Fault currents, KAIC, and the series of fuses and breakers

[Carultch]

When given a fault current, sourced on the line side of a set of fuses, with downstream circuit breakers, can you take any credit for the fuses in terms of reducing the fault current to the downstream circuit breakers?

I know about the "up, over, down" method, which seems intuitive, but I also see a lot of criticism of its validity. Is there any other calculation you can do with the time-current curves and specifications of fuses and breakers to see if there is a reduction in fault current due to using the fuses?

Or is it the case that if it is not a listed series combination for the full fuse KAIC rating, then it is not an approved series combination for ANY fault current larger than the breaker's KAIC capacity?

Example:
Given 27 kA of available fault on the line side of RK5 fuses, what is the minimum KAIC of a breaker would you need on the load side? Neglect distance of conductor runs, and assume no load side fault sources.

 

[augie47]

Based on 110.22, you would need engineering documentation or a equipment manufacturer series rating documentation. With neither of those your 27k would officially be 27k at the panel.

 

[jim dungar]

When given a fault current, sourced on the line side of a set of fuses, with downstream circuit breakers, can you take any credit for the fuses in terms of reducing the fault current to the downstream circuit breakers?

For almost 35 years, every major fuse manufacturer has said no. Every breaker manufacturer has said no. UL has said no. The NEC has said no.

Yet licensed electricians keep asking.

The answer is: The specific combination of fuse and breaker must have been tested.

 

[templdl]

Based on 110.22, you would need engineering documentation or a equipment manufacturer series rating documentation. With neither of those your 27k would officially be 27k at the panel.

I had provided breaker curves and let through currents many times in the past to OEMs such that they could get a good idea as to what to expect the results would be when they did their series testing for their UL listings.
As you stated those curves by themselves are not to be misunderstood to be in any varification of a series ratings as well as enclosed control kaic ratings.

 

[jeremy.zinkofsky]

When given a fault current, sourced on the line side of a set of fuses, with downstream circuit breakers, can you take any credit for the fuses in terms of reducing the fault current to the downstream circuit breakers?

I know about the "up, over, down" method, which seems intuitive, but I also see a lot of criticism of its validity. Is there any other calculation you can do with the time-current curves and specifications of fuses and breakers to see if there is a reduction in fault current due to using the fuses?

Or is it the case that if it is not a listed series combination for the full fuse KAIC rating, then it is not an approved series combination for ANY fault current larger than the breaker's KAIC capacity?

Example:
Given 27 kA of available fault on the line side of RK5 fuses, what is the minimum KAIC of a breaker would you need on the load side? Neglect distance of conductor runs, and assume no load side fault sources.

Fuses act relatively slowly in picking up any instantaneous fault current, they are better at sensing lower faults at a longer time. So the answer to your question is that it depends on the fault. If it is big and fast, then the fuses probably won't see it before the breakers downstream. If it is smaller and last longer then the fuses should pick it up. You can always install fuses with shorter TCC curves but then you risk miscoordination with downstream protection and will cause nuissance tripping. Basically, all you can hope to do is install fuses or breakers that protect cables and equipment from the availbale short circuit current.

 

[Carultch]

Fuses act relatively slowly in picking up any instantaneous fault current, they are better at sensing lower faults at a longer time. So the answer to your question is that it depends on the fault. If it is big and fast, then the fuses probably won't see it before the breakers downstream. If it is smaller and last longer then the fuses should pick it up. You can always install fuses with shorter TCC curves but then you risk miscoordination with downstream protection and will cause nuissance tripping. Basically, all you can hope to do is install fuses or breakers that protect cables and equipment from the availbale short circuit current.

So is there any calculation you can do with manufacturer's data for the fuses and breakers, knowing that the fault current is relatively low compared to the fuse's total value, to show that the fuse will fail benignly before the breaker fails catastrophically?

 

[GoldDigger]

So is there any calculation you can do with manufacturer's data for the fuses and breakers, knowing that the fault current is relatively low compared to the fuse's total value, to show that the fuse will fail benignly before the breaker fails catastrophically?

Sure, but those calculations will not satisfy the NEC.

 

[jim dungar]

Fuses act relatively slowly in picking up any instantaneous fault current,....

What type of fuses are you thinking of? The industry standard RK1 and RK5 fuses clear almost always clear instantly for fault currents in their 'current limiting range'.

 

[Carultch]

What type of fuses are you thinking of? The industry standard RK1 and RK5 fuses clear almost always clear instantly for fault currents in their 'current limiting range'.

I'm looking at Bussmann FRN-200R fuses, which are class RK5 with a panelboard downstream that has Square D - QOB breakers. I'm trying to show whether the situation is correct or incorrect, even though the listed series combination indicates class J fuses as the "only option". And if it is incorrect, I'd rather be able to show more than that it is incorrect by omission of information.

 

[jim dungar]

So is there any calculation you can do with manufacturer's data for the fuses and breakers, knowing that the fault current is relatively low compared to the fuse's total value, to show that the fuse will fail benignly before the breaker fails catastrophically?

The problem is that molded case circuit breakers begin to open about the same time as, or even before, the fuse begins to melt. as the breaker contacts separate, the impedance from the arc could affect current through the fuse thereby changing its melting time.