Current Limiting Fuses to address SCCR Underrated Panels

Jraef

Moderator
Staff member
Can current limiting fuses, upstream of panel, be used to reduce available fault current to the panel? My research seems to indicate that UL does not recommend this. This rather old article from EC&M also suggests it's not recommended. Unless of course the fuse has a series rating with the panel's breakers that achieves the higher SCCR.

https://www.ecmweb.com/content/dont-be-felled-higher-fault-currents
It's been discussed in this forum many times now. The answer is, probably not.

If you are a contractor wanting to do this in the field, then it's definitely no. Using CL fuses is NOT an accepted method of correcting for an improperly rated piece of equipment in the field.

If you are or hire a registered PE willing to stake your/their professional reputation on it, technically that might be acceptable to an AHJ, but you should check first. A PE can do a proper in-depth STUDY of the fuse let-through curves and the withstand ratings of ALL of the down stream power devices, then stamp that study and you can submit that to the AHJ as an option. But if you are hiring the PE, you had better know in advance that it's going to be acceptable, because you will have to pay for it either way.

If you are a UL panel shop, there is a process under UL508A Supplement SB by which you can apply those principles as well, and then label the panel accordingly. But that would apply to custom assembled "Industrial Control Panels", not things like panelboards and load centers.

But if this is about a panelboard or load center, definitely check with the mfr, because in a LOT of cases they have already "series listed" the equipment with up stream fuses.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Can current limiting fuses, upstream of panel, be used to reduce available fault current to the panel? My research seems to indicate that UL does not recommend this. This rather old article from EC&M also suggests it's not recommended. Unless of course the fuse has a series rating with the panel's breakers that achieves the higher SCCR.

https://www.ecmweb.com/content/dont-be-felled-higher-fault-currents

2017 NEC 240.86.A does indeed allow Series Ratings , Selected Under Engineering Supervision in Existing Installations. And yes, one has to, "ensure that the downstream circuit breakers(s) that are part of the series combination shall remain passive during the interruption period of the line side fully rated, current-limiting device."

As I recall, it hasn't changed since 2008.

And yes, I am certain UL wouldn't like it. Gets them right where it hurts - right in the wallet.

A 1999 EC&M article wouldn't worry me too much. I've never seen much evidence they get anything peer reviewed.
So, dust off your stamp, review your E & O insurance. Plan on hiring a test lab for a couple of tests. And have at it.

I'm thinking you likely already know all this. So what exactly is it you are wondering about?

the worm
 

ptonsparky

Senior Member
Funny. I just went to get my hours for CE. Two of the presenters told us that current limiting fuses technically were not to used for this purpose. Then with after all the CYA disclaimers they could think of using, suggested they could possibly help. My take was, after you’ve done the study and find out that your POCO has changed the available fault current levels, put the current limiting fuses in. Then get the wheels moving.
 

drktmplr12

Senior Member
If you are a UL panel shop, there is a process under UL508A Supplement SB by which you can apply those principles as well, and then label the panel accordingly.
Something along the lines of a marking that read "This panel must be fed by xx Amp Class RK1 Current Limiting Fuses" ?
 

iceworm

Curmudgeon still using printed IEEE Color Books
I'm sure all know this, however, just to clarify:
NEC (2008 to 2017) 240.86.A only covers existing installations. New installations such as the article linked in post 4 are not included.

The only times one should consider using 240.86.A is if an existing panel (or equipment) has a feeder transformer change (or service change) and the available SCC has increased past the existing equipment specification. Then one may consider adding a current limiting device.

If one is specifying new equipment, then get suitable ratings. The code does not allow patching design/purchase errors.
 
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