What if the I/P voltage across the coil is 230v AC, to be more specific to control the speed of fan we use regulator( step type in my case), as we rotate the regulator knob, for fraction of second the supply to the fan is cut OFF during this time the current flows in coil in reverse direction. How can we prevent this current from entering main I/P supply.

 

If the supply to the fan is interrupted, then how can there be a current back to the supply?
What fan is it ? What power supply is it? Inverter, mains?

I know that some inverters can have a problem with intermittently connected fans (that are still running). For mains there should be no problem.

 

What if the I/P voltage across the coil is 230v AC, to be more specific to control the speed of fan we use regulator( step type in my case), as we rotate the regulator knob, for fraction of second the supply to the fan is cut OFF during this time the current flows in coil in reverse direction. How can we prevent this current from entering main I/P supply.

There will be a momentary arc across the AC isolate switch contacts which will naturally extinguish in the same way that occurs when you isolate any appliance having an inductive load component from the AC mains supply.

Isolating an inductive load doesn't cause the current to reverse. Perhaps you are thinking of the back (induced) emf.

If you are thinking about speed control of a fan then a triac based controller would be a typical (& probably simpler) solution.