Fuse downstream let-through Current

[kcunningham59]

I will keep this question quick and short. I know there is a lot out there on this topic but I can't get a straight answer. If there is a fused disconnect on the outside of a building and the available fault current at the switch is say 15k and the let-through is say 8k, can the first panel after the switch be rated at 10k or do we assume 15k is still the worst case passing through the switch to the next device. If it is not allowed, an NEC section would be appreciated. I know there are some UL guidelines as well that may affect this answer.

 

[electrofelon]

No, you cannot use let thru current of a CL fuse to reduce AFC of a downstream OCPD. 240.86.

 

[jim dungar]

As has been stated on this forum repeatedly, NEC 240.86 says all series rating with breakers must be tested. This has been true for almost 40 years now.

About the only significant recent NEC changes happened back in 2005 when control panels and other items were added with the inclusion of SCCR as well as AIC into the NEC. Here is a link to a Ferraz document from that period.

http://www.electricalmanuals.net/pdfs/Application%20Handbooks/Ferraz-Shawmut/BR-NEC-001.PDF

Fuse manufacturer literature likes to talk about current limiting with Panels, but their summaries rarely say they only mean control panels/equipment built to UL508A versus distribution/branch circuit paneboards using breakers listed to UL489.

 

[winnie]

IMHO you can look at this another way:

Obviously, the current limiting fuse reduces the peak current that flows during a fault. This is exactly why a series combination works to allow a breaker rated less than the available short circuit current.

However the only way to determine if the fault current reduction is sufficient for safety is via a series rating. The issue is that a breaker opening during a fault presents a 'dynamic impedance', and thus the fuse let-through is not clearly defined. If you have two devices in series, both acting to open and limit the 'let through', it isn't clear which device is 'seeing' the available fault current.

The fuse let through calculation is valid for components which do not have dynamic impedance, such as conductor damage curves or bus bar mechanical bracing.

 

[jim dungar]

Obviously, the current limiting fuse reduces the peak current that flows during a fault. This is exactly why a series combination works to allow a breaker rated less than the available short circuit current.

It is totally possible to have a downstream device operate and clear the fault prior to the upstream fuse beginning to melt much less enter its current limiting region.

Published fuse and breaker curves, be they Timed current or Let-through, are only valid for the parameters used to construct them, such as .01 secs or an increasing fault current, anything else and curves alone are no longer sufficient for determining the resultant OCPD behavior.