NFPA 79 Short-circuit current rating 5

[PaulKraemer]

Hi,

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 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,
Paul

 

[xnuke]

1. Yes, 30 A.
2. Control panel SCCR would be determined using the methods in UP 508A Supplement SB.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[RRaghunath]

Your short circuit protection for both phase faults and ground faults will be 30A rated fuses.
The control panel short circuit current rating will be Let-through current of these 30A fuses as these fuses are at the entrance to the control panel. This information can be had from the fuse catalogs.
You don't need to consider the fault level of the customer panel upstream and calculate the fault level at the incoming to the control panel.
Disconnect at the inbound side is not supposed to be operated when there is fault and hence, it has no short circuit current rating.

 

[jraef]

My copy of NFPA 79 is now getting a little long in the tooth (2002), but I'm not sure that the requirement for an SCCR is (or is ONLY) related to NFPA 79, it is in the NEC (NFPA 70) article 409 and one might consider that having primacy anyway.

If you paid no attention to this requirement when building your panel, you must not be a UL508A listed shop because that would have been part of those requirements as well. So now you might be faced with some challenges in what you have built. Without going into the gory details, your choices are to EITHER build it and have everything tested (very VERY expensive), or use specific components that have ALREADY been tested and listed together by the manufacturer(s). This only applies to devices in the POWER circuit, anything below the primary fuses of a Control Power Transformer (or Power Supply) is irrelevant. But EVERYTHING in the power circuit must be listed with an SCCR that is higher than the Available Fault Current, or you are forced to use the LOWEST value of any component, or finally, if you can't find that info, you get relegated to what's often called the "courtesy" listing of 5kA, and that is honestly all but useless in the real world.

So for example if you used a contactor and OL from one mfr and a Circuit Breaker from someone else, they will NOT have been tested and listed together and you will have to change one of them. Fuses are often easier because most power devices will get listed behind fuses, but you have to find out which SPECIFIC fuses they were listed with, you cannot substitute. It's often the little details that bite you too. For example if you used power terminals in the circuit, THEY must be listed with whatever OCPD you use, and many are not. Drives are often problematic because you may have to use fuses if the VFD mfr didn't bother to list them with breakers, which happens a lot with VFDs sold by companies that don't make breakers. The list of pitfalls goes on.

But to the point about using components listed together, most equipment mfrs now provide information on what their components are listed with, sometimes referred to as a "Series Listing". So for example a contactor might be listed at 10kA SCCR by itself, and OL relay might be only 5kA by itself, but with a (specific) CB in a Series Listing arrangement, the combination is listed for 65kA, and with current limiting fuses, maybe even 100kA. Ask your supplier for SCCR data on the components, they should be able to get you that. If not, find a better supplier.


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

 

[RRaghunath]

jraef, your posts are always comprehensive and backed by in depth knowledge. Thanks!

 

[wbd]

Requirements for equipment to be properly rated are NFPA 70 Article 110.10 and OSHA 1910.303(b)(5).

 

[PaulKraemer]

Hi RRaghunath, jraef, xnuke, and wbd -

Thank you for your responses. Wow - this is a lot to absorb. I am not a UL UL508A listed shop, and I was not made aware of this requirement to list a Control Panel SCCR on our machine nameplate when we got the order to build this machine. The reason for this requirement is because this particular customer (in California) is required to have a third-party inspection for all new equipment. I am not sure if this is a California regulation or a regulation for their particular industry (battery manufacturing). I was not made aware of this requirement when we accepted the order. I can tell you that this customer already has one of our machines that we built 10 years ago. They purchased it on the used market and installed it on their own. When we design/build all of our machines, we do try to use only components that are UL and CE certified, but we never went to the extent of having a machine as a whole either UL or CE certified. We also never paid specific attention to SCCR. When this customer installed the 10 year old machine, they made their own nameplate and listed a SCCR of 5 kA. When I asked their in house (mechanical) engineer who handles equipment installations (using outside electrical contractors) how they came up with this rating, his response was as follows:

"The SCCR is based on the lowest rated component in the enclosure (aka "weakest link"). In the case of the coater/dryer enclosure (not the Indeeco), the SCCR was 5kA for the side-mounted disconnect. If a component is installed that has a SCCR of less than 5kA, it must be replaced/upgraded as it not allowed by NEC. From my notes, NEC-2005 110.10 , but I don't have a copy of the code."

I think at this point, my best course of action is to send them spec sheets on the disconnect and the fuses I used on the new machine for their review. He can show this to whoever will be doing the inspection and let me know if they foresee any issues. I will not feel financially responsible for any corrective actions they suggest because this was not expressed as a requirement when the order was placed. Of course, I will do my best to provide whatever information is required and to assist in the corrective actions any way I can (without incurring excessive cost).

I will post here as I get more feedback. I appreciate all of your help.

Best regards,
Paul

 

[jraef]

Just for your future reference:
Claiming you "didn't know" is kind of a dodging of responsibility. The National Electric Code (NFPA 70) is a requirement in all 50 states and article 110.2 states:

110.2 Approval. The conductors and equipment required or
permitted by this Code shall be acceptable only if approved.
Informational Note: See 90.7, Examination of Equipment
for Safety, and 110.3, Examination, Identification, Installation,
and Use of Equipment. See definitions of Approved,
identified, Labeled, and Listed.

Now, the term "approved" is defined as "acceptable to the Authority Having Jurisdiction" (AHJ), which is what can and does vary state-by-state. But ALL of the 11 Western States (CA, OR, WA, AK, ID, NV, AZ, NM, UT, CO, WY) as well as many others have clauses in their state Electrical Codes that add the requirement for any electric equipment to have an NRTL listing applied if there there are more than "x" number of devices in an assembly (the "x" varies a little state-to-state, but is usually 3-5, including the enclosure itself).

Even when not required by a particular state code amendment, the NEC can be interpreted to require it for many assemblies by reference to needing "listed equipment" and then the definition of that term is in Article 100 as well, which is basically what these states have adopted. This has been like this since the 1980s that I am aware of.

Article 409, which is where the term "Short Circuit Current Rating" first came up, began in the 2005 NEC and has been the law of the land since then.


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

 

[LionelHutz]

Yes, not knowing and "trying" to use listed components is a rather weak excuse. If I was a customer, I'd reject the panel unless it was listed.

Based on the ratings you posted for that fused? disconnect, it appears to be some type of supplemental protection device that isn't suitable for use as the main disconnect when following UL508A. Even if it was acceptable, the aM or gG fuses that it requires to get a fault rating are IEC fuses and not UL listed which kills any hope of putting a SCCR on the panel.

 

[controlsdude]

Your company being an OEM its been my experience that usually most OEMs come to the conclusion that every panel should be UL stickered and the KA ratings on motor components are at a certain level. That way when your selling into different areas of the country the local electrical inspector if they see the panel is UL stickered your chances of it failing inspection drops below 10%. And the arc flash concerns are minimized if your components are rated to certain level.

Otherwise your fighting a losing battle on every job due to above not true with the local electrical inspector.