fuse type for disconnect switches to bolster SCCR rating of same

[mshields]

Owing to the extremely low SCCR rating of unfused disconnect switches (something like 7,500A, we routinely “own” fuses (assuming 480V) to eliminate the concern.

The contractor on a recent project came back asking for a type fuse. Even when no other contractor has come back to me with this quesstion, it's nevertheless a fair one.

Given that the disconnect is sized for 125% of FLA (and then next standard size up) and that the OCP upstream could be anything up to 250%, I imagine the answer is that these should be 125% FLA (and next size) but that they should provide short circuit protection only; no overload capability.

1. Do you agree with that and
2. What type fuses should I be specifying?

 

[jim dungar]

If the only purpose of this fuse is to protect the non-fused switch it can be the same size as the maximum rating of the switch. Because it needs to series rate with the non-fused switch it must be of the proper Class and type tested by the switch manufacturer.

 

[petersonra]

protecting the unfused switch for excessive short circuit current has nothing whatsoever to do with FLA.

The manufacturer of the switch will have instructions that tell you what fuse you can use to increase its SCCR, and the maximum rating of that fuse to get the desired SCCR.

You could have a 100 A fused switch protecting a 30 A unfused switch.