Hello all,

I completed a study for a small generation plant, and their staff disagrees on determining the fault contribution on a generator breaker.

I am being told when determining a breaker's interrupting duty it only needs to be rated for the larger kA of between the system contribution or the generator contribution.

(Using SKM) The bus at the breaker when faulted is 37.5kA, in which 18.5kA comes from the utility and 19kA is from the generator. The breaker is rated for 30kA, so SKM notes as a failed device. This plant engineer claims I shouldn't use the combined 37.5kA fault value, but instead the 19kA generator contribution fault value only when determining the gen breaker is failed or not.

Is he right here? Should I disregard the total fault contribution to the breaker? Why does SKM use total contribution instead for device evaluation?

Under what conditions would the breaker see the combined fault current?
Is this a tie breaker or one feeding a transfer switch?

The generator fault current will flow through the breaker during a utility side fault,
The utility fault current will flow through the breaker during a generator side fault.
What is the current rating of the breaker when fault is internal to the breaker and it is not able to interrupt?

I imagine the breaker sees both the generator and utility contributions in the case of an internal fault within the breaker.

The breaker is the only device between the generator and the secondary low side of a 3 winding transformer.

The breaker symmetrical rms short circuit rating is the 30kA. That worst case fault on the breaker would be 38kA total, and the breaker frame wouldn't be able to withstand it, it could fail to open in that case. That is what I am thinking about the SKM software's fault value given.

If I understand this properly you have the utility on one side of this breaker and the generator on the other. If this is the case, the interrupting contacts within the breaker will never be exposed to the summation current. With a closed breaker and an internal ground fault you will have summation current flowing into the ground, but the contacts will still only see the contribution from from one source or the other depending on the fault location within the breaker. I'm not an SKM user, so I cannot say why it reports the way it does. Could it be that the asymmetrical fault current is too high?