Calculators
Calculate network impedance from short-circuit power
The network operator quotes a short-circuit power, but the design needs an impedance in ohms. This calculation translates between the two, and back again when only the short-circuit current is known or comes from a measurement.
Result
- Network impedance
- —Ω
- Short-circuit current I_k″
- —kA
- Short-circuit power S_k″
- —MVA
- Resistance R
- —Ω
- Reactance X
- —Ω
Please fill all fields with valid numbers.
A guide, not a design to standard. We give no warranty for the correctness of the results. The applicable standards and case-by-case verification govern.
The grey values are an example. Simply enter your own.
Matching services
Calculating is the easy part.
A formula gives you a number. Designing a plant also demands installation method, grouping, discrimination, the standards in force and a look at the installed base. That is what we take on.
Method and questions
Network impedance
Upstream impedance from the network operator figure, in both directions.
Discuss a projectWorked example
The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.
Inputs
- Known quantity
- Short-circuit power S_k″
- Nominal system voltage
- 400 V
- Short-circuit power S_k″
- 15 MVA
- Short-circuit current I_k″
- 21.7 kA
- Voltage factor c
- 1
- R/X ratio of the source
- 0.1
Result
- Network impedance
- 0.011 Ω
- Short-circuit current I_k″
- 21.7 kA
- Short-circuit power S_k″
- 15 MVA
- Resistance R
- 0.001 Ω
- Reactance X
- 0.011 Ω
Formula and standards
Formula
- From power Z = c · U² / S_k″
- From current Z = c · U / (√3 · I_k″)
- Reverse I_k″ = c · U / (√3 · Z)
- Split X = Z / √(1 + (R/X)²), R = (R/X) · X
Assumptions and standards
- The voltage factor c and the R/X ratio are entered here. Both depend on the voltage level and the case considered. The standard you apply governs.
- The calculation gives the symmetrical initial short-circuit current, not the peak short-circuit current.
- The impedance applies at the point for which the network operator gave the figure. Everything downstream adds to it.
Which value should I use for c?
It depends on what you are asking. For the maximum short-circuit current, that is, for the breaking capacity of protective devices, a value above 1 is used. For the minimum short-circuit current, against which the disconnection condition is checked, 1.0 is usual. The values are in the standard you apply.
Why do I need the split into R and X?
Because impedances cannot simply be added. Your cable impedance is mostly resistance, the network impedance mostly reactance. Only with both separated can they be added correctly and then recombined into a magnitude.
Is a loop impedance measurement the same thing?
Not quite. The measurement covers the whole loop up to the measuring point, that is the network plus every cable ahead of it. For the upstream impedance alone you have to measure at the point of supply or subtract the cable contributions.
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