram

Breaking capacity is the maximum fault current that can be safely interrupted by the breaker without damaging the breaker contacts.