Install Current Limiting Fuse to Reduce KAIC

ptonsparky said:
Why do Class CC fuses limit the damage of components if they do not limit the current?
Or do they just open quicker?
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They limit the current by opening and clearing quicker.
Many breakers often begin to open as fast as fuses melt, thereby limiting the current, however they do not clear as fast therefore they cannot be called current limiting per the definition.
 
electrofelon said:
That does not seem to be what is said in this document from bussmann. The document seems to be dated 2005, is this out of date?


For example it says,



Them further down it offers a more detailed list of "circuit components"
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They have used almost the same wording since the early 80s and it goes on to say:
 If both the let-through currents (IRMS and Ip) of the current-limiting fuse
and the time it takes to clear the fault are less than the withstand rating
of the electrical component, then that component will be protected from
short circuit damage

I know of no breaker or equipment manufacturer that publishes the Ip rating of their equipment. It is probably somewhere in the UL procedures for SCCR and AIC but I haven't seen it.

If you continue reading this bulletin you will see, in their UL 508 write up, that they say you cannot use Up Over Down.
 
electrofelon said:
It seems the confusion is in the definition of "available fault current", which the NEC does not provide.
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You need to look at the industry standard definitions.
UL does a good job of defining "current limiting"
You probably need to look at NEC 110.10 and IEEE for information on calculating available fault current as there are many commonly used methodologies, although none of them include any current limitation provided by protective devices. Heck, even the Eaton literature you referenced contains one procedure that ignores fuses in the circuit.
 
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ramsy said:
Have you seen any PE specify up-stream line reactors, that are rated within 110.10?
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Yes. I have done this.
I don't like it because there is often a large voltage drop. I prefer transformers because they have taps that can be used to negate this.
 
JuliaTruchsess said:
It does, theoretically at least. But the NEC does not allow it to be used for SCCR purposes without PE analysis. It's that simple.
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It seems he NEC prohibits it for AIC purposes, but not for SCCR purposes. That is the way I read it anyway. I complain about this nearly every time this topic comes up, but I wish the NEC would be clearer that it is not allowed. Perhaps deleting the definition they provide for CL fuse, and I don't see what purpose that serves other than to confuse and mislead people.
 
electrofelon said:
Perhaps deleting the definition they provide for CL fuse, and I don't see what purpose that serves other than to confuse and mislead people.
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Especially because the definition in 240.2 is not the same as the definition in the UL standards. Many of the new generation (past 20 years) of circuit breakers meets the simple definition of reducing the current magnitude. However the UL definition contains a speed requirement as well as a maximum magnitude, so the industry does not consider most breakers as being CL.
 
jim dungar said:
AIC - the amount of short circuit current a protective device can interrupt without self destructing.
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Does a fuse not self destruct when it blows? I thought the reason for a fuse to have a high enough kAIC rating was so that the fault current won't continue to arc through it after the element has melted.
 
jim dungar said:
Not if it is Listed. The internal element melts but the fuse barrel is supposed to remain intact.
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Well, OK, but I would consider a blown fuse to be destroyed since it is no longer functional or reparable.
 
ggunn said:
Does a fuse not self destruct when it blows? I thought the reason for a fuse to have a high enough kAIC rating was so that the fault current won't continue to arc through it after the element has melted.
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I would not say it self destructs. The element opens up but I am pretty sure the case is required to not rupture when the element opens by the product standards.
 
There is a lot of really good discussion here. I do apologize but I am still confused.

I also would like to through this out there. In MN, some electrical inspectors got together and formed a small business to provide code training classes. In their classes they teach that installing a CL fuse is acceptable to lower available short circuit current when a piece of equipment with a low SCCR rating is provided. I have been telling the state this is wrong, mainly because of IEEE 242-1986. But then the arc flash item dawned on me, and I read the updated 242-2001 and the past belief that CL fuses shouldn't be used in such a manner seems to be contradictory. It all seems wrong, but it just can't explain why. I have seen many contractors now doing this and electrical inspectors approving it. I want to help educate the inspectors, but I need to educate myself first which is why I am looking for info on this.

1) A CL fuse cannot be used to reduce short circuit current in a circuit for the purpose to reduce the current for a downstream piece of equipment with a low SCCR rating.....why?

2) A CL fuse can be used to reduce short circuit current downstream in a circuit for arc flash purposes....but not for the above item 1) ...why?

3) Then the IEEE Buff Book 242-2001 basically states that it is acceptable practice to do item 1) if I read it correctly.... so why is item 1) not acceptable?
 
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My simplified understanding is that you have two classes of 'things' that need to be considered for short circuit currents:
1) passive things that carry current such as cables, bus bars, non-automatic switches, and other components that don't change state during the short circuit event.
2) dynamic impedances such as circuit breakers, which actively respond and change state during the short circuit event.

Short circuit events can create huge magnetic/mechanical stresses, and components must survive these stresses. See, for example:

Continuing my simplified understanding:
Current limiting fuses operate so quickly that they limit the total I^2T that components will be subject to. For passive components such as cables this reduces the total impulse that the components must tolerate. Thus for passive components a current limit fuse can be treated as reducing the available fault current.
For dynamic impedances the device is actively changing the circuit characteristics during the time that the fuse is supposed to be operating. This _might_ increase the clearing time of the fuse, or the opening contacts of the dynamic impedance might have a much shorter time constant for damage than passive devices. For these devices, the current limiting fuse cannot be treated as reducing the available fault current without actual testing.

This is my simplified understanding of the situation. I've never had to actually go through the engineering documentation to really understand what is going on with current limiting fuses; just casual reading and reading of discussions here.
 
Mr. Pickle said:
1) A CL fuse cannot be used to reduce short circuit current in a circuit for the purpose to reduce the current for a downstream piece of equipment with a low SCCR rating.....why?

2) A CL fuse can be used to reduce short circuit current downstream in a circuit for arc flash purposes....but not for the above item 1) ...why?
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This is a comparison of apples and oranges. With a little myth/misunderstanding thrown in.

SCCR and AIC, above 5kA, are determined by UL procedures and often involve actual testing using specific fault currents and parameters. These events are typically over within 3 cycles.

Arc Flash Incident Energy is determined by calculation only, typically using IEEE formulas. These events often stretch over periods up to 125 cycles.

But the biggest issue is the fault level. SCCR and AIC are based on full bolted fault levels, while AFIE is calculated at an arcing fault level. This means that during arcing events it is extremely common that fuses never enter their current limiting region and thus it is simply their normal operation that limited the energy.

In literature current limiting fuses are often listed as possible ways to reduce AFIE. But they are being compared with the old style one-time fuses not breakers.
 
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winnie said:
My simplified understanding is that you have two classes of 'things' that need to be considered for short circuit currents:
1) passive things that carry current such as cables, bus bars, non-automatic switches, and other components that don't change state during the short circuit event.
2) dynamic impedances such as circuit breakers, which actively respond and change state during the short circuit event.

Short circuit events can create huge magnetic/mechanical stresses, and components must survive these stresses. See, for example:

Continuing my simplified understanding:
Current limiting fuses operate so quickly that they limit the total I^2T that components will be subject to. For passive components such as cables this reduces the total impulse that the components must tolerate. Thus for passive components a current limit fuse can be treated as reducing the available fault current.
For dynamic impedances the device is actively changing the circuit characteristics during the time that the fuse is supposed to be operating. This _might_ increase the clearing time of the fuse, or the opening contacts of the dynamic impedance might have a much shorter time constant for damage than passive devices. For these devices, the current limiting fuse cannot be treated as reducing the available fault current without actual testing.

This is my simplified understanding of the situation. I've never had to actually go through the engineering documentation to really understand what is going on with current limiting fuses; just casual reading and reading of discussions here.
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I think that basically covers it however the engineers among us seem to be saying you can't use a CL fuse even for a passive device. I just still don't understand it, it seems like that is exactly what the fuse manufactur's literature is saying they do, but then the response is they're not actually saying that or you still can't use it 🙃
 
electrofelon said:
I think that basically covers it however the engineers among us seem to be saying you can't use a CL fuse even for a passive device. I just still don't understand it, it seems like that is exactly what the fuse manufactur's literature is saying they do, but then the response is they're not actually saying that or you still can't use it 🙃
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CYA scenario?
 
electrofelon said:
I think that basically covers it however the engineers among us seem to be saying you can't use a CL fuse even for a passive device. I just still don't understand it, it seems like that is exactly what the fuse manufactur's literature is saying they do, but then the response is they're not actually saying that or you still can't use it 🙃
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I don't know which engineers are saying you can't use a CL fuse to protect a passive device like a conductor.

My point is that a control panel or a piece of equipment containing electronics are not passive devices. Take a look at any CL fuse manufacturer and their discussion of protecting UL508A equipment.
 
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jim dungar said:
I don't know which engineers are saying you can't use a CL fuse to protect a passive device like a conductor.

My point is that a control panel or a piece of equipment contains electronics are not passive devices. Take a look at any CL fuse manufacturer and their discussion of protecting UL508A equipment.
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Ok, yes, maybe we are on the same page then. I agree UL508A not only likely contains non passive devices, but the listing process / rules have specific requirements for SCCR and upstream OCPD's that you can't ignore.

So what about something like a typical automatic transfer switch?

What about something like a package HVAC unit, do essentially all of those have a UL 508 panel inside of them?
 
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