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Available fault current ratings
3

Available fault current ratings

Available fault current ratings

(OP)
We have a new machine that came in. It has a 5ka rating. My available fault current is higher than that. I assume I need to install a disconnect and fuses to get it lower than the 5000. Anyone know what style or type of fuse I can use?

RE: Available fault current ratings

Assuming from the description that you are in the US somewhere...

Nope. No can do in a Code compliant way. This was a failure of whomever bought the machine to tell the machine supplier that this was a requirement. If the tender document stated that it was the SUPPLIER'S responsibility to ensure that the "SCCR" (Short Circuit Current Rating) meet or exceeded your Available Fault Current, it would have triggered a set of conversations about your needs and the supplier COULD HAVE very simply designed and listed their control panel to meet that requirement. But unfortunately there is no simple or low cost way to fix this after the fact.

Your options are:
1. Remove the control panel and find a local UL-508A listed panel shop willing to gut and rebuild that panel, putting their OWN new UL listing on it with the proper SCCR.

2. Send the machine back to the manufacturer and tell them to do it.

3. Hire a registered PE in your area to do a deep dive evaluation of ALL of the components in the power circuit of the machine, then come up with a STAMPED drawing or document stating that they have done a thorough evaluation and approved the use of a SPECIFIC current limiting fuse ahead of this. Good luck on finding that PE by the way (I know, I've tried). They would be staking their professional license on this, so they are not going to do so inexpensively, if at all.

Take note of that initial statement I made for any and all future machinery purchases. IF YOU DON'T SPECIFY WHAT YOU NEED, SUPPLIERS WILL OFTN IGNORE THIS ISSUE. It's easy to just put a "default" rating of 5kA on a panel, because you do NOT do any of the relatively simple and inexpensive steps that it takes to get a decent SCCR. One of those simple steps is to use devices that are ALREADY listed in series with each other, so that you do not have to do the expensive testing. That however means the component MANUFACTURERS have had to do that expensive testing, and one mfr is not going to do testing of their components in series with a competitor's components. That then means that if you use Siemens breakers, you have to use Siemens Contactors and Siemens Overload Relays etc. etc. Cheap panel builders and OEMs want to use the cheapest parts they can find for every component, i.e. Siemens breaker, Schneider contactor, ABB terminal block etc., but that then means they cannot get more than the "courtesy" 5kA SCCR. If YOU, the buyer, tell them that it MUST have an SCCR of 65kA (or whatever you need), then they CAN just spend a little extra for the parts that all match up.


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

RE: Available fault current ratings

(OP)
Thanks jraef. Yeah I've tried and tried to convey that message to the boys up top who buy the machines. I've even made copies of what to request from the manufacturers, like it needs a UL Listing, 480v preferable, good schematics and free software if there is a PLC, and now I guess I'll put on the new list proper SCCR ratings. It usually falls in the brown file container though.

RE: Available fault current ratings

Current limiting type HRC fuses could limit the fault current as seen by the machine at its terminals.
You should be able to verify the let-through current of selected fuses.

RE: Available fault current ratings

This might not fix until something gets installed that the local AHJ rejects. That tends to shine a spotlight on things.

RE: Available fault current ratings

Quote (RRaghunath)

Current limiting type HRC fuses could limit the fault current as seen by the machine at its terminals.
You should be able to verify the let-through current of selected fuses.
While in theory this is possible from a strictly engineering perspective, here in the US that does not satisfy the National Electric Code requirements for connecting equipment to systems that have a greater Available Fault Current than the listing of the equipment, because it changes neither. As I said, a PE could possibly crunch the numbers and provide a letter stating that it is OK to do under their auspices, but again, I have tried that route several times in the past and was unable to find a PE that was willing to do it, at least for less than it would have cost to just rip out the panel and send it to a UL-508A panel shop to be rebuilt and listed by them..


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

RE: Available fault current ratings

Thank you for that insight jraef.
I wasn't speaking about UL listed panels in my post and I am not that much conversant with American practice.
Thanks again.

RE: Available fault current ratings

It helps get better answers if the question indicates if the location is a IEC or National Electric Code. The two regions reach safety by dramatically different paths.

RE: Available fault current ratings

deetz Likely a US location.

RE: Available fault current ratings

(OP)
That is correct, US, so NEC. Also MN, so extra or more enforced NEC rules than most of the US. I constantly get asked why we need to have the new machines UL listed, when the manufacturer says no one else we sell to asks us. Now it's the SCCR or AFC, and now machine manufacturers are just putting a generic 5ka, like what jraef explained. Saves them money if most customers don't need it. I get it. Sucks to be us.

RE: Available fault current ratings

You could possibly get an inspector to approve a higher rating on site by using UL508A supplement SB.

RE: Available fault current ratings

An isolation transformer will limit fault current.

RE: Available fault current ratings

Quote (LionelHutz)

You could possibly get an inspector to approve a higher rating on site by using UL508A supplement SB.
Assuming all of the components match up to what the mfrs tested them at. Plus last time I had to call for a UL field inspection it cost me $15k and that was 20+ years ago. I would think you could find a panel shop locally that will re-list the panel for less than that.

Quote (FacEngrPE)

An isolation transformer will limit fault current.
Provided an AHJ accepts that as an "approved method", and then you have to live with the losses of that transformer forever.

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

RE: Available fault current ratings

Nothing to do with mfrs testing, it's simply applying series fuses and getting a higher rating. Pretty limited possibilities, but it can work. I also wouldn't go right to calling in UL, go over it with the local AHJ first.

RE: Available fault current ratings

(OP)
Okay so we have to have a company come in and List the panel anyways.. Looking inside and at the drawings and schematic, the company seems to have documented everything pretty well so estimate on the UL listing is around 6K. We asked them as long as they are doing that if they could reconfigure the AFC. The AHJ is working with us and not against us on this so that's good. Will keep you posted as to what happens. Thanks again for the input.

RE: Available fault current ratings

The requirement for equipment to have an SCCR rating is not specifically tied to UL listing the panel, but it is one of the "approved methods" called out for in Article 409 of the National Electric Code.

Quote (NEC)

409.22 Short-Circuit Current Rating. An industrial control
panel shall not be installed where the available fault
current exceeds its short-circuit current rating as marked in
accordance with 409.110(4).

409.110 Marking. An industrial control panel shall be
marked with the following information that is plainly visible
after installation: ...

(4) Short-circuit current rating of the industrial control
panel based on one of the following:
a. Short-circuit current rating of a listed and labeled
assembly
b. Short-circuit current rating established utilizing an
approved method
Informational Note: ANSI/UL 508, Standard for Industrial
Control Panels, Supplement SB, is an example of an
approved method.

240.86(A) is the only possible exception in terms of getting a "series rating" with a current limiting device, and that is where the Code establishes the requirement for it to be done and stamped by a registered PE.


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

RE: Available fault current ratings

It's not possible for a company to do a UL listing of a control panel on site. The panel must be built and listed at the manufacturers plants as listed in their UL file. I'm pretty sure that applied for pretty much anything UL listed - only possible at the factory it was produced in.


jraef - sure the code may say that, doesn't mean the inspector won't work with you and allow something else. The inspector always has final say.

RE: Available fault current ratings

Quote ('UL’s Field Evaluation (FE) Services' (attached)

United States’ Occupational Safety and Health Administration (OSHA) requirements mandate that all electrical equipment in the workplace be “certified” or subjected to a complete and thorough evaluation before use (29 CFR 1910.303 and 1910.399). Additionally, many city, counties, other municipalities, states,provinces and regions have regulations requiring building, gas fired and electrical products to be certified or evaluated. A UL Field Evaluation is an accepted approach to accomplish this required evaluation. UL’s Field Evaluation program meets all requirements of the ANSI/NFPA 790 and ANSI/NFPA 791 Standards for competency and field evaluation procedures.

This should be good enough.

RE: Available fault current ratings

Sure, field inspection can often work. It's not a listing and not available from just anyone. You've got to prove what the short circuit rating should be, properly label the box and then the inspector will slap that sticker on it.

RE: Available fault current ratings

(OP)
Thanks FacEngrPE, I'll just copy that and get it printed on some label material and stick it on there. Just kidding. Yeah we actually have a company now in town that does UL listing. They have done a couple of smaller machines for us already. It's good with the Inspector too.

RE: Available fault current ratings

I probably should have figured out how to add a "specimen" watermark on the image before posting.blush

RE: Available fault current ratings

Quote (FacEngrPE)

An isolation transformer will limit fault current.

Quote (jraef)

Provided an AHJ accepts that as an "approved method", and then you have to live with the losses of that transformer forever.

Why do you suggest an AHJ would need to approve this in regards to the acceptability of installing the panel at this location? It is simply lowering the "available fault current" as mentioned in 409.22.

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