Which of the following should be used under no-load conditions?

• A. Circuit breaker
• B. Rewireable fuse
• C. Isolator
• D. Air-break switch

Answer: (C)

Isolator or disconnecting Switch: An isolator is a switch that is designed to open a circuit under no-load condition. Its main purpose is to isolate one portion of the circuit from the other and is not allowed to be opened while current is flowing in the line. Such switches are used on both sides of a circuit breaker so that its repair works or replacement could be done.
Note that an isolator is never opened until the circuit breaker in the circuit is opened and it is closed before the circuit breaker is closed. If an isolator is switched OFF when a high current is flowing through the circuit, a heavy spark will be produced. This heavy spark may break the supporting insulator of the isolator which may cause a fatal accident to the operator.