**SYMMETRICAL FAULT**:

As we already known that, symmetrical faults are* a) short circuit between three phase and b) three phase to earth fault.*

These types of fault raise symmetrical fault current with 120˚ phase displacement.

Though these types of fault are rarely occurred in power system but it imposes heavy duty on the circuit breaker.

The fault current in system is limited by the impedances of the system. This impedance includes impedance of transmission line, transformer, and generator.

As maximum impedances are reactive and nearly 3 times of resistance, reactance plays the vital role, whereas resistances are negligible.

**SHORT CIRCUIT CURRENT:**

As resistance is negligible, reactance is the only element of the circuit,

So, short circuit current, I_{SC}= V/X where, X = reactance.

Now, we know that, transformer, reactor, generators are usually shown by percentage reactance for short circuit calculation,

So, %X=IX/V*100

%X can also expressed in KVA,

%X=$$\frac { KVAX }{ { 10KV }^{ 2 } }$$

or, X=%XV/I100 where I = full load current,

X = reactance in ohm/ ph,

V = voltage.

So, short circuit current, I_{SC} = 100I/(%X)

We can say, short circuit current of the system is full load current multiplied by 100/%X.

**BASE KVA, WHAT IS BASE KVA?**

We have seen that, %X depends on the equipments KVA rating. As there are several equipments with different KVA rating, it is necessary to calculate %X in a common KVA rating. This common KVA rating is known as base KVA.

It should be remembered that the value of base KVA is quite unimportant as short circuit current does not depends on the value of base KVA. For simplicity of calculation the value is chosen.

**SHORT CIRCUIT KVA**:

At the point of fault, the product of short circuit current and system voltage expressed in KVA, known as short circuit KVA.

Short circuit KVA in single phase circuit is = VI_{SC}/1000.

For 3 phase, =3 VI_{SC}/1000.

As, I_{SC}= 100I/%X,

= 3VI*100/1000*%X

= base KVA*100/%X

So, short circuit KVA is base KVA multiplied by 100/%X.

**REACTOR CONTROL:**

As we know, the short circuit currents are abnormally high, it may damage several equipments, so, to limit the short circuit current in the permissible value of circuit breaker, reactors which is nothing but an additional reactance is connected in series with the system. It limits the faulty current for which we can install lower rating circuit breaker.

This reactors are used in series with Generator, in bus bar or in series with each feeder.