Fuse Curves

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augie47

augie47

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I have an inspection where the contractor is attempted to reduce a 42K SCA to supply equipment with a 5K SCCR. The supply is 400 amps.
Ignoring wire length, etc for conversatuon sake, is there a 400 amp fuse that will reduce 42 to 5 ?.
I've not found one.
 
augie47 said:
I have an inspection where the contractor is attempted to reduce a 42K SCA to supply equipment with a 5K SCCR. The supply is 400 amps.
Ignoring wire length, etc for conversatuon sake, is there a 400 amp fuse that will reduce 42 to 5 ?.
I've not found one.
Click to expand...
This could be a loaded question. What equipment or device is being feed that requires a maximum of 5ka of available SSC?
This may require a series rating where the devices must be tested and listed by UL as such.
 
Cooper Bussmann Current-Limiting Fuae Let-through Data publication

Cooper Bussmann Current-Limiting Fuae Let-through Data publication

My go-to source for something like this is the Bussmann Let-through charts, and a quick review of that publication shows you could get down to a little over 6kA with a type 400-amp type JJN fuse. But a 200-amp fuse would get you to 5kA.
 
mayanees said:
My go-to source for something like this is the Bussmann Let-through charts, and a quick review of that publication shows you could get down to a little over 6kA with a type 400-amp type JJN fuse. But a 200-amp fuse would get you to 5kA.
Click to expand...

NEC generally requires the fuse to be tested with the downstream device. 240.86

Sometimes you can use the letthrough curves for the engineered option in 240.86, but I don't know many PE's that would put their license on the line as required.
 
templdl said:
This could be a loaded question. What equipment or device is being feed that requires a maximum of 5ka of available SSC?
This may require a series rating where the devices must be tested and listed by UL as such.
Click to expand...


I also think it might depend on what the equipment is. When I first saw "equipment", and "5K", I'm assuming the "equipment" is a rooftop unit, HVAC unit, or something similar. They usually have a 5K limit.

With HVAC equipment, I'm under the impression that a fuse that limits the available let through current to less than 5K is all that is needed.

If the "equipment" is another protective device, switchboard, panel, etc, then I would agree with Ron.
 
augie47 said:
I have an inspection where the contractor is attempted to reduce a 42K SCA to supply equipment with a 5K SCCR. The supply is 400 amps.
Ignoring wire length, etc for conversatuon sake, is there a 400 amp fuse that will reduce 42 to 5 ?.
I've not found one.
Click to expand...

I have a similar situation on my job.

There is one manufacturer that makes HVAC equipment has a higher rating than 5K. All others are 5K.

My confusion is this. And please enlighten me because my understanding is far less than I am comfortable with on this subject:

article 110.9 of the NEC requires overcurrent protective devices to have interrupting ratings sufficient for the available fault current and voltage at the equipment it is intended to
protect. According to Littlefuse.

So you need to know the let through current of a fuse or a breaker, yet the fuses and breakers tell you the maximum interrupt rating and the higher the interrupt rating the less let through there is. They don't tell you the let through current. So, if I have a 65KAIC breaker in a switchboard 100 feet away from the equipment, how does one ( especially the average electrician, maintenance person or inspector even determine what the actual available current will be at a piece of equipment that has say a 5K SCCR? more so, allegedly a 65KAIC breakers let's less current through than a 22KAIC breaker. How much less? Where is this information? They talk about current limiting breakers and fuses, but I don't see where it actually says how much current it limits. Before I started trying to wrap my limited brain around it, I thought that the 65KAIC meant something regarding the current limiting. How?

Lastly, please tell me if I simplistically understand the issue:

My assumption is that a piece of equipment or feeder wire, can sustain a certain amount of current for a certain amount of time before bursting in to fire with threat to others. The breaker or fuse must disconnect before this current before that time is exceeded.

SCCR and AIC deal with instantaneous trip which is in fact not instantaneous, but as close as currently possible with technology.

Say a 10 KAIC breaker is subject to 25KAIC, then it would fuse together in an "instant" situation thereby letting the 25KAIC through so we must depend on a large KAIC breaker up stream interrupt this current before it burns things up. Thereby, coordinated tripping. Not to be confused with selective tripping where a coordination study determines that a downstream breaker is given the time to clear a fault more quickly than a larger upstream breaker.

The entire system being designed to interrupt the maximum let through of the power source (transformer, generator etc.) before it saturates or burns up.

So given all of this, how does the breaker limit the amount of current available at a piece of equipment?


Sorry if this commandeers your question Augie, but I have had this on my list of things to do and I think the answers may be an in depth answer to your more specific question.
 
Strathead said:
My assumption is that a piece of equipment or feeder wire, can sustain a certain amount of current for a certain amount of time before bursting in to fire with threat to others. The breaker or fuse must disconnect before this current before that time is exceeded.
Click to expand...
Yes, this is in 110.10 of the NEC.

Strathead said:
SCCR and AIC deal with instantaneous trip ...
Click to expand...
This is not true in the strictest sense.
SCCR is how much current a device can handle for a certain amount of time. Most Listed equipment needs to 'stay together' for 3 cycles, while some other stuff is rated for 30 cycles.
AIC is the current that a protective device can interrupt.

Strathead said:
Say a 10 KAIC breaker is subject to 25KAIC, ...
Click to expand...
A device with an instantaneous function, like most molded case breakers, would try to open this fault, and could easily fail explosively.

Strathead said:
So given all of this, how does the breaker limit the amount of current available at a piece of equipment?
Click to expand...
When the current through a protective device exceeds a certain level, in a certain amount of time, it begins to 'open'. As soon as it begins to open an arc is developed inside of the device. There is more impedance (e.g. resistance) in an arc than in closed contacts, so less current will flow. The faster the arcing impedance grows, the greater the current is limited.
 
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I had to reply. Jim, that is a very informative response. Nice job. As Steve said, this answer depends on the definition of "equipment". I guess
we sill have to wait on Augie to summon enough energy to respond.:lol:
 
The equipment in an Industrial Control Cabinet.
Originally supplied with a C-E certificate and thus unacceptable to TN regulations.
Contractor & Customer had a NRTL evaluate the equipment and in doing so the supplied a nameplate indicating the SCCR is 5k.
Customer has a 43k Utility supply and added fusible switches in an attempt to reduce the SCA to the 5k SCCR withstand. I have not seen the NRTL report but am told the "weak link" was the bussing in the cabinet.
 
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