How to calculate electrical fault or Short circuit current?

Short circuit current calculation. 

Purpose

It plays a very important role to protect & also safety of commercial and industrial electrical equipment like circuit breakers,transformers …etc.

What is short circuit current?

When two or more conductors are different phases come in contact with each other in a line, transformer or any other power element, then the part of the impedance is shunted out of the circuit due to which a large current flow in the un-faulted phases, such current is called the short circuit current.

It reduces the effect of impedance in the circuit while the current in the circuit rises.

  • The flow of large current will overheat the equipment.
  • The flow of short circuit current in the current carrying parts produces a force of electrodynamics interaction which may destroy or damage the equipment.

Fault MVA    = (Base MVA / % Z  ) × 100  

MVA     = KVA / 1000

Where,

Base MVA= Transformer Rating

%Z = Impedance + 10 % tolerance of z

       Isc    =Fault MVA / (√3 × Base KVA )

Where,

          I SC  = Value of fault current or short circuit current in KA

Base KVA= Transformer is secondary output voltage (like 6.6kv/440 v)

short circuit current
Single line Diagram

So let’s take Quick Observation

Calculate Fault current at each stage of following Electrical System SLD having details of.

  • Transformer Rating is 2.5 MVA.
  • Transformer Impedance is 7.25 %.

Solution:

Use the above  formula to convert MVA to KVA

2500/1000= 2.5 MVA

As per above formula

Fault MVA      =2.5 / (7.25 % + 10% tolerance)  ×  100

= 2.5 / (7.25 +0.725) × 100

=31.32 MVA

Now calculate Short circuit current

Isc    =     31.32 / (√3 × 0.44)    = 41.09 KA

Short circuit current  is  41.09 KA.

 

 

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