The impedance voltage uk is the percentage of rated voltage you must apply to the primary to drive rated current through the secondary while the secondary is short-circuited. It is a single number that decides both how far the output voltage sags under load and how large a fault current the transformer lets through. Most distribution units sit between uk = 4 % and uk = 6 %.
What the percentage actually measures
Measuring uk means measuring the transformer internal series impedance, expressed relative to its own rating. A 630 kVA unit with uk = 4 % needs 4 % of rated voltage to push full-load current into a bolted short, because the winding resistance and — dominantly — the leakage reactance oppose it. Because it is a relative value, two transformers of different ratings with the same uk do not have the same impedance in ohms.
The practical consequence follows directly. The prospective symmetrical short-circuit current at the secondary terminals is roughly the rated current divided by uk expressed as a fraction. At uk = 4 % that is 25 times rated current; at uk = 6 % about 16.7 times. A 630 kVA, 400 V transformer at 4 % therefore delivers in the order of 22 kA into a terminal fault, against roughly 15 kA at 6 %. Upstream network impedance reduces both figures a little, but the transformer usually dominates.
The trade-off you are actually making
Low impedance gives a stiff supply: less voltage drop when a large motor starts, better regulation across the load range and easier compliance with voltage limits at the far end of a long LV network. It also makes every downstream fault more violent.
High impedance limits fault current, which allows switchgear and busbar systems with a lower short-circuit withstand — often a genuine saving — and reduces the mechanical stress on the transformer itself. The price is a larger voltage drop under load, more sensitivity to motor starting and a slightly worse regulation figure.
Choosing a value
- Start from the short-circuit withstand of the LV switchgear and busbar system, in kA rms for 1 s; the transformer must not exceed it.
- Check the voltage regulation at maximum load, including motor starting, welding plant and EV-charging steps.
- If two units may ever run in parallel, match their impedances within about 10 %, otherwise load sharing is unequal.
- Confirm the protection still sees the smallest expected fault: raising uk lowers fault current and lengthens clearing times.
Manufacturers shift uk by changing the winding geometry and the gap between the HV and LV windings, so non-standard values such as 8 % or 10 % are available on request. It is a design decision taken before manufacture, not something that can be adjusted afterwards.
Short-circuit withstand is a separate promise
A given uk tells you how much current will flow. IEC 60076-5 governs whether the transformer survives it. Units are designed, and where specified tested, to withstand the resulting thermal and electromagnetic forces for a defined duration of typically 2 s. Those forces scale with the square of the current, which is why impedance and short-circuit withstand are always discussed together.
Treat uk as a system parameter, not a transformer detail. Fix the switchgear rating and the acceptable voltage drop first, then specify the impedance that satisfies both — and state it on the enquiry, because a 4 % and a 6 % unit of the same kVA are not interchangeable.
