Hopefully we have some NEC experts here because I am not sure about this particular application.
In the control panels we build, we generally we have a fused main disconnect, several motors starters, and several contactors controlling multiple zones of resistance heaters. Each motor starter and heater zone gets its own set of dedicated fuses that are rated for branch circuit protection. This is the way we have always done it.
Fast forward to this project. The customer has specified solid state relays for the resistance heaters. The solid state relays require fast-acting semiconductor fuses to keep the I^2T below a specified value. Since the semiconductor fuses are not rated for branch circuit protection, I designed in two sets of fuses: Bussman KTK-R (branch-circuit rated) and FWC (semiconductor, supplementary) in front of each solid state relay. Now the project manger is questioning the dual sets of fuses and wants to just use the semi-conductor fuses and not the branch circuit fuses. He contends that the main disconnect provids the required branch circuit protection in the event of a bolted fault.
I've done some reading in NFPA 70, but I am still not sure if I can get rid of any fuses. How do I know when branch circuit protection is required? I called Bussman, Shawmut, and Littlefuse. I got different answers from all three. Bussman said semiconductor fuses by themselves were suitable. One person from Shawmut said it depends on tap rules found in NEC 240. Another person from Shawmut said that since the switching device for the heaters is physically located inside the control cabinet, supplementary protection by itself is sufficient. Littlefuse said to read the NEC.
I'm thoroughly confused. Any insight you can offer is most appreciated!
In the control panels we build, we generally we have a fused main disconnect, several motors starters, and several contactors controlling multiple zones of resistance heaters. Each motor starter and heater zone gets its own set of dedicated fuses that are rated for branch circuit protection. This is the way we have always done it.
Fast forward to this project. The customer has specified solid state relays for the resistance heaters. The solid state relays require fast-acting semiconductor fuses to keep the I^2T below a specified value. Since the semiconductor fuses are not rated for branch circuit protection, I designed in two sets of fuses: Bussman KTK-R (branch-circuit rated) and FWC (semiconductor, supplementary) in front of each solid state relay. Now the project manger is questioning the dual sets of fuses and wants to just use the semi-conductor fuses and not the branch circuit fuses. He contends that the main disconnect provids the required branch circuit protection in the event of a bolted fault.
I've done some reading in NFPA 70, but I am still not sure if I can get rid of any fuses. How do I know when branch circuit protection is required? I called Bussman, Shawmut, and Littlefuse. I got different answers from all three. Bussman said semiconductor fuses by themselves were suitable. One person from Shawmut said it depends on tap rules found in NEC 240. Another person from Shawmut said that since the switching device for the heaters is physically located inside the control cabinet, supplementary protection by itself is sufficient. Littlefuse said to read the NEC.
I'm thoroughly confused. Any insight you can offer is most appreciated!