Short-circuit impedance uk: what 4 % or 6 % means for your installation

On a PNACHO data sheet you will find uk = 4 % for oil-immersed units up to 630 kVA and uk = 6 % above that and for all cast-resin units. Few parameters are as important for the design of the low-voltage switchgear – and few are as often ignored.

Definition

The short-circuit impedance (impedance voltage) is the primary voltage, expressed as a percentage of rated voltage, that drives rated current through the transformer when the secondary is short-circuited. It is measured in the routine test at 75 °C reference temperature. It combines the resistive part (load losses) and – mostly – the leakage reactance between the windings.

Consequence 1: fault current on the LV side

With an infinitely strong MV network, the maximum symmetrical short-circuit current at the LV terminals is simply

Ik = In / uk

Rating (0.42 kV)	Rated LV current In	uk	Max. LV fault current Ik
250 kVA	344 A	4 %	8.6 kA
630 kVA	866 A	4 %	21.7 kA
1 000 kVA	1 375 A	6 %	22.9 kA
1 600 kVA	2 199 A	6 %	36.7 kA
2 500 kVA	3 437 A	6 %	57.3 kA

The LV switchboard, its busbars and the circuit breakers must be rated for this current (plus the contribution of motors). This is why the impedance rises to 6 % at larger ratings: at 4 % a 2 500 kVA transformer would deliver 86 kA – beyond most standard switchgear.

Consequence 2: voltage drop under load

The same impedance causes a voltage drop across the transformer when current flows. At full load with cos φ = 0.8 the drop is roughly 0.8 × ur + 0.6 × ux; for a 630 kVA unit with uk = 4 % (ur ≈ 0.7 %, ux ≈ 3.9 %) that is about 2.9 % – within the ±5 % most installations tolerate, but worth checking for long LV feeders and for PV plants, where the voltage rises under generation.

Consequence 3: parallel operation

Transformers in parallel share the load in inverse proportion to their impedance. IEC practice: uk of the units should not differ by more than 10 % of each other, and the power ratio should not exceed 3:1. A 630 kVA (4 %) and a 1 000 kVA (6 %) unit can be paralleled only if the 630 kVA is accepted to run overloaded first.

Can uk be changed?

Yes, within limits. A higher impedance (e.g. 6 % instead of 4 % on a 630 kVA) reduces the fault current and lets you reuse an existing switchboard; a lower impedance reduces voltage drop. Both are custom windings with a longer lead time. The standard values on the PNACHO-O and PNACHO-D product pages follow IEC 60076-5 and the usual grid-operator specifications, so they fit standard switchgear without recalculation.