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Topic Title: SCCR vs Icw/Ipk Short Circuit Withstand
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Created On: 11 July 2013 06:35 PM
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 11 July 2013 06:35 PM
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clarky280

Posts: 84
Joined: 10 June 2008

Hi,

I'm looking for some help regarding NEC short circuit current ratings.

I'm reasonably familiar with the short-circuit rating definitions under IEC 61439;

* Icw is the RMS value and duration of the fault current that the assembly can withstand before experiencing thermal damage
* Ipk is the peak current that the assembly can withstand before experiencing mechanical damage.

These two values can be used to determine if an assembly is adequately rated given the fault current characteristics as well as the let-through characteristics of an upstream protective device.

For example, the I2t withstand of the assembly can be calculated using the Icw rating and the duration, and this can be compared against the energy let-through of the device at the maximum short-circuit level (determined from the energy limitation curve of the device). Then, the peak withstand value (Ipk) of the assembly can be compared against the peak current let-through of the device (determined from the current limitation curve of the device). If these two ratings are higher than the let-through values, then the assembly is adequately rated.

I am less familiar with the NEC term short-circuit current rating (SCCR), and do not know how to apply this in the same way. This value is never stated with a duration of fault, so I'm not sure how the thermal withstand of the assembly can be determined. Also, it does not provide a peak withstand value - is there a standard multipler to get the peak rating as per IEC 61439?

How can I determine if this assembly is adequately protected when there is a current limiting device upstream?

If anyone could provide clarity on this it would be much appreciated.

Thanks.
 11 July 2013 08:31 PM
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OMS

Posts: 19900
Joined: 23 March 2004

As I understand it, the SCCR is based on compliance with the UL methodology.

If the panel as a whole has been UL tested to a given SCCR it will (or at least may have) taken account of any energy limiting protective device that forms part of that assembly. ie it then cannot have a further claim made on for the effects of upstream device I2t (as the upstream device I2t is peculiar to the device and did not form part of the tested assembly.

To answer your question, I don't believe you can determine the rating of SCCR based on I2t of the upstream device - you have to go with the prospective value as if that device was not present.

It's why you tend to find UL switchgear with an active incoming device rather than an inactive device ie non auto device.

That's my view on it but I don't a lot with NEC systems - at least not at the level of detail you want. We do have a team out in US though, I'll try and find out from them what thier approach is - although when we've worked on mid east jobs together using both NEC and IEC codes, this is an area where I've had many an argument with the cousins across the pond. It's my view that the panel is exposed to the current limiting effect of the upstream breaker and as such, that's what the SCCR needs to meet - they disagree.

Regards

OMS

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