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Short-circuit current at the secondary terminals of a MV/LV distribution transformer

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General rules of electrical installation design
Connection to the MV utility distribution network
Connection to the LV utility distribution network
MV and LV architecture selection guide for buildings
LV Distribution
Protection against electric shocks and electrical fires
Sizing and protection of conductors
LV switchgear: functions and selection
Overvoltage protection
Energy Efficiency in electrical distribution
Power Factor Correction
Power harmonics management
Characteristics of particular sources and loads
PhotoVoltaic (PV) installation
Residential premises and other special locations
ElectroMagnetic Compatibility (EMC)


The case of one transformer

  • In a simplified approach, the impedance of the MV system is assumed to be negligibly small, so that:

Isc = \frac {In \times 100}{Usc}


In = \frac{S \times 10^3}{U_{20}\sqrt 3}


S = kVA rating of the transformer
U20 = phase-to-phase secondary no-load voltage
In = rated current of the transformer
Isc = short-circuit current in amps
Usc = short-circuit impedance voltage of the transformer in %.

Typical values of Usc for distribution transformers are given in Figure G33

Transformer rating (kVA) Usc in %
Oil-immersed Cast-resin dry type
50 to 750 4 6
800 to 3,200 6 6

Fig. G33Typical values of Usc for different kVA ratings of transformers with MV windings ≤ 20 kV


400 kVA transformer, 420 V at no load

Usc = 4%

In =\frac{400 \times 10^3}{420\times \sqrt 3} =550A

Isc =\frac{550 \times 100}{4} = 13.7 kA

The case of several transformers in parallel feeding a busbar

The value of fault current on an outgoing circuit immediately downstream of the busbars (see Fig. G34) can be estimated as the sum of the Isc from each transformer calculated separately.

It is assumed that all transformers are supplied from the same MV network, in which case the values obtained from Figure G33 when added together will give a slightly higher fault-level value than would actually occur.

Other factors which have not been taken into account are the impedance of the busbars and of the cable between transformers and circuit breakers.

The conservative fault-current value obtained however, is sufficiently accurate for basic installation design purposes. The choice of circuit breakers and incorporated protective devices against short-circuit and fault currents is described in Selection of a circuit-breaker .

Fig. G34Case of several transformers in parallel