When should a motor-circuit switch be used?

A motor-circuit switch is specifically designed to interrupt the current of a motor. This type of switch is crucial in applications that involve motors because it provides a safe and reliable means to disconnect power to the motor circuit, ensuring that maintenance or servicing can be conducted without the risk of electric shock or equipment damage.

When a motor is stopped, having a dedicated switch that can handle the typical inrush currents and operational demands of the motor is essential. Interrupting a motor’s current requires switches that are rated for motor loads and can handle the inductive kickback produced when the motor is turned off. That's why the use of a motor-circuit switch is specifically indicated during the interruption of motor current scenarios, making option B the appropriate choice.

Using a motor-circuit switch for regular operation of any electrical load or for controlling lighting circuits only does not align with the switch’s intended purpose. Similarly, while an isolating switch may be necessary in certain contexts, it does not specifically address the unique demands and safety considerations associated with motor operation, which the motor-circuit switch is designed to fulfill.