NFPA 79 Short-circuit current rating 5? 1

[ColtonCameron]

I am just about ready to power up, program, and test a machine my company built. It has a single control panel with a power disconnect. My power requirements are 208 VAC / 3 ph / 60 Hz / 30 amps.

My customer will provide the incoming power circuit, which will be landed at the inbound side of my disconnect. I have the outbound side of the disconnect fused for 30 amps. Downstream of these 30 amp fuses, I have additional branch fuses with lower ratings that I use to protect the various electrical components (drives, 24 VDC power supply, PLC, etc).

My customer has informed

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me they have specific requirements based on NFPA 79 for the information I have to include on the equipment nameplate. Two of these requirements are causing me some confusion:

(1) Maximum ampere rating of the short-circuit and ground-fault protective device, where provided.

Other than my 30 amp main fuses and the lower rated branch fuses, I do not have an additional ground-fault protective device. Would this mean that my short circuit ampere rating is 30 amps (the rating of my main fuses)?

(2) Short-circuit current rating of the industrial control panel

Would this be something I would find in the specifications for my disconnect? My disconnect specs list a few things that seem like they might be relevant ...

[Icm] rated short-circuit making capacity = 2.1 kA 400 V at Ipeak
[Icw] rated short-time withstand Rms current = 756 A 400 V 1 s
Rated conditional short-circuit current = 10 kA 400 V 63 A aM; 10 kA 400 V 63 A gG
Breaking capacity = 500 A 400 V AC-21A; 500 A 400 V AC-22A; 500 A 400 V AC-23A

... would one of the above values be what I should use as the short-circuit current rating of my control panel? Or should I be looking elsewhere?

Thanks in advance,

 

[wbd]

I believe that for (1) it would be the amperes of the main fuses and there is no ground fault so that would be N/A.

(2) is a bit more tricky as the short circuit rating of the panel would be:
1. The rating of the lowest rating in the panel. So for example if a drive has a rating of 10kA and that is the lowest of all components, then the panel is rated at 10kA. OR
2. A rating obtained by testing the panel at various currents to determine the rating. OR
3. 5kA rating which is courtesy rating allowed when no rating has been determined.

I am sure someone else will chime in with more details or experience than me on this subject but the above is how I believe it works.

 

[LionelHutz]

This is something that can't be answered here. This is design work, not an engineering tip.

UL508A is basically the standard with instructions for building a control panel that is listed. It also requires you to have a file open with UL and have met their other requirements. I would suggest as a minimum that you read and understand that standard if you want to build a panel that is marked with a fault rating. Better would be to go through the process to get setup with UL so you can build panels with the UL mark. There are lots of things you need to do correctly to achieve the fault rating besides just looking at the fusible ratings.

 

[FacEngrPE]

I will side with LionelHutz here. The goal is to make sure the panel can withstand the available supply fault current.
Here are the sort of things to work through with the UL 508 rules.
If your disconnect is separate from your panel, and has a published Short Circuit Current Rating(SCCR), the SCCR will apply only to the disconnect switch. If the disconnect switch is inside the panel, the component SCCR might or might not be the deciding factor in the panel rating.
(edit)
jraef 's explanation is much better than mine.

 

[jraef]

(1) Maximum ampere rating of the short-circuit and ground-fault protective device, where provided.

Other than my 30 amp main fuses and the lower rated branch fuses, I do not have an additional ground-fault protective device. Would this mean that my short circuit ampere rating is 30 amps (the rating of my main fuses)?

The term "short-circuit and ground-fault protective device" is just the NEC terminology for the main over current protective device; i.e. fuses or circuit breaker, "ground fault" in this context just means it will clear on a short circuit line-to-ground, in addition to line-to-line. This causes people a lot of confusion, but it has been this way for decades, long before specific "Ground Fault Circuit Interrupters" or "Ground Fault Equipment Protection" devices existed. Not the same thing, don't worry about it.

(2) Short-circuit current rating of the industrial control panel

Would this be something I would find in the specifications for my disconnect? My disconnect specs list a few things that seem like they might be relevant ...

No, "Short Circuit Current Rating" (SCCR) has little to do with the specs on your disconnect, in fact most fused disconnects end up having an SCCR based on the interrupt rating of the fuses they hold anyway. SCCR for the PANEL assembly (required per NEC article 409 since 2005) is a calculated or tested value of ALL of the POWER devices in a circuit (other than transformers). As mentioned, you can use the "weakest link" method and give your assembly an SCCR based on the lowest rated component, or you can just use the "courtesy" rating of 5kA for untested equipment. The problem with those methods is that the SCCR must meet or exceed the Available Fault Current at the installation site. So in most cases if this is anything other than residential equipment, a 5kA or 10kA SCCR rating ends up not being able to be installed. And no, contrary to popular belief, the installer cannot just slap another fused disconnect with current limiting fuses ahead of it and call it good, it doesn't work that way.

So your other two alternatives are to:
A) have the panel built by a UL508A listed panel shop and have them do an SCCR evaluation and listing per the UL procedures in Supplement SB of UL508A, or
B) you can do it yourself by CAREFUL and thorough planning of your Bill Of Materials to take advantage of "Series Listings" of components.

In that last option, what you would do is to evaluated ALL of the power circuit devices and as a modification to the "weakest link" method, select ONLY those components offered by manufacturers in "Series Listed" combinations that provide you with a reasonably high SCCR value that you can then put on your label and have the documentation to back it up (if challenged).

So for example let's say you have, in your POWER circuit, that Fused Disconnect, fuses of course, 3 or 4 contactors and motor overload relays, then some terminal blocks for field power connections. You will likely find that the disconnect will carry the fuse rating, i.e. 100kA, so that's fine. But the contactor and/or OL relay and/or terminal blocks may only be listed for 5kA SCCR all by themselves. However they might (likely) be listed by the manufacturer (via an expensive testing process) in series with a SPECIFIC fuse at 65kA SCCR, so you may have to change to a different fuse (and thereby disconnect) to take advantage of it. In addition, you may have used one brand of contactor and a different brand of OL relay because it was cheaper and you are wiring separate anyway, but they may not be listed TOGETHER in a series combination, so you may have to select the SAME BRAND for all of your components. This gets especially true when using circuit breakers, because it is HIGHLY unlikely that a company that manufactures contactors, OL relays AND circuit breakers will pay for the necessary series testing and listing of their contactors with a competitor's circuit breaker. Why would they? So for the last decade or so, component manufacturers (with a conscience) now provide you with a method to look up which components are series listed and with what. The process differs from one to another, and some of the "bottom feeder" cheap stuff you buy on the internet may not have it, so the only slight detriment is in limiting your supplier choices, which is really not a big deal.

So the only thing that has changed is that it's not actually all that difficult in general to attain a high SCCR value for a panel, even if you don't need a UL508A label, but you just can't be lazy about it any more.

And...PLEASE PLEASE PLEASE don't just default to the 5kA listing , it's a ROYAL pain in the rear for the poor people who have to try to install it. In this case your customer is being smart and forcing you to do it up front (they have likely been burned). So it might behoove you to make a call and ask them what they NEED before you start. If they need 65kA, it might make a difference in your selection compared to only needing 25kA.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[EdStainless]

Your customer did have this requirement on the PO, correct?
WE have bought panels like this and people whine about the cost, because you really can only do it using UL listed components for everything. But it does preclude the use of cheep junk that looks OK but hasn't been tested.

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