Relay

This depends on the specific overcurrent relay you are using.

You have to find the manufacturer's information for that relay and the time / current curves supplied there.

Not enough information.

It also sounds like you need to increase the time setting you allocate to your homework!

Unfortunately, this guy appears to have a job.

Improving Earthing

Makes ya wonder what he did in class, doesn't it?

thankx a lot for ur good comments......................

but if we will use normal inverse curve then what will be the time after which relay will operate???????????

thankx.

There is no "normal" inverse curve that applies to all devices. You have to get the curve for the specific device you have, from the manufacturer.

If this is homework or a learning / practice situation and a specific device has not been mentioned, then I'd suggest you get the curve for any device, use it, and in your answer specify that your answer is based on the curve for such and such a device.

One of the other posters posted links to some curves for some specific devices for a specific manufacturer.

Please describe the curve that is in use.

It depends upon:

i) The type of tripping charecteristics you have selected in the relay (viz) Definite Time or Inverse Time and even within inverse-time, what is the slope - Normally Inverse or Very Inverse or Extremely Inverse or Long Time Inverse, etc.

ii) The acutal magnitude of the fault current

Although I don't usually vote a GA for responses that are links to other people's work, I feel that the two articles you cited were to the point and appropriate to the OP's level of understanding.

He would do well to take your advice seriously, and seek it in the future.