we use crowbar circuit to protect the load from over voltage by shorting the output to ground so can some one explain why we do not use over volatge protection so when we get over voltage the unit will turn off instead of crowbar

 

If the control circuitry for the power supply were working properly, there would not be a need for the crowbar. If a failure mode of the power supply results in lack of control and an increase in the output voltage, the crowbar simply shorts the output and trips overloads or blows fuses. Also, crowbar circuits don't need very many components. It is an economical protection for anything powered by the power supply.

What did you have in mind as an over-voltage protection?

 

A crowbar is a protection of last resort.

It is there for when a regulator worth several hundred dollars has failed and a rack worth 10's of thousands of dollars or more is at risk.

 

If a power supply goes overvoltage, turning it off, if that is even possible, will probably not drop the voltage below the safe level in a timely fashion, due to charge storage on filter capacitors, etc. The purpose of a crowbar is to bring the voltage below the maximum tolerable voltage as soon as possible. The quickest way to do this is generally by applying a short circuit across the power supply, which is what a crowbar does.