Question

what is the use of plug key in circuit

Answer 1

A plug key is an electrical component which acts like a switch. A small metallic key is provided which is inserted in a metallic base to switch on the circuit and if it is taken off from its base then the circuit get switched off. 

Answer 2

it used to complete the circuit if it is open then the current will not flow if it is closed then current will flow.Also it works as a switch for the circuit.