UL508A Current Limiting Fuse in Branch Circuits

[Flattcatt]

I'm looking for an explanation on the reason why a current limiting fuse must be in the feeder to raise the overall SCCR of a panel when using UL508A's supplement SB to calculate SCCR. For example, lets say a panel has no feeder fuse. There are just terminal blocks with jumpers that feed individual branch circuit loads and the panel is in a system where the available short circuit current is 100kA. The branch circuits use class J current limiting fuses (with 200kA interrupt rating) but the rest of the components in the branch circuits have a 10kA rating. The panel has a 10kA SCCR because of those branch circuit components and because UL508A says the current limiting fuse must be in the feeder. Now add a Class J fuse upstream of those terminal blocks (in the same panel) feeding the branch circuits and now you have a 200kA SCCR for the panel. To really focus the point, now consider a situation with a panel with a single branch circuit. You'd essentially have 2 fuses of the same size in series feeding the branch circuit components to get you a 200kA SCCR but with only one fuse you'd have 10kA. Why isn't the peak let through current of the current limiting OCPD not considered for raising the SCCR of each branch?

 

[jim dungar]

Why isn't the peak let through current of the current limiting OCPD not considered for raising the SCCR of each branch?

How does an internal fuse protect the line side components and connections in the control panel?

 

[Flattcatt]

I see your point the current limiting fuse in the feeder is needed to protect devices in the feeder. I guess my question is only an issue in my second scenario where there is only 1 branch circuit in the panel. If the supply is landed directly on the fuse, there is no feeder component to protect but the branch circuit would still be 10kA until an additional fuse is added ahead of the branch circuit OCPD, even if it is an identical fuse. This would be an uncommon situation but not impossible right?

 

[petersonra]

I see your point the current limiting fuse in the feeder is needed to protect devices in the feeder. I guess my question is only an issue in my second scenario where there is only 1 branch circuit in the panel. If the supply is landed directly on the fuse, there is no feeder component to protect but the branch circuit would still be 10kA until an additional fuse is added ahead of the branch circuit OCPD, even if it is an identical fuse. This would be an uncommon situation but not impossible right?

I have been known to create multiple feeders inside a control panel by putting fuses in series so one of the feeders can be used to feed a branch that needs it to meet the SCCR.

It's a goofy rule. I don't know why putting it in a feeder circuits is magic and when it is in a branch it is not magic.

 

[Flattcatt]

That's my feeling too. Thank you for your thoughts on this.