How to calculate short circuit current for transformer

Short circuit current for transformer:

Generally we know short circuit current is nothing but low impedance path having abnormal contacts between two points of different  potentials. so short circuit calculation is very important to protect the equipment and people from high mechanical force and high heat stress. so we need to calculate the short circuit current rating of the equipment and choose the right breaker for protection of the equipment’s.

short circuit current transformer


The short circuit current calculation of  transformer we need following details,

  1. KVA/MVA rating of transformer
  2. Secondary voltage of the transformer
  3. impedance value of the transformer

short circuit current Isc= Isec / Z

Isc – short circuit current

Isec – secondary side Full load current of the transformer

Z – Impedance of the transformer

Let us assume we are going to calculate short circuit current of the following transformer 3.3kv/440v , 100kva rating, impedance Z=5%

so first we need to calculate secondary full load current Isec,

Isec   = kva x 1000 / 1.732 x Vsec

=  100 x 1000 / 1.732 x 440

Isec   =   131.2 Amps.

short circuit current of transformer,

Isc  =  Isec / Z

= 131.2 / 0.05

= 2624.31

Isc = 2.6KA

in this way we can easily calculate the short circuit current of the any rated transformer.

short circuit current rating almost 20 times greater than full load current of the transformer.

so we need to choose the circuit  breaker with breaking capacity of at least 2.7KA.


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