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Determining the Short Circuit Current Rating (SCCR) of an unrated panel that has power and control

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bdn2004

bdn2004

Electrical
Jan 27, 2007
792
US
I'm asked to determine the SCCR of what's been deemed an "Industrial Control Panel" and must meet the qualifications of NEC 409. I understand to use the "weakest link method" UL508A to do this. But in NEC 409 there is this exception to 409.110 (4): "Short-circuit current ratings are not required for industrial control panels containing only control circuit components".

My question then is this:

The box I'm looking at is a custom built heat tracing panel. It has contactors and a panelboard that's connected to the heat tracing circuits. And it has bunch of 120VAC hard-wired control relays that interact with the external thermostats, the pushbuttons, indicating lights, etc and the contactors that power the heat trace circuits.

If this were just a control panel - no rating is needed. Can I assume then that the 120VAC control portion of this panel is ok and doesn't figure into the "weakest link" ?



 
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in the UK, Control Panels have to undergo S/C withstand analysis checks too, in accordance with BS EN 61439 (also in the EU generally)

Unsure what NEC provisions are, but from a safety point of view, you need to know this panel will safely stand the effects of a short circuit at the point of connection

I.Eng MIMMM MIET MIPowerE AIOSH
 
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Here's some more technical data on
Contactor_Rated_Operational_Current_udoffi.jpg
this panel, Rated Operational Current:
 
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I take this to mean 190A at 208V....so the SCCR of this particular part is 8 X Ie = 8 X 190A = 1520A.
That's a far cry from the 5kA that the panel is said to be rated at. This is a part number ABB AF 190-30-11-13 contactor.
 
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The panel must be considered in its entirety - the fact it has different internal sections (form types) for power and control is irrelevant for certification, certainly against BSEN61439

Your extract from NEC409 references purely control panels (purely PLC + I/O etc, no power devices such as transformers, breakers, contractors etc)

BS EN 61439 contains some small control panel exemptions from S/C withstand testing, namely:
a) Assemblies having a rated short-time withstand current or rated conditional short-circuit current not exceeding 10 kA r.m.s.
b) Assemblies, or circuits of assemblies, protected by a current-limiting device having a cut off current not exceeding 17 kA at the maximum allowable prospective short-circuit current at the terminals of the incoming circuit of the assembly.
c) Auxiliary circuits of assemblies intended to be connected to transformers whose rated power does not exceed 10 kVA for a rated secondary voltage of not less than 110 V, or 1,6 kVA for a rated secondary voltage less than 110 V, and whose short-circuit impedance is not less than 4%.


I.Eng MIMMM MIET MIPowerE AIOSH
 
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Wattyeng....

"Your extract from NEC409 references purely control panels (purely PLC + I/O etc, no power devices such as transformers, breakers, contractors etc)"

I know...

I'm just saying by that same logic....eliminate all that same small control stuff in a panel like this from making it's overall rating. This has about 50 items in this panel. I've looked I don't see that info on the standard data sheets anyway. It's all connected with #14 wire. How can the hazard be that much there?

But do evaluate the power circuits including that ABB contactor shown. Which at first glance looks too small to me for a 5kA rating...unless it's dependent on that fuse listed. I don't know if that fuse is built in or it has to be installed in series with it.
 
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I don't have any coverage of the NEC aspects of it, but a couple of considerations:
1. Feeding supply to heat tracing circuits would not be considered a control aspect, rather a power supply aspect to me. Relays and so on for indication to me would be control aspects, but if the contactors used to switch the power supply are in the cabinet then I wouldn't see it as eligible for the exemption.

5kA rating to me implies that it both needs to be able to withstand 5kA for a through-fault beyond the enclosure, as well as have a protective device able to break 5kA. I am not surprised that a contactor does not have this rating, as they are normally paired with an appropriate protective device.

jraef has mentioned this 5kA trap previously in regards to selection of rated, tested components as an assembly, although I don't recall the details.

EDMS Australia
 
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bdn2004 - yes agree - with BSEN61439, if the control panel is small and fits within the exempted criteria, then no S/C withstand needs to be assessed / calculated / tested, provided the supply network at the POC is also less than stated exemption limits - it is not acceptable to install a 10kA RMS rated panel at a POC where the PSCC is 11kA!

The info you present does show the ABB Contactor rated well below the panel designated rating, however, that is quite normal (and is purely the contactor rating), providing the panel enclosure itself has been tested to withstand 5kA, from say an integral busbar or internal power cable fault

BSEN61439 predecessor used to allow TTA/PTTA with individual components rated themselves, rather than as a complete panel - now the panel has to be verified with all components in situ inside the panel at its specified SCCR

I.Eng MIMMM MIET MIPowerE AIOSH
 
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Well, you are looking at an IEC number for UL and any IEC data is useless and meaningless when trying to apply UL508A.

Find the UL data for the contactor. Actually, find it for all components. If a component is not UL listed then the panel can't be rated by following UL508A.

UL508A defines what is control circuit and what is power circuit. That determines what can be ignored when calculating the SCCR rated as per UL508A.

 
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I don't know if this panel was tested or not. It came to the facility with an engraved nameplate that says SCCR: 5kA. I'm aware that this is the minimum rating for a lot of unmarked things per that UL508A standard.

When we run the EasyPower software on this panel the available short circuit value at the point of connection is about 7kA.

So the question is what do we do to remedy that? Knowing that the 5kA value is the minimum you wonder: was this panel really tested or was the 5kA rating just stamped onto it? Retrofitting things is costly and time consuming.


 
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Ok I contacted ABB and they sent me an SCCR Coordination Table. The part is UL listed with a Listing card number: UL-E36588. I look at this sheet in that table....

ABB_SCCR_Compatible_2_h3d1du.jpg
 
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Yes, that gives co-ordinated ratings. Any UL listed contactor has to meet a minimum SCCR requirement based on HP, which 5kA is the lowest rating.

You could possibly put another set of fuses in front of the panel protection following supplement SB to increase the SCCR rating.

Note that changes will result in an unlisted control panel that should be field inspected in theory. The original panel listing doesn't cover field changes.
 
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Per the coordination chart the ABB T5_400_W circuit breaker will protect it. But it's already got a panel mounted MCCB upstream of it. See the drawing. That's item #25. But that's a SQ-D product. So I assume the only thing that is compatible with the ABB contactor (Item #5),is that ABB MCCB listed in the coordination chart? The fuses in the coordination chart look like they are more generic.

So it looks like our choices are: 1) put in protection external to the panel 2) install new fuses upstream of the contactor in series with the existing MCCB 3) change out the MCCB to an ABB 4) change out the contactor to a Sq D if they have something similar. Can't think of much else.



ABB_MCCB_nytaxr.jpg
 
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The rest of the power circuit goes to an internally mounted power panel that is the source for the heat tracing circuits. And then there is a bunch of 120V controls for buttons and lights and horns, etc.

I think the moral of the story to most designers should be: use the same manufacturer for as much stuff as possible to avoid this situation.
 
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bdn2004 said:
I don't know if this panel was tested or not. It came to the facility with an engraved nameplate that says SCCR: 5kA. I'm aware that this is the minimum rating for a lot of unmarked things per that UL508A standard.

When we run the EasyPower software on this panel the available short circuit value at the point of connection is about 7kA.

So the question is what do we do to remedy that? Knowing that the 5kA value is the minimum you wonder: was this panel really tested or was the 5kA rating just stamped onto it? Retrofitting things is costly and time consuming.
Click to expand...

The SCCR rating of the panel is 5kA as that is what is on the panel. That is what you have to live with.

Just because people confuse terms such as available fault current, Have you modeled the system using the available fault current obtained from the utility? This would not be on a chart that the utility gives you. If you are not, then the fault current may be less than the 7kA.

If not, one method to reduce the fault current would be to put an isolation transformer upstream of the panel. You will have to size it and then model to see what impedance is needed to reduce the fault current below 5kA.
 
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wbd,

They are living with the SCCR (5kA) that's on the panel - and that's what's needs a remedy. And yes the modeling is per the Utility's provided data on the primary side. This panel is 208Y/120V and is fed through an 112.5 kVA isolation transformer. That's 312A FLA. It feeds that 200A panel mounted breaker. Why they didn't pick a 75kVA with a 208A output - I don't know. It is what it is.

If we can't modify the panel to buck up the rating, and then re-label it, what's the use of all these standards and requirements ? This is a custom built one of a kind thing.

Just at first glance, the cheapest option of those I'm seeing is to add the recommended fuses upstream of the contactor. Main breaker - new fuses - contactor.




 
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