Which is a part of the circuit that consists of a conductor that melts easily and breaks the connection when the current exceeds the predetermined value? 

The correct answer is option 3.

Rewireable fuse

• A fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit.
• It is a metal wire or strip that melts when too much current flows through it, thereby stopping or interrupting the current.

Isolator

• An isolator is a manually operated mechanical switch that isolates the faulty section of the substation.
• It is used to separate a faulty section for repair from a healthy section in order to avoid the occurrence of severe faults.

Circuit breaker 

• A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit.
• Its basic function is to interrupt current flow to protect equipment and to prevent the risk of fire.

Air-break switch

• An air break switch is a known break in the power line system that can be manually opened or closed.
• If closed, electricity flows past the break and on down the line.
• If the switch is open, electricity from the substation stops at that point.