07 — Tools
Short-circuit current at the transformer
The short-circuit current determines the breaking capacity the downstream protective devices need. For a first estimate, three figures from the transformer nameplate are enough.
Result
- Short-circuit current at the terminals
- —kA
- Rated current
- —A
- Ratio Iₖ / Iᵣ
- —
Please fill all fields with valid numbers.
A guide, not a design to standard. The applicable standards and case-by-case verification govern.
Worked example
The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.
Inputs
- Rated apparent power
- 630 kVA
- Nominal voltage, low-voltage side
- 400 V
- Short-circuit voltage uₖ
- 6 %
Result
- Short-circuit current at the terminals
- 15.2 kA
- Rated current
- 909 A
- Ratio Iₖ / Iᵣ
- 16.7
Formula
- Iᵣ = Sᵣ / (√3 · Uᵣ)
- Iₖ = Iᵣ / (uₖ / 100)
Assumptions and standards
- The calculation assumes an infinitely strong upstream source – the real network limits the current further, so the result is on the safe side.
- Only the transformer is considered, not cables, busbars or motor contribution.
- A binding design requires calculation to IEC 60909.
Frequently asked
Why is the value higher than in the network study?
Because the upstream network impedance is neglected here. In practice it reduces the short-circuit current, often by 5 to 15 %. For selecting breaking capacity, erring high is the right direction.
What does uk on the nameplate mean?
The voltage that has to be applied on the high-voltage side so that rated current flows on the low-voltage side under short circuit – given as a percentage of nominal voltage. A small uk means a high short-circuit current.
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.
Contact
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