The electrical energy is being sold to the consumers all over the World by either government, Semi Government, Public Limited or by Private Utilities. All of them have the same goal i.e. "Reliable, Power Supply to the Consumers"
No consumer whether domestic, commercial or industrial wants interruption in their supply. We see that in spite of the fact that neither the producers nor the consumers like interruptions, they still occur.
Interruptions are result of short circuits which can occur for many reasons and result in equipment failure. The effects of such short circuits and extend of the fault can be minimized by suitable design of the power system and related protective equipment.
Proper co-ordination of the operating and tripping characteristics is essential to achieve selective tripping of protective devices connected in series in the network and thus assure the high level of reliability of supply required by consumers.
The subject is essential for the proper selection of circuit interrupting devices ( circuit breakers, fuses, and fuse switches load breaking devices) and the associated apparatus such as bus bars connections, current transformers, cables and boxes etc. The main source of fault power is generating plants but large contribution could be made by induction motors of larger sizes or by many induction motors of smaller sizes.
The faults Power supplied by induction Motors are maximum in the first half cycle of short circuit. The source of excitation however is rapidly consumed so that the fault current decays from its peak value in the first half cycle until, after say five cycles or so, it is relatively significant. The magnitude of fault current is controlled by the Combined impedances of all machines, transformers cables etc. In case of generators, transformers , synchronous motors and reactors the resistance is very small compared with its inductance and therefore can be ignored.
