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SHORT CIRCUIT FAULT:
In power system, if the current flowing path is shorted by touching the earth or touching another conductor which causes huge current flows, is known as short circuit fault.
Short circuit faults are five types. 1) Single phase to earth. 2) Between two phases, 3) Two phase and earth, 4) between three phases, 5) Three phase and Earth.

In case of 1st 3 faults, the value of fault currents are not same in 3 phases are known as unsymmetrical fault. Majority of faults are these types.
In case of last 2 faults, the values of fault current are same in 3 phases, known as symmetrical fault. This fault current has 120˚phase displacement.

When a short circuit occurs, a heavy current called short circuit current flows through the circuit.

Relay setting of the protection system, made by the value of unsymmetrical fault.
Rating of circuit breaker depends on the value of symmetrical fault.

Probabilities of fault:
Single phase to ground —15%
Phase to phase —15%
Two phase to ground —10%
Three phase to ground —5%
Three phase short —5%

Causes of short circuit fault:
Short circuit fault occurs in the system due to
1) Break down of insulation of generator, transformer, etc.
2) Breakdown of transmission lines or any equipment.
3) Due to ageing, insulation may become poor.
4) Due to improper installation.
5) Insulation failure due to lightning surges.
6) Mechanical damage by externally.
7) Excessive heating due to over loading.
8) Touching trees with transmission line.

Effect of short circuit fault:
When a short circuit fault occurred in a system, two conditions happen,
a) The current increases rapidly to excessive high value.
b) As well as system voltage decreases to a very low value.

Now this abnormally high current causes excessive heating, may result fire which may damage equipments. Sometimes this heavy current may produce an arc, which can melt the conductor too.
On the other hand if voltage becomes very low for some time, generator becomes unstable for the system. Motor may become off.
So, due to avoid the effect of short circuit fault, it is desirable to disconnect the faulty section as quickly as possible.

Why calculation of short circuit current is necessary?
Majority of fault in power system is short circuit fault. Calculation of short circuit is required due to
i) As short circuit is cleared by circuit breakers, it is necessary to know the maximum value of short circuit current for the rating of the installed circuit breaker.
ii) Relay setting also depends on the value of fault current.
iii) To install any protective reactors, magnitude of short circuit current is vital.
iv) Other equipments selection like bus bar, current transformer etc. are depends on the value of short circuit current.

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