In order to switch-off and EHV circuit for maintenance, the following sequence is adopted:
Right Answer is:
Open the circuit breaker, open the isolator, operate the earth switch
Switching sequence – to turn on or off a bus bar, equipment, power plant requires proper steps and skill. A single mistake would cause an accident.
Step 1: Open the circuit breaker: CB is a device that can make and break the circuit under normal as well as under faulty conditions manually or automatically.
Step 2: Open the isolator: Isolator is a device that always operates under no-load condition. This is because it has no provision for arc quenching.
Its function is to isolate the circuit after operation of the circuit breaker and discharge the grapes charges to earth through the earth switch.
Step 3: Operate the Earth switch: After opening the circuit breaker and manually disconnecting the isolator the next is to engage the earth switch. Its function is to discharge any remnant charge due to residual magnetism which might be lethal to humans and expensive testing equipment.