SCCR for existing installation

[wbdvt]

All the CL for a fuse is based on the fault current that it is seeing. So far the conversation seems to be centered on a bolted fault current and determining the let thru based on that. But what if it is an arcing fault current that is less than the bolted fault current but not in the current limiting range of the fuse and results in a fault current greater than the SCCR of the equipment?

 

[jim dungar]

But adding a definition for SCC with methods described would certainly clarify, but no one is looking for clarification on using a transformer or reactor.

Impedance is specifically called out by the NEC in 110.10.

Protective devices are also mentioned in 110.10, however they are generally not used the way you are suggesting.
One problem with fuses is that they are easily interchangeable and are often replaced with the lowest cost option rather than the specific part number required. UL series rating testing uses generic 'umbrella' fuses rather than the 'off the shelf' products, just because of this.

Ask the fuse manufacturers for references like white papers or IEEE articles showing the application of fuse let through curves for you application. I cannot remember ever seeing anything other than a single glossy page in fuse application guides.

 

[drktmplr12]

Minor clarification for anyone reading this thread because I see the two terms being used interchangeably in some cases.

SCCR - Short Circuit Current Rating - the capability of electrical equipment to withstand a fault of a certain magnitude. This is determined by the manufacturer of said equipment according to rules set forth in the respective UL category.
AFC - Available Fault Current - the result of analyzing the power system and determining the maximum fault current possible at a given point

SCCR of equipment must exceed AFC at that equipment.

StackPath

This article illustrates the problem with up-over-down.

 

[drktmplr12]

Another thought..current-limiting fuses are typically used to improve the SCCR equipment (within the confines of UL listed assembly) or change incident energy levels for arc flash considerations. Using peak let thru curves to lower available fault current is a misapplication of current-limiting fuses, in my view.

 

[electrofelon]

All the CL for a fuse is based on the fault current that it is seeing. So far the conversation seems to be centered on a bolted fault current and determining the let thru based on that. But what if it is an arcing fault current that is less than the bolted fault current but not in the current limiting range of the fuse and results in a fault current greater than the SCCR of the equipment?

Valid concern. But couldn't it still work if the SCCR of the widget and the CL range of the fuse were properly coordinated?

Minor clarification for anyone reading this thread because I see the two terms being used interchangeably in some cases.

SCCR - Short Circuit Current Rating - the capability of electrical equipment to withstand a fault of a certain magnitude. This is determined by the manufacturer of said equipment according to rules set forth in the respective UL category.

AFC - Available Fault Current - the result of analyzing the power system and determining the maximum fault current possible at a given point

I dont recall those terms being mixed up here. I have been using SCC as equivalent to AFC.

SCCR of equipment must exceed AFC at that equipment.

StackPath

This article illustrates the problem with up-over-down.

I read that article. Most of it is about OCPD's in series (which everyone keeps coming back to, and I know that does not work and why). The article does say there are valid applications for passive circuit breakers and other passive devices, for example it says, " Busway may be used on circuits having available short-circuit currents greater than the three-cycle rating of the busway rating when properly coordinated with current-limiting devices."

 

[jim dungar]

Valid concern. But couldn't it still work if the SCCR of the widget and the CL range of the fuse were properly coordinated?



I dont recall those terms being mixed up here. I have been using SCC as equivalent to AFC.



... for example it says, " Busway may be used on circuits having available short-circuit currents greater than the three-cycle rating of the busway rating when properly coordinated with current-limiting devices."

Part of the problem is in finding busway which can be applied in this manner. Most plug-in busway manufacturers show the SCCR from their UL labeling.

 

[petersonra]

Another part of the problem is that what you're suggesting is that basically you would have to have to available fault current ratings for your electrical Network. One that would only apply to overcurrent protection devices, and one that would apply to passive devices. That would add confusion to something that's already pretty convoluted.

A lot of passive devices already have short circuit current ratings that are substantially increase when current limiting fuses are put in series with them.

Part of the answer to your desire to use lesser rated passive devices is to get manufacturers to have those devices be tested and listed for a certain short circuit current rating if protected by a current limiting fuse. That's already done with a lot of things. They just can't be used for the most part except in specific cases or as part of listed devices like industrial control panels.