Short-circuit current rating (SCCR)
Short-circuit current rating (SCCR) is the maximum short-circuit current a component or assembly can safely withstand when protected by a specific overcurrent protective device(s) or for a specified time. Short-circuit current ratings provide the level of fault current that a component or a piece of equipment can safely withstand (based on a fire and shock hazard external to the enclosure).
Without knowing the available fault current and short-circuit current rating, it is impossible to determine if components or equipment can be safely installed.
Individual panel components have an SCCR or interrupting rating. The component with the lowest SCCR in the panel limits the overall panel assembly rating. This SCCR rating has to be displayed on the nameplate and is usually calculated by the manufacturer or assembler of the control panel.
The manufacturer often does not know the switchgears future operating conditions (feeding transformer, type and length of cable network, change of conductor resistance caused by the temperature change, additional power to the grid, caused by motors, etc.)
It is the responsibility of the consulting or electrical design engineer to verify that all industrial control panels are applied in a manner such that the panel’s SCCR is greater than the system’s available fault current.
It is the responsibility of the industrial control panel manufacturer to provide accurate SCCR information to the consulting engineer and authority having jurisdiction through required labels and published technical information.
Determining the rated short-time withstand current (Icw) of a circuit of an assembly
The original manufacturer of the switchgear system, is responsible for the verification of the short circuit withstand capacity of the system components, e.g. the Icw value of the busbars.
Rated short-circuit withstand current is determined by the values Ik“, Icw, Icp, Icu. A verification of the short-circuit withstand strength is not required for the following:
- Assemblies having a rated short-time withstand current or rated conditional short-circuit current not exceeding 10 kA r.m.s.
- Assemblies protected by current-limiting devices 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 assemblies.
- 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%.
All other circuits shall be verified.” (IEC61439-1, 2009, p. 73)
The example based on the network in the Figure above shows how to determine the short-time withstand current Icw.
|Title:||Short-circuit withstand current rating for low voltage switchgear – Thesis for Bachelor’s Degree of Matthias Christian Schütt at Savonia University Of Applied Sciences|
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