petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
That is true but an electrician cannot just add CL fuses in the feeder circuit and have it mean anything.
UL says you can within the context of a UL listed control panel.
SCCR rating on HVAC equipment control panels
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UL says you can within the context of a UL listed control panel.
Same method.
Read the Bussmann (now Cooper) fuse protection handbook. It is obviously biased against circuit breakers but still covers the subject very well including how to calculate SCCR when fuses and breakers are involved. This includes figuring in cable lengths. The book is free as a PDF. It even includes arc flash protection (fuses only).
- Location
- Illinois
- Occupation
- retired electrician
Then once it leaves the factory, there is no way to install the equipment where the supply circuit has a higher available fault current than the marked SCCR. The installer cannot use the provisions of UL 508 as it is not an adopted code.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
This is the crux of the matter. The information put out by the fuse mfrs is related to the FACTORY doing an SCCR evaluation of the EQUIPMENT before labeling it, using TESTED AND LISTED series combinations with the fuses, per the applicable standards for whatever NRTL they are using (which are usually UL's standards regardless of the NRTL). So yes, a PANEL SHOP can use fuses to change the SCCR shown on the label of the PANEL. AND it is possible that the panel supplier's label can say "Suitable for connection to a system having xxxxxx Available Fault Current if protected by yyy fuse" or equal. But if the label on the panel already SAYS 5,000A and the Available Fault Current at the terminals is a higher value, adding current limiting fuses in the field to feed that panel does NOT change the Available Fault Current nor does it change the label on the panel.
The idea of a "series rating" of OCPDs for untested combinations is actually SPECIFICALLY addressed in 248.86(A) in saying that a registered PE can perform the calculations and make the determination, but that is the ONLY way it can be acceptable. On a couple of occasions where I was trying to help someone do this, we could not find a PE willing to do it. That's not to say it doesn't happen, but it's not simple to pull off. The PE is essentially staking his license to practice his profession if he makes a mistake and something goes wrong, so it's not something they take lightly.
- Location
- Illinois
- Occupation
- retired electrician
But is it a "series rating" if there are no OCPDs in the control panel. I thought the issue with series ratings in the dynamic impedance of OCPDs that are in series with each other.
If there are no OCPDs, why can't we use the let peak let through current on the load side of the current limiting fuse?
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Mostly because the code says you can't unless you are a PE and willing to stake your license to it. And I thought it only allowed this for existing systems. I think a new system would not qualify for this provision.
You can get UL to come do a field evaluation. Or you can do it the right way and just put the required SCCR in the RFQ and PO. I don't know why people are opposed to this simple solution.
- Location
- Illinois
- Occupation
- retired electrician
Where does the code say that? As far as I know, the code does not address the issue of using a current limiting device to limit the fault current at passive downstream equipment. It only address that when you are trying to create a "series" system of OCPDs.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
I don't recall how it's worded so you may be right. If it was that easy people would just do it though. I'm not sure what kind of passive equipment you want to try and protect though. A control panel has a short circuit current rating on it that specified by the manufacturer.
Unless a marking says so, adding current limiting fuses upstream does not change the short-circuit current rating of the control panel.
I'm not sure what other passive components you are talking about. If you put a contactor and a overload relay together you have an industrial control panel and are thus required to get it listed. If it's listed, it has to have a short circuit current rating. Which cannot be changed in the field.
- Location
- Illinois
- Occupation
- retired electrician
I guess I am incorrect, it was my understanding that you could use current limiting OCPDs for this purpose as long as the downstream control panel does not have dynamic impedance. By 'passive device" I mean a device that does not have dynamic impedance. It was my understanding that the only thing that has dynamic impedance is an OCPD as that is what the rule in 240.86 addresses.
The code needs to be changed to make it clear that you can't use current limiting OCPDs to protect control panels with a lower SCCR.
The code needs to be changed to make it clear that you can't use current limiting OCPDs to protect control panels with a lower SCCR.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
The code is already clear on that point. The code very clearly says that an industrial control panel must have a short circuit current rating marking and that the available short circuit current cannot exceed the marking on the industrial control panel.
As far as I know none of the accepted means of determining what the short circuit current is at a particular place in an electrical network uses the let through of a fuse or current limiting circuit breaker as part of that calculation.
- Location
- Illinois
- Occupation
- retired electrician
I just don't understand how using the let through current of a current limiting device is really any different from installing one to one transformers, reactors, on increasing the conductor length to reduce the available fault current at the control panel. If the current limiting device actually changes the available fault current, how it different from adding impedance on the line side of the panel.
I also don't understand how we can use the upstream current limiting fuse to lower the incident energy at the equipment, but we can't take that same current limiting and use it to protect the equipment.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
it is done all the time. It just can't be done in the field by electricians. It has to be part of a listed combination. There are not many individual electricians who are set up to do listed control panels. There are however, a fair number of electrical contractors that are. It isn't especially cheap to get set up to be a UL 508 a panel shop, but it may be something that electrical contractors are going to eventually be forced to do.
SceneryDriver
Senior Member
- Location
- NJ
- Occupation
- Electrical and Automation Designer
It's because the UL overlords have deemed it so. We used to be a 508A panel shop; we're dropping our status next month. UL's rules often have little to no engineering behind them, and amount to a money grab from panel shops and manufacturers. Case in point: I wanted to use a certain UL-Recognized power supply in a 508A panel. I couldn't, but I could use it in a UL-listed "sign." Why? Because they hadn't investigated it, and wanted over $10k to do a paperwork shuffle.
Add to that their field inspectors not having any idea how to inspect, enforce, or answer technical questions about their own standards. One of my (many) field inspectors actually told me he has almost no idea how to inspect a 508A panel; he was a fire door guy. UL went to crap when they became a for-profit company.
SceneryDriver
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
UL field inspectors are not by and large technically astute. It is not their job to train you or your people on how to design and build UL 508a panels, nor to answer technical questions.
There are good reasons why you can't use just any power supply in a UL listed control panel. They may not seem like good reasons to you, but they are good reasons, mostly. Most UL investigations involve just adding parts to your procedure and don't cost $10,000. You may have selected a power supply that they have to do actual testing on. But why would you want to use a recognized power supply anyway? There are plenty of listed power supplies available.
- Location
- Illinois
- Occupation
- retired electrician
But there is nothing prohibiting the Electrical Contractor from reducing the incident energy in the field by installing a current limiting device....the prohibition on doing it for equipment is bogus, in my opinion.
SceneryDriver
Senior Member
- Location
- NJ
- Occupation
- Electrical and Automation Designer
And this is where I have issue: if they're not technically astute, how can they be expected to properly interpret and understand a technical standard? It also seems like few people at UL can interpret their own standards, and finding someone who will return your phone call in less than a geological timescale is neigh on impossible.
To the OP's point: SCCR is a bit of a nightmare, and it's my belief that UL keeps it so in order to justify the way they do things. Either a fuse will limit current or it won't; there's no engineering rationale that can justify allowing it to be installed in the factory but not in the field. Either the math works or it doesn't.
I was quoted almost $10k to investigate the possibility of adding the power supply to our procedure; no testing required. As to why: it all came down to form factor and available space. Further, no one at UL could actually explain why it couldn't be used in a 508A panel. It was a case of "just because we said so." If there is an actual engineering reason, so be it. That's a good reason, and they should be able to explain it. But it was just paperwork / money grab, and that's what I have a problem with.
SceneryDriver
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
I don't think you understand the purpose of the field inspectors. They are not there to determine whether you did the job right or not. why you may think that's what they're there for they are primarily there for administrative purposes. They are not there to understand or interpret any standard. They seem to get a quarterly requirement to investigate certain things that have been a problem to make sure that manufacturers are doing things correctly. For instance one of the things they made a point of checking not too long ago was the use of 600 volt insulated ethernet cables where required. A lot of manufacturers were not separating ethernet cables from 480 volts as is required. So the inspectors were trained to take a look at this. but it's not like they were training to look at anything else.
It's hard to know what your situation involved but it's also hard for me to understand why the form factor would matter for a power supply there are plenty of listed power supplies in just about any form factor I suppose you could come up with some goofy design that had it worked out so no listed power supply would work but one would think that it would not be all that difficult to modify the design to use something that wouldn't cost you $10,000 to add to your procedure.
It's hard to know what your situation involved but it's also hard for me to understand why the form factor would matter for a power supply there are plenty of listed power supplies in just about any form factor I suppose you could come up with some goofy design that had it worked out so no listed power supply would work but one would think that it would not be all that difficult to modify the design to use something that wouldn't cost you $10,000 to add to your procedure.
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