SCCR calculation - Published current limiting fuse data versus Table SB4.2

[chash]

When is it (or is it) permissable to use manufacturer's published let through current data for Listed current limiting fuses versus using Table SB4.2 of UL508A? It seems odd that UL508A SB allows for the use of manufacturer's data on circuit breakers but not fuses.

 

[petersonra]

When is it (or is it) permissable to use manufacturer's published let through current data for Listed current limiting fuses versus using Table SB4.2 of UL508A? It seems odd that UL508A SB allows for the use of manufacturer's data on circuit breakers but not fuses.

You really have to follow the rules in the supplement if you want to use UL's method.

In any case, you can only use UL's method on UL listed industrial control panels.

 

[chash]

Thanks. I agree that to meet and be listed to UL508A the SB process would need to be followed. However, NEC states that SCCR can be defined by UL508A or another approved process. I have read in other papers that the AHJ can approve other processes. Since UL 508 SB defines the let through current based on the UL 248-X maximum let through current that any Listed fuse of a given class would pass. This may be overly conservative for a given installation that uses a specific fuse or specific set of fuses. Just wondering if anyone may have had experience with an AHJ in this type of situation. I assume there would be some additional requirement to label the fuseholder with a requirement to use a specific model of fuse.

 

[petersonra]

Thanks. I agree that to meet and be listed to UL508A the SB process would need to be followed. However, NEC states that SCCR can be defined by UL508A or another approved process. I have read in other papers that the AHJ can approve other processes. Since UL 508 SB defines the let through current based on the UL 248-X maximum let through current that any Listed fuse of a given class would pass. This may be overly conservative for a given installation that uses a specific fuse or specific set of fuses. Just wondering if anyone may have had experience with an AHJ in this type of situation. I assume there would be some additional requirement to label the fuseholder with a requirement to use a specific model of fuse.

As far as I know, there is no other method, other than for specific products and specific combinations of products by the same manufacturer.

In any case, the UL procedure is for determining the SCCR of an industrial control panel, and can't be used for anything else.

(4) Short-circuit current rating of the industrial control
panel based on one of the following:
a. Short-circuit current rating of a listed and labeled
assembly
b. Short-circuit current rating established utilizing an
approved method
Informational Note: UL 508A-2001, Standard for Industrial
Control Panels, Supplement SB, is an example of an
approved method.

Note what the paragraph says. Then look at the informational note that says supplement SB is an example of something that can be used to evaluate an industrial control panel.

I would suggest that in a highly legalistic way, only a UL listed industrial control panel can use supplement SB. One might be able to argue it could also be used for unlisted industrial control panels as well. The first paragraph of UL508a very clearly says it only applies to industrial control panels.

Since both the NEC and UL508a both state quite clearly that supplement SB can only be used to evaluate the SCCR of an industrial control panel, I do not see any way to extend that to anything else.

 

[jim dungar]

Other than the procedure in UL508A, for industrial control panels, there is no generally accepted field methodology for determing how fuse and molded-case circuit breakers will interact during fault conditions which are not represented on standard Time Current Curves.

I suggest you look into the condition called "dynamic impedance".