How to define no. of earth switch for isolators in switchyard

Check with your local regulatory agency. Frame a more clear question for them.

It all depends upon the level of safety that you would like to have. The best is to have earth switch on both sides of an isolator.

You must consult local electrical and substation standards and regulations for this answer. If you don't know, don't guess!

Consult an electrical consultant or a more senior substation engineer or manager on this project for more guidance.

The earth switch in an isolator is used as a safety factor........ generally isolators on the incomer side have earth switches... if both sides of an isolator has imp equipments then mostly 2 earth switches are used......... this is jus a rough idea........

generally for one isolator we have only one earth switch, the main purpose is to discharge the carrying current that is been present in the nearby equipment.

the main principle of earth switch working is O-O-C, AND C-C-O, i.e., when the circuit breaker is open then only the isolator should be opening so when the isolator is opened the current present in the equipment must be discharged to earth so the earth switch will close, same process will take when the breaker closes.