Connecting an Industrial Control Panel with a Low SCCR

[richwaskowitz]

Suppose your customer contracts you to install an industrial control panel (covered under Article 409) in their plant. You of course look at the manufacturer's marking and see that it has a pretty low SCCR, of say only5 kA. Maybe you don't actually calculate the available fault current, buy experience tells you that for the size of the bus duct and transformer feeding it, you're fairly sure that there is a lot more than 5 kA available. How do you proceed? What kind of available fault currents do you folks see in mid-sized to large industrial plants?

The reason I am asking this is, we are the manufacturer of the industrial control panel in this scenario. What if due to the design of the panel or because of some particular components in the panel, its SCCR comes out really low, like the 5 kA mentioned above. Without knowing where this panel is going, or what the available fault current at the point of connection might be, I am concerned that we are leaving ourselves open for a real problem if the installer says he can't connect it. In your experience, what levels of fault current do you typically see in larger facilities on 1600 to 2000 amp buses? Please advise.

 

[ron]

It depends on the building and how far up the distribution the panel is connected.
For example, in NYC, the fault current available at the service entry is 200kA. If your 1600A bus comes off of a service entrance or just 1 or 2 distribution levels down, the fault current would still be 65kA+.

 

[richwaskowitz]

That's what I'm talking about.....So what do you, the installer do in that case? Has anyone ever had this happen? How was it resolved?

 

[GeorgeB]

You of course look at the manufacturer's marking and see that it has a pretty low SCCR, of say only5 kA.

When my job included design and commissioning of control panels, we ALWAYS used either class (R)K5 or J (200kAIC) fuses in a fused disconnect to eliminate any issues there. Individual (motor and control) circuits either used the same or the "KTK" class CC.

We probably didn't adequately brace wiring from the disconnect load side, but didn't know anything about that at the time (this was early 1980s). Line side wiring was minimal length with the disconnect as close as practicable to the installers choice of rear/top/side conduit entry.

Do you have options to add/change fuses and use current limiting devices?

 

[richwaskowitz]

Based on UL508A supplement SB, simply putting current limiting fuses in the panel disconnect cannot always improve a low SCCR of another component in the panel. Even putting J class fuses in the bus plug can't bring the available fault current down to 5 kA. The peak let through of a set of LPJ100s is 9kA (UL 508A table SB4.2). What I'm saying here is that it has already been determined that there is nothing feasible (or at least cost effective) which can be done to the panel to bring the "assembly SCCR" of the panel up to say 45 or 100 kA and now it gets to the job site and it can't be connected. Now what?

 

[ron]

Does the Control panel UL standard let you use let through current to theoretically determine series listing for your panel?
Typically the code requires tested combinations. We are not able to use the let through "up over down method" anymore when equipment hasn't been tested with that current limiting device.

 

[kc8dxx]

Once bought a large drive panel that needed much more that 5KVA SCCR, based on the drive size. Vendor shipped it to us blindly applying that seemingly universal 5KVA SCCR sticker. We pointed out that the drive couldn't function with such a small SCCR, rejected the shipment, and returned it.

We put it like this to them, "You can't ship us a 3/4 ton pickup, then stick a label on it that says it can only haul a couple bags of groceries."

 

[jim dungar]

Does the Control panel UL standard let you use let through current to theoretically determine series listing for your panel?
Typically the code requires tested combinations. We are not able to use the let through "up over down method" anymore when equipment hasn't been tested with that current limiting device.

Yes, UL508A allows the use of some 'let-through' currents. However, using the specific UL508A values for Ipeak and I?t is not the same thing as the general 'up-over-down' method.

 

[augie47]

Would the correct path not be to have an engineer select protection to meet the withstand rating of the equipment somewhat in line with 110.22 ?

 

[jim dungar]

Would the correct path not be to have an engineer select protection to meet the withstand rating of the equipment somewhat in line with 110.22 ?

Almost apples and oranges.
110.22 is about AIC ratings (110.9).

The assembler of the control panel 'determines' the SCCR of that panel (409.110(3)). The most common method used, is the procedure outlined in UL508A.
The installer must make sure that the available SCA at the panel does not exceed the SCCR on its label (110.10).

 

[augie47]

Almost apples and oranges.
110.22 is about AIC ratings (110.9).

The assembler of the control panel 'determines' the SCCR of that panel (409.110(3)). The most common method used, is the procedure outlined in UL508A.
The installer must make sure that the available SCA at the panel does not exceed the SCCR on its label (110.10).

Jim, Is there any reason, NEC wise, that the E/C or customer could not use enginering specs to select the means of reducing the SCA by installing the engineered fuse/ reactor, etc. ?

 

[ron]

Gus,
Engineering specs such as added impedance is fine.
A fuse (or other current limiting protective device) will not do the trick unless it is a tested combination rating with the down stream equipment.

 

[augie47]

Ron, That was what had me wondering. So, I am at the OP's question again. What are the acceptable steps to address this. It is an everyday occurrence in our local industry. They never specify a required SCCR for their equipment. As an inspector I can enforce 409.110 and obtain the SCCR on the equipment.
Heretofore we have had the E/C supply documentation from a registered engineer that steps have been taken to assure the SCA is acceptable.
Historically this has been accomplished by current limiting fuses.

 

[jim dungar]

Jim, Is there any reason, NEC wise, that the E/C or customer could not use enginering specs to select the means of reducing the SCA by installing the engineered fuse/ reactor, etc. ?

Ideally, the SCCR label on the equipment, per 409.110(3), already takes into account an 'engineered/designed' solution.

According to UL508A (2001 version) a 30A current limiting type J fuse will let through 6kA when up to 50kA is available. Working backwards, UL says that a 30A (max) Class T fuse is the only 'branch circuit' protective device that will limit the let through current to 5kA.

So, if the label says an SCCR of 5kA, reducing the available SCA is the only possible remedy short of having the equipment re-investigated/redesigned.

 

[richwaskowitz]

To answer your question, Ron "Does the Control panel UL standard let you use let through current to theoretically determine series listing for your panel?". The answer is no.

Once again, I know that it is not difficult to obtain 100 or even 200 kA SCCR by using the correct components and design practices BEFORE the panel is built and shipped. I am interested in the fallout that occurs when that is not done for one reason or another, either out of necessity or ignorance, and what if anything the customer or installer can or has done to fix the problem.

 

[augie47]

Rick, I'm on the same page as you.
Jim and Ron seem to have a handle on this that I don't.
I encounter the same situation often. The customer knows zilch about SCCR, they just bought the thing and want it set up. It was not mentioned by supplier or purchaseer at the time of purchase.
The E/C knew nothing until he got the call that the equipment is set.
Assuming we know the available AIC and the equipment SCCR, what is the "acceptable" NEC method to install the equipment.
For years the locally acceptable means has been to allow the E/C to select a fuse with an high enough AIC rating for the supply and a let-thru below the equipment SCCR, but, obviously that's not the correct way.

 

[mcclary's electrical]

I bid a job on a bakery recently and I submitted three bids because the fault current at this time is unknown. (same as your job) I had one price for 5-12kAIC, a second price for 12-25k AIC, and a third price for 25-65k AIC. I explained this plainly to the customer and it seems to pose no problem.

 

[jim dungar]

Rick, I'm on the same page as you.
Jim and Ron seem to have a handle on this that I don't.
I encounter the same situation often. The customer knows zilch about SCCR, they just bought the thing and want it set up. It was not mentioned by supplier or purchaseer at the time of purchase.
The E/C knew nothing until he got the call that the equipment is set.
Assuming we know the available AIC and the equipment SCCR, what is the "acceptable" NEC method to install the equipment.
For years the locally acceptable means has been to allow the E/C to select a fuse with an high enough AIC rating for the supply and a let-thru below the equipment SCCR, but, obviously that's not the correct way.

The only remedy is to, do something to add impedance (which will reduce the amount of available fault current) or have the equipment rebuilt with the required SCCR.

Simply adding current limiting fuses has never been (well at least not in the 30+ years I have been in the industry) the 'correct' solution, although it was commonly done and accepted. My understanding, is this action was part of the reason SCCR labeling requirements were include when article 409 was added to the code.

In most cases, but not all, a 5kA SCCR label means the manufacturer has not 'tried' to design their product by taking advantage of possible listed series combinations.

 

[ron]

Ideally, the SCCR label on the equipment, per 409.110(3), already takes into account an 'engineered/designed' solution.

According to UL508A (2001 version) a 30A current limiting type J fuse will let through 6kA when up to 50kA is available. Working backwards, UL says that a 30A (max) Class T fuse is the only 'branch circuit' protective device that will limit the let through current to 5kA.

So, if the label says an SCCR of 5kA, reducing the available SCA is the only possible remedy short of having the equipment re-investigated/redesigned.

Jim's statement works if the research to get this listing occurs at the UL panel builders facility and comes on the equipment label. You cannot perform this analysis in the field.
I make the installing folks run additional length of wire, that almost always works.

 

[jim dungar]

Jim's statement works if the research to get this listing occurs at the UL panel builders facility and comes on the equipment label. You cannot perform this analysis in the field.

Says who?

Article 409.110(3) says whoever assembles the panel needs to provide its SCCR. But, I agree you can probably not field determine an SCCR that is different than the one already on the panel, but the original assembler should be able to.