Need help with Fuse Confusion

agarb

Member
Join Date
May 2006
Location
USA
Posts
309
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!
 
I'll concur with you on that we use I^2T fuses after the normal branch circuit fuse.

In fact, a company called HEAT (HEAT EXCHANGE AND TRANSFER, Inc.) does it this way in the control cabinets they supply to us. They make electric heaters / controllers.

However, the use of
I^2T fuses is to protect expensive SCR controllers if a heater element shorts out.It has nothing to do with branch circuit protection. I do not know of any NEC code that talks about supplemental I^2T fusing.

Do not get rid of your
KTK-R fuses. If replacing the SSR is of low cost / consequence then you might forgo the I^2T fuses.

You would also examine the mfg data sheet for the SSR's and see what they specify. If it's not installed per them, then it may not meet UL509 code.

You could post this over at the MikeHolt.com forum to get some very expert feedback on tap rules and branch circuit protection.
 
I believe the fused disconnect switch in your panel would be considered as protection for the feeder within your panel. To be considered branch circuit protection, it would have to be the final overprotection fuse or circuit breaker for the load to be served. That would require ALL wiring after the fused disco to be sized for the ampacity of the disco OCP, and your OCP would have to protect each of the devices. Not likely for a panel with multiple circuits.

Also, from UL508A, Industrial Control Panels:

31.6 SIZING OF BRANCH CIRCUIT PROTECTION FOR HEATER LOADS
31.6.1 Resistance heating element loads shall be provided with branch circuit protection sized:
a) Not less than 125% of the heater load current;
b) Not larger than 60A; and
c) Not larger than the ampacity of the field wiring to the heater load.

Kent
 
Thanks for the responses.

I also found this:

NFPA 79 Electrical Standard for Electrical Machinery

7.2.11 Resistance Heating Branch-Circuit Overcurrent Protection

7.2.11.3 The additional overcurrent protective devices shall include all of the following:
(1) Installed within or on the machinery or provided as a separate assembly
(2) Accessible but need not be readily accessible.
(3) Suitable for branch-circuit protection

The main conductors supplying these overcurrent protective devices shall be considered branch circuit conductors.

That said, I really don't see how semiconductor fuses (supplemenatary) are able to meet the code requirements.
 
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I was hoping for some other responses to learn on this question myself.

I am somewhat surprised at the different responses you received from 3 fuse manufacturers.

It is also interesting that the responses here add more guidance ie UL508, NFPA 79 in addition to NEC. The question now becomes do they all apply and which one can if possible override others.

Glanced thru NEC art 422. Seems appropriate and duplicats some of the stuff quoted here.

cooper bussman has a manual on this based on 2008 NEC. It is at
http://www.cooperbussmann.com/2/SPDTableofContents.html


Dan Bentler
 
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The branch fuses should protect the cabling and the fast ones should be in every SSR if needed, a fast fuse is expensive, and because it are heaters, they can not take more current and do not have peak loads, on them, so i would not use them, only for motors etc.
 
Dont know if it meets your codes over there but Siemens do certain SSR's that can be protected by B type breakers rather than fast acting semiconductor fuses. The breakers do protect the SSR and cables, we use them alot.
 

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