Install Current Limiting Fuse to Reduce KAIC

[wwhitney]

Back to the actual topic of the thread, say I get an electric kiln that contains resistance heating elements, some contactors, and a digital temperature controller, and it is marked SCCR 5 kA. Are those components all "passive"? And if so, does that mean I can install a CL fuse or CL breaker to satisfy 110.10 if my AFC at the equipment would otherwise be greater than 5 kA?

Cheers, Wayne

 

[petersonra]

Back to the actual topic of the thread, say I get an electric kiln that contains resistance heating elements, some contactors, and a digital temperature controller, and it is marked SCCR 5 kA. Are those components all "passive"? And if so, does that mean I can install a CL fuse or CL breaker to satisfy 110.10 if my AFC at the equipment would otherwise be greater than 5 kA?

Cheers, Wayne

The ONLY way you can take advantage of a CL fuse to increase the SCCR of a component is if UL blessed the combination. This blessing will be noted in the instructions or other documentation for the component.

The instructions will tell you what class of CL fuses you can use for a specific part number and the max rating. For instance it may say 30 Amp class CC or 60 Amp class J.

It might also have an entry for a CB. Or it might not. Because it has to be tested with the specific CB, only the contactor manufacturers CBs would be listed if any. They are not going to pay to test their contactors with someone else's CB.

The other problem you may have is that if the kiln comes with a UL listed control panel, nothing you do can change what the SCCR is UNLESS the labeling of the control panel says so. For instance, it might say 5kA but 65 kA with max 30 Amp class J fuses. This is not common.

 

[electrofelon]

Back to the actual topic of the thread, say I get an electric kiln that contains resistance heating elements, some contactors, and a digital temperature controller, and it is marked SCCR 5 kA. Are those components all "passive"? And if so, does that mean I can install a CL fuse or CL breaker to satisfy 110.10 if my AFC at the equipment would otherwise be greater than 5 kA?

Cheers, Wayne

While I am clearly not an expert and am struggling with understanding this issue, my take so far:

1. There seems to be vague definitions and standards as to what a CL fuse does to available fault current. Although a CL fuse "reduces the current flowing in a faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor", it seems that a CL fuse does not reduce "available fault current". Those two phrases sound the same to me but apparently they are not?

2. Some equipment that is listed and tested to a product standard will have specific instructions and requirements for SCCR ratings with different ocpds. I suppose it is reasonable that those override any attempts at engineering your own combination. However in your example of a kiln, I do not think there is any testing or product standard that applies.

 

[electrofelon]

The ONLY way you can take advantage of a CL fuse to increase the SCCR of a component is if UL blessed the combination. This blessing will be noted in the instructions or other documentation for the component.

Code reference?

The other problem you may have is that if the kiln comes with a UL listed control panel, nothing you do can change what the SCCR is UNLESS the labeling of the control panel says so. For instance, it might say 5kA or 65 kA with max 30 Amp class J fuses.

How much equipment has a UL listed control panel? Do most HVAC units and chillers have listed control panels?

 

[wwhitney]

The ONLY way you can take advantage of a CL fuse to increase the SCCR of a component is if UL blessed the combination.

I guess what I was driving at with my question was whether there is a category of equipment that is obviously passive and for which you can use a CL fuse to increase the SCCR without any UL testing or information from the manufacturer? Would such entirely passive equipment come have an SCCR in the first place?

Cheers, Wayne

 

[jim dungar]

1. There seems to be vague definitions and standards as to what a CL fuse does to available fault current. Although a CL fuse "reduces the current flowing in a faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor", it seems that a CL fuse does not reduce "available fault current". Those two phrases sound the same to me but apparently they are not?

You seem to be reading the NEC definition as applying equally to all items that 'substantially reduced current'. However, not all current limiting fuses are created equal.

Most instructions that list a fuse as protection also include the specific class of fuse required. You cannot use a Class R fuse if the rating requires a Class J. The performance of the fuse is critical to its ability to protect equipment.

 

[ramsy]

You cannot use a Class R fuse if the rating requires a Class J.

And, we all know thats exactly what maintenance monkeys do.

Selective coordination becomes SNAFU the first time it gets replaced.

 

[electrofelon]

Most instructions that list a fuse as protection also include the specific class of fuse required..

Okay but what about things that do not have such instructions and that is not part of any listing or product standard? I like Wayne's approach, let me go even simpler let's just say I install a contactor in a box and it has a 10K sccr.

 

[JuliaTruchsess]

a CL fuse does not reduce "available fault current"

It most definitely does, as fuse manufacturers will be happy to tell you. But you cannot use it to satisfy the SCCR requirements of the NEC unless the specific combination has been tested, as others have stated.

 

[jim dungar]

Okay but what about things that do not have such instructions and that is not part of any listing or product standard? I like Wayne's approach, let me go even simpler let's just say I install a contactor in a box and it has a 10K sccr.

Back to, is it a Listed contactor or is it a Recognized component?

If it is a recognized component, you can use the let-through curves to chose a fuse which you then submit for testing and Listing. Or you can follow the steps in UL508A, but it would be best if you were authorized to do so. i think 508A pretty much maxes out around 30A Class CC.

Have you tried asking a fuse manufacturer this question? Pick a contactor and a fault current, then ask what fuse they would recommend?

 

[electrofelon]

It most definitely does, as fuse manufacturers will be happy to tell you. But you cannot use it to satisfy the SCCR requirements of the NEC unless the specific combination has been tested, as others have stated.

Code reference?

 

[electrofelon]

Back to, is it a Listed contactor or is it a Recognized component?

If it is a recognized component, you can use the let-through curves to chose a fuse which you then submit for testing and Listing. Or you can follow the steps in UL508A, but it would be best if you were authorized to do so. i think 508A pretty much maxes out around 30A Class CC.

I do not believe there is any NEC requirement for contactors to be listed, although some certainly will be. So if it is not listed, I it seems like it should be fine. If it is listed, does that result in documentation that says ONLY these provided fuse and sccr combinations are allowed? (I admit I have never dug into contactor literature documents).

Have you tried asking a fuse manufacturer this question? Pick a contactor and a fault current, then ask what fuse they would recommend?

I have not, perhaps that would be a good exercise.

 

[petersonra]

Code reference?

The SCCR of a component is part of the product standard, not the code.

How much equipment has a UL listed control panel? Do most HVAC units and chillers have listed control panels?

Virtually all HVAC and chiller equipment have UL listed control panels. Most are listed at 5 or 10kA though.