SCCR for existing installation

[adamaj]

Hello,

I'm wondering what codes, if any, would require that equipment with an inadequate SCCR be made to exceed the SCCA for an existing system?

OSHA 1910.303 has language that indicates this, but I'm not sure if this is an installation code or if the requirements must be met at all times?

We had our SCCA increase due to upgrades performed by the utility on their system. Unfortunately, original engineers designed for the SCCA that existed at the time, despite the size/impedance of the utility transformer having the potential for a much higher SCCA.

Thanks in advance for your help.

 

[Jraef]

So your choices are to lower the Prospective Short Circuit Current (PSCC) by adding a current limiting reactor, transformer etc., increase the length of conductors feeding it (if it's only a small amount needed), or try to find a PE willing to engineer and certify a fuse system with a let-through that will satisfy the needs. That last one is something that people think they can do on their own after reading some of the literature put out by the fuse mfrs,, but that's not how it works unless the EQUIPMENT you are looking to protect has ALREADY been series listed with fuses for that purpose.

 

[paulengr]

You don’t need a regulation to tell you that overdutied equipment is a bad idea. It is very common to see drives blown up when a motor goes to ground, contactors welded shut or turned to slag, and breakers similarly obliterated when a fault occurs far worse than the damage of the fault itself. Bus bars turned into art work as they bend and rip loose from the insulators is impressive but less common. It’s not like it won’t trip and burn down a building but the damage is much more immediate.

And if you think there is some wiggle room think again! SCCR is not like thermal damage. Effects are proportional to the square of the current so a small amount under is much more severe than expected.

 

[GoldDigger]

And there is not much of a time element except for the very small inertia of the components that fly apart and the length of time over which that force is applied.

Sent from my Pixel 4a using Tapatalk

 

[wbdvt]

OSHA regulations need to be met all the time. So if your equipment is overdutied, you are in violation of OSHA 1910.303.

I am not sure if you are using the infinite bus short circuit current which is typically provided by the utility and based on your wording, " We had our SCCA increase due to upgrades performed by the utility on their system. Unfortunately, original engineers designed for the SCCA that existed at the time, despite the size/impedance of the utility transformer having the potential for a much higher SCCA. " This would seem to indicate infinite bus. If it is, you should get the available short circuit current from the utility. You may find that your equipment is not overdutied using the available short circuit current.

You do not specify what type of equipment is overdutied but if panels/breakers there may be series ratings that can help.

 

[electrofelon]

That last one is something that people think they can do on their own after reading some of the literature put out by the fuse mfrs,, but that's not how it works unless the EQUIPMENT you are looking to protect has ALREADY been series listed with fuses for that purpose.

Where exactly is this stated? Is this in the listing documents? Who is to say that a properly selected CL fuse doesn't create a "new" and legitimate fault current value? Obviously the NEC has the tested combination requirements for circuit breakers that are interrupting fault current, but they is no requirement that I can see, at least in the NEC, for SCCR.

 

[drktmplr12]

Where exactly is this stated? Is this in the listing documents? Who is to say that a properly selected CL fuse doesn't create a "new" and legitimate fault current value? Obviously the NEC has the tested combination requirements for circuit breakers that are interrupting fault current, but they is no requirement that I can see, at least in the NEC, for SCCR.

I think UL is to say. Unless the equipment has been tested with the combination, it cannot be combination rated. That is to say, unless the manufacturer has paid to have said combinations tested, they cannot be relied upon. At the end of the day, if the PE wants to stake their license on it, they price the risk accordingly and that is their business.

here is an informative article, although somewhat dated, it does touch on subjects brought up here.

StackPath

to the OP:
Looking at the fault current available at the secondary terminals of the transformer, based on napkin math, is not accurate. The available fault current needs to take into account all fault contributions including utility transformer, impedances due to conductors, motor contribution and OCPD settings.

You might consider hiring a firm to do a facility-wide arc flash, short circuit and coordination study-or just a short circuit study. It might seem like overkill, but it would certainly be a good investment if the result demonstrates you are not over duty. They might even help you with remediation options. There's much to consider. Replacing equipment only to increase interruption ratings will be costly and certainly disrupt whatever operations you have going on. Adding a reactor will have energy costs spread out over time.

 

[adamaj]

Thanks to all for the responses, especially wbdvt, who gave me the answer I was looking for.

I've already explored most of the mitigation options outlined here. Just writing up a project justification and was looking to see if there was a regulatory component I could add to it, beyond the obvious point of unsafe operating conditions. The company I work for is very safety oriented, so I'd imagine that will be plenty, but want to provide as much detail as possible.

 

[adamaj]

Past my edit time for the above, but wanted to add that the short circuit current I'm using for the utility feeder is not infinite bus (utility provided based off of their ASPEN model). All SCCAs in our system have been found using EasyPower. So little doubt about the accuracy.

Where exactly is this stated? Is this in the listing documents? Who is to say that a properly selected CL fuse doesn't create a "new" and legitimate fault current value? Obviously the NEC has the tested combination requirements for circuit breakers that are interrupting fault current, but they is no requirement that I can see, at least in the NEC, for SCCR.

On the topic of Current Limiting Fuses. I couldn't find anything that strictly forbid using Current Limiting Fuses to decrease SCCA to downstream non-interrupting devices. I also didn't find any examples of it being done. It would seem to me that fuse manufacturers would be all over touting that capability if it were possible.

We had an interesting situation where we had a number of MCCs that were rated 42kA (due to bus bracing), while all of the buckets had a 65kA rating (the MCC as a whole is a UL 508 assembly). SCCA at the MCC was ~55kA. Now to me it would seem that this would be an ideal situation from an electrical design standpoint to utilize a current limiting fuse that interrupts on the first half cycle, but would I stick my neck out for it? No way. Do I expect that a PE would? Not a chance.

Manufacturer provided no options for series rating, so I started looking down alternate paths.

 

[jim dungar]

... but they is no requirement that I can see, at least in the NEC, for SCCR.

I have always considered 110.9 for protective device AIC and 110.10 for equipment SCCR.

I know of no tools, such as manufacturer guidance or engineering software, except some UL standards that can be used to apply the current limiting effect of fuses or breakers.

 

[electrofelon]

On the topic of Current Limiting Fuses. I couldn't find anything that strictly forbid using Current Limiting Fuses to decrease SCCA to downstream non-interrupting devices. I also didn't find any examples of it being done. It would seem to me that fuse manufacturers would be all over touting that capability if it were possible.

We had an interesting situation where we had a number of MCCs that were rated 42kA (due to bus bracing), while all of the buckets had a 65kA rating (the MCC as a whole is a UL 508 assembly). SCCA at the MCC was ~55kA. Now to me it would seem that this would be an ideal situation from an electrical design standpoint to utilize a current limiting fuse that interrupts on the first half cycle, but would I stick my neck out for it? No way. Do I expect that a PE would? Not a chance.

I know of no tools, such as manufacturer guidance or engineering software, except some UL standards that can be used to apply the current limiting effect of fuses or breakers.

Isn't this exactly what the fuse manufacturers say we can do utilizing their peak let thru charts?

Start by locating the 100,000 A available fault-current on the
bottom of the chart (Point A) and follow this value upwards
to the intersection with the 600 amp fuse curve (Point B).
Next, follow this point horizontally to the left to intersect
with the A-B line (Point C). Finally, read down to the bottom
of the chart (Point D) to read a value of approximately 18,000
amps.
Can the fuse selected properly protect the equipment for
this application? Yes, the POWR-PRO® LLNRK 600 ampere
RK1 current-limiting fuses have reduced the 100,000
amperes available current to an apparent or equivalent
18,000 amps. When protected by 600 amp LLNRK RK1
fuses, equipment with short-circuit ratings of 22,000 amps
may be safely connected to a system having 100,000
available rms symmetrical amperes.


This method, sometimes referred to as the “Up-Over-and-
Down” method, may be used to:
1. Provide back-up short-circuit protection to large air
power circuit breakers.
2. Enable non-interrupting equipment such as bus duct to be installed in systems with available short-circuit currents
greater than their short circuit (withstand) ratings.

What am I missing here?

 

[adamaj]

Interesting. I had always read that as a pitch at panel builders that wanted to achieve a higher SCCR for the panel they were building (which would eventually get listed).

Certainly could be in the context of protecting downstream equipment in a distribution system. Definitely deferring to someone else's experience on that one.

 

[paulengr]

OSHA regulations need to be met all the time. So if your equipment is overdutied, you are in violation of OSHA 1910.303.

I am not sure if you are using the infinite bus short circuit current which is typically provided by the utility and based on your wording, " We had our SCCA increase due to upgrades performed by the utility on their system. Unfortunately, original engineers designed for the SCCA that existed at the time, despite the size/impedance of the utility transformer having the potential for a much higher SCCA. " This would seem to indicate infinite bus. If it is, you should get the available short circuit current from the utility. You may find that your equipment is not overdutied using the available short circuit current.

You do not specify what type of equipment is overdutied but if panels/breakers there may be series ratings that can help.

Be careful of HOW this calculation is done. Every SCCR calculation uses some assumptions. For instance the infinite bus assumption doesn’t model the real (not infinite) source well nor cable impedances nor the effect of loads. So it is usually lower but could be higher! ANSI and some IEC methods handle inductive sources and sources but tend to underestimate cable impedances by assuming only inductance matters and not resistance.

The point by point method is suitable for spreadsheets and relatively simple for mere mortals, not just engineers with overpriced software.

 

[jim dungar]

What am I missing here?

Note the examples at the bottom of your post.

The fuse manufacturer's publications are living in the past.
This method is for large power circuit breakers that are intended to stay closed during most faults, sometimes these are called 30 cycle breakers or iron frame breakers. they were common 50 years ago, but they would rarely be found in new construction.
The other example is large bus bar assemblies, like the ones found in open-air substations or electrical room. They are not talking about the bussing in panelboards, switchboards and plug-in busway. Again this is for stuff seen in historical pictures rather than in new construction.

I do remember using the up-over-down methodology when I first entered this business and these types of installations were still fairly common. But haven't used it in more than forty years.

 

[Jraef]

...
We had an interesting situation where we had a number of MCCs that were rated 42kA (due to bus bracing), while all of the buckets had a 65kA rating (the MCC as a whole is a UL 508 assembly). SCCA at the MCC was ~55kA. Now to me it would seem that this would be an ideal situation from an electrical design standpoint to utilize a current limiting fuse that interrupts on the first half cycle, but would I stick my neck out for it? No way. Do I expect that a PE would? Not a chance.

Manufacturer provided no options for series rating, so I started looking down alternate paths.

Side note:
Bus bracing is a little different than interrupt or withstand ratings. The bus bracing must hold UNTIL the other devices act and stop the current flow, yet have no role in the actual interrupting. So you can’t do “series ratings” like you can with breakers and fuses. The bracing is rated for the value given to it and literally engraved into the MCC nameplate.

Word to the wise on MCC bus bracing: I made that same mistake once, it was a PITA to fix. The difference between 42kA and 65kA bracing on most brands of MCCs is very very little, in one case I know of, it is one extra sheet metal screw per section, but it cannot be changed in the field, the UL label on the MCC states the bracing and that is that. We ended up digging a trench and wrapping the incoming cables around the shipping container housing the MCC to increase the cable resistance. Seems simple, but it was 1200A so we had to increase the conductor size for the VD we also created. I won’t make that mistake again... I now always spec 65kA whether I need it or not (unless it’s higher of course).

How did that happen you say? The MCC specialist at the distributor asked me what the available fault current was, I said I didn’t know yet, so he put down 1,000A in his software because the software needs a value to move on and 42kA is the minimum. When I finally got the number, which was only 48kA by the way, he never went back and redid the quote. Even though it was a big MCC, the difference was only going to be about $400 total, I probably spent more than $4k on the extra cable.

 

[mayanees]

Where exactly is this stated? Is this in the listing documents? Who is to say that a properly selected CL fuse doesn't create a "new" and legitimate fault current value? Obviously the NEC has the tested combination requirements for circuit breakers that are interrupting fault current, but they is no requirement that I can see, at least in the NEC, for SCCR.

It's NEC 240.86 Series Rating
Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating.... and it gives two options for acceptance: A) Selected under engineering supervision in Existing Installations, which comes with a bunch of stipulations, and (B) Tested Combinations, which applies to new and existing. The difficulty with (A) is that the downstream device must remain inactive while the upstream, higher-rated device opens to avoid the presence of a dynamic impedance. The let-through charts assist with that determination.

 

[electrofelon]

It's NEC 240.86 Series Rating
Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating.... and it gives two options for acceptance: A) Selected under engineering supervision in Existing Installations, which comes with a bunch of stipulations, and (B) Tested Combinations, which applies to new and existing. The difficulty with (A) is that the downstream device must remain inactive while the upstream, higher-rated device opens to avoid the presence of a dynamic impedance. The let-through charts assist with that determination.

But that doesn't apply to SCCR which is what I am talking about in regards to the let thru charts and CL fuses. For that situation, you do not have dynamic impedance nor 240.86 telling you you can't.

 

[mayanees]

But that doesn't apply to SCCR which is what I am talking about in regards to the let thru charts and CL fuses. For that situation, you do not have dynamic impedance nor 240.86 telling you you can't.

But I'd still us the let-through charts to see if the let-through permits continued usage of the panel based on the SCCR.

 

[electrofelon]

Note the examples at the bottom of your post.

The fuse manufacturer's publications are living in the past.
This method is for large power circuit breakers that are intended to stay closed during most faults, sometimes these are called 30 cycle breakers or iron frame breakers. they were common 50 years ago, but they would rarely be found in new construction.
The other example is large bus bar assemblies, like the ones found in open-air substations or electrical room. They are not talking about the bussing in panelboards, switchboards and plug-in busway. Again this is for stuff seen in historical pictures rather than in new construction.

I do remember using the up-over-down methodology when I first entered this business and these types of installations were still fairly common. But haven't used it in more than forty years.

Jim, I am not seeing anything in my quotation or that article (which is from Littlefuse) that restricts or even mentions that type of equipment.

 

[adamaj]

Be careful of HOW this calculation is done. Every SCCR calculation uses some assumptions. For instance the infinite bus assumption doesn’t model the real (not infinite) source well nor cable impedances nor the effect of loads. So it is usually lower but could be higher! ANSI and some IEC methods handle inductive sources and sources but tend to underestimate cable impedances by assuming only inductance matters and not resistance.

The point by point method is suitable for spreadsheets and relatively simple for mere mortals, not just engineers with overpriced software.

Not sure if this was directed at me, or just put out there so that anybody viewing this thread gets the message, but above I outlined at how I'm arriving at these SCCA values. See below.

Past my edit time for the above, but wanted to add that the short circuit current I'm using for the utility feeder is not infinite bus (utility provided based off of their ASPEN model). All SCCAs in our system have been found using EasyPower. So little doubt about the accuracy.

EasyPower defaults to ANSI methods, but they provide a couple other calculation methods for finding SCCA.

OSHA regulations need to be met all the time. So if your equipment is overdutied, you are in violation of OSHA 1910.303.

I am not sure if you are using the infinite bus short circuit current which is typically provided by the utility and based on your wording, " We had our SCCA increase due to upgrades performed by the utility on their system. Unfortunately, original engineers designed for the SCCA that existed at the time, despite the size/impedance of the utility transformer having the potential for a much higher SCCA. " This would seem to indicate infinite bus. If it is, you should get the available short circuit current from the utility. You may find that your equipment is not overdutied using the available short circuit current.

You do not specify what type of equipment is overdutied but if panels/breakers there may be series ratings that can help.

After re-reading my post, I could see how that may be interpreted as using infinite bus. Just to be clear, the utility increased wire size to provide a neighbor of ours with increased capacity. This led to an increase in SCCA at our equipment which caused it to become inadequate (still short of infinite bus, but enough to become a problem).

This is a good learning experience for anyone working on 5-year Arc Flash revalidation that you should always contact the utility to get the most up-to-date value. In my case, I saw them changing out the wire, so I knew that I should ask the question immediately.

You don’t need a regulation to tell you that overdutied equipment is a bad idea. It is very common to see drives blown up when a motor goes to ground, contactors welded shut or turned to slag, and breakers similarly obliterated when a fault occurs far worse than the damage of the fault itself. Bus bars turned into art work as they bend and rip loose from the insulators is impressive but less common. It’s not like it won’t trip and burn down a building but the damage is much more immediate.

And if you think there is some wiggle room think again! SCCR is not like thermal damage. Effects are proportional to the square of the current so a small amount under is much more severe than expected.

You paint a vivid picture, and no doubt I'll be doing the same. However, if you're staring down the barrel of a million dollar mitigation project, it's best to show up with all the facts in place, is it not? The safety implications are justification enough in my mind, just want to have the full story when I present the information.

There's been a lot of great info in this thread, but the original intent was to chase down any specific regulations that apply to this situation.

I know that this is primarily an NEC forum, but the NEC is an installation code, and in this particular case we aren't changing anything. The equipment became underrated by actions performed by the utility, so it would seem to me that the NEC doesn't apply to this particular situation.