ron
Senior Member
- Location
- New York, 40.7514,-73.9925
When you are calculating available fault currents at different parts of the distribution system, do YOU take this value into account when choosing a non-fused disconnect (maybe used for Article 430 compliance) or transfer switches. Most non-fused disconnects have a withstand rating of 10kA. BTW, a withstand rating is a component's ability to withstand the fault current if it can't automatically open itself (somewhat similar to the AIC rating for circuit breaker/fuses, panels, swgr, etc).
Very often I design into the system, oversized fused disconnect switches (I don't want the fuse to open on an overload) as a local motor disconnect, even though I don't need the overcurrent protection of the fuse. The fused switch gives a much higher rating as compared to the withstand rating of the non-fused switch itself and is properly rated when I have fault current in excess of 10kA.
I always get the question, do you REALLY want 60A fuses in the local 60A disconnect that serves the 15HP 480 motor? And my answer is that the protection for the circuit and motor is further upstream in the motor starter, the fused disconnect at the motor is just a local disconnect in which we have higher than 10kA calculated fault current.
Most folks ignore the rating on disco's and transfer switches, and I wanted to pass the word.
Very often I design into the system, oversized fused disconnect switches (I don't want the fuse to open on an overload) as a local motor disconnect, even though I don't need the overcurrent protection of the fuse. The fused switch gives a much higher rating as compared to the withstand rating of the non-fused switch itself and is properly rated when I have fault current in excess of 10kA.
I always get the question, do you REALLY want 60A fuses in the local 60A disconnect that serves the 15HP 480 motor? And my answer is that the protection for the circuit and motor is further upstream in the motor starter, the fused disconnect at the motor is just a local disconnect in which we have higher than 10kA calculated fault current.
Most folks ignore the rating on disco's and transfer switches, and I wanted to pass the word.
