Rated Short-Time Withstand Current(Icw) & Conditional Short-Circuit Current(Icc)

The definition, clauses 5.3.4 and 5.3.5 both refer to an ASSEMBLY (I have BS EN 61439-1:2011 which is identical to IEC version)

Could you be more specific where it differs?

Icc: Rated conditional short circuit current. This is the rating that is associated with SCPD (Short Circuit Protective Device), the short circuit current initially flow and cleared by SCPD.

Icw: Rated short time withstand current. This is the rating of the Breaker that the assembly can withstand for a set time duration(say micro second) and breaker need not to be tripped.

rogerggbr, rkguptab, 7anoter4 --- thanks a lot for your prompt replies.

I'm referring to IEC 61439-1&2, Edition 2, 2011-08 which defines 'Icw' of a circuit of an assembly and 'Icc' of an assembly.

The point I'm confused about is the actual meaning of these two values.

As per my understanding is below, please correct me wherever I've been wrong.

Icw: is the withstand strength of an assembly without protective devices (protective devices (SCPDs) in incoming as well as outgoing circuits will be replaced with shortened copper bars during an actual test if conducted). Since the weakest point in a system defines the actual strength of an assembly, this value is defined for a circuit (in effect the smallest outgoing circuit).

Icc: is the strength of an assembly for a time that the incoming SCPD(located within or outside the assembly)breaker take to trip and isolate the assembly. This value can be made higher than Icw by introducing a suitable SCPD in incoming circuit. Here also I feel the strength of smallest circuit matters but not mentioned in IEC definition.

I couldn't understand the importance of "of a circuit of an assembly" part in IEC definition of Icw while its being ignored for Icc.

Please help me understand it better.

Thanks in advance.

Hi, thanks for reply, your reply has given me more information on the tests and concepts.

If a tested equipment name plate carry only one value of 'Icw' and 'Icc' what should we understand.

thanks in advance

Quoting Chris Smith, DIRECTOR at CIS ASSOCIATES Ltd

IEC 61439-1/2, and for that matter IEC 60439-1, withstand rating of busbars is based on the concept that the assembly must be able to withstand the prospective fault level at the point of connection. Thus an assembly has only one Icw rating.
When authoring IEC 61439-1/2 we were very much minded to facilitate the simplest possible design verification for the broadest range of manufactured variants possible from a given design. The objective being to eliminate the need for assemblies to have application specific design elements which were not verified or based on a verified design, as was often the case with IEC 60439-1 assemblies.
For assembly designs were a wide range of assemblies are to be made using verified designs each module of the design is verified and then the modules required for a given application brought together in the built assembly. Some module ratings may be derived from those which have been verified by testing using the rules/procedures defined in the standard. The principle remains that all elements of a IEC 61439-1/2 assembly should be verified designs.
It is therefore true to say that an assembly might be made up using, say, a horizontal busbar rated at 80kA 1s and a series of distribution busbars rated at 50kA 1s but then the assembly can only be rated at 50kA 1s because it is as strong as it's weakest point.
In much the same way many assemblies if tested without devices achieve a Uimp of 12kV but because the devices fitted are normally rated at 6kVimp or 8kVimp the assembly when complete can only be rated at 6 or 8kVimp depending on which devices are fitted.

Thanks for the discussions.

For accepting an LV Switchboard, are both Icw and Icc are required or any one of the 2 in the verification test is acceptable? IEC 61439-1 requires one or more of the 2.

I have recently come across a situation, the specification requires a low voltage switchboard with rated voltage 440V, 100kA/1 sec. The Manufacturer is able to provide a verification report for a switchboard rated at 415V, 100kA/1 sec. (ASTA test report indicates Icw 100kA/1sec and Icc 100kA/1sec @415V) For Icw, voltage is not the criteria, however, for Icc, the test voltage must be 1.05 times the rated voltage. Therefore, a switchboard tested for Icc at 415 V will not meet 440V specification.

Much appreciate your input to the above.