A knife switch should not be used to stop a motor because:

A knife switch should not be used to stop a motor because:

• A. it does not give thermal overload protection
• B. It may be damaged by arcing
• C. Its dual element fuse may blow
• D. It cannot carry the locked-rotor current

Answer: (A)

Stopping a motor safely requires protection against overheating. A knife switch is just a manual disconnect and has no mechanism to sense motor current or heat. Without thermal overload protection, the motor can overheat during overload or stall and suffer winding damage even after you kill the supply. That protective function—detecting excessive current and opening the circuit to protect the motor—is essential, and a knife switch cannot provide it by itself. Arcing can occur when breaking current in a motor circuit, which is another concern, but the main reason this device should not be used to stop a motor is that it does not give thermal overload protection.

Stopping a motor safely requires protection against overheating. A knife switch is just a manual disconnect and has no mechanism to sense motor current or heat. Without thermal overload protection, the motor can overheat during overload or stall and suffer winding damage even after you kill the supply. That protective function—detecting excessive current and opening the circuit to protect the motor—is essential, and a knife switch cannot provide it by itself. Arcing can occur when breaking current in a motor circuit, which is another concern, but the main reason this device should not be used to stop a motor is that it does not give thermal overload protection.