**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

I_{sc} =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**)

**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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