How are Short Circuit Power (Sk) and Impedance (Zk) Calculated?

Short circuit power Sk = √3·U·Ik, and the network impedance Zk = U/(√3·Ik) For example, at 400 V and a short-circuit current of 24 kA, Sk ≈ 16.6 MVA and Zk ≈ 9.6 mΩ. If Sk is given, Ik is calculated inversely using Ik = Sk/(√3·U).

Sk = √3·U·Ik · Zk = U/(√3·Ik)
Example calculation: 400 V, Ik=24 kA → Sk = √3×400×24000 ≈ 16.6 MVA; Zk = 400/(√3×24000) ≈ 9.6 mΩ.

Short Circuit Power & Impedance (Sk / Zk)

Calculate the short-circuit power (Sk) and the mains impedance (Zk) from the short-circuit current at a point; or vice versa.

Short circuit power Sk
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Short circuit current Ik
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Impedance Zk
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Notes: Sk = √3·U·Ik · Zk = U/(√3·Ik). Either Ik or Sk is entered, and the other is calculated. The values are for a symmetrical short circuit; the actual fault current varies with the R/X ratio and distance. The circuit breaker breaking capacity (Icu) should be selected based on Ik.
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How is it calculated?

  1. Enter the voltage (V).
  2. Enter the short-circuit current (kA) or short-circuit power (MVA).
  3. Press Calculate; Sk, Ik, and Zk will be displayed.

Frequently Asked Questions

What is short-circuit power?
It is the apparent power of a potential short circuit at a point (Sk = √3·U·Ik); it indicates the power/reliability of the network at that point.
How to find the current from a short circuit power?
Using the formula Ik = Sk/(√3·U), the distribution company usually provides the Sk value for the connection point.
Why is ZK important?
Mains impedance determines fault current and voltage drops; it is used in compensation and motor starting evaluations.

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