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# Switchgear Short Circuit Rating Calculator

Free calculator estimate Switchgear Short Circuit Rating with contribution from upstream transformer and downstream rotating motor load.

Transformer
Secondary Voltage
Impedance
%

 Secondary Rating Current – Transformer SC contribution – Motor SC contribution – Switchgear SC Current –

## Basis and Assumption for Switchgear Short Circuit Rating Calculator

The calculator is based on following assumption:

• Neglect power source at primary side of Transformer,
• All cable or bus duct impedance is consider zero,
• Motor load contribution is estimated as recommended by IEEE Std 141 (chapter 4.5.4.1),
• Voltage factor c as IEC 60909 is ignored.

Clearly, the result from calculator is conservative when all above parameters accounted (i.e. cable impedance will lower short circuit current).

## Transformer Impedance

Typical impedance of transformer can be found as below table

Rated Power (kVA)Impedance (%)
25 to 6304
631 to 12505
1251 to 25006
2501 to 63007

## Transformer Short Circuit Current

Transformer short circuit current is calculated with data from short circuit impedance:

$\dpi{110}\textbf{I}_{sc}&space;=&space;\frac{I_{sec}}{Z}$

where:

• Isc : Short circuit at transformer secondary side
• Isec : Rating current at transformer secondary side
• Z : Transformer short circuit impedance (% or pu)

For other related calculator, pls access Transformer Rating Current Calculator or Transformer Efficiency Calculator.