Which device is commonly used to verify that a guard is closed before starting a machine?

• A. A pushbutton
• B. A limit switch
• C. A thermal overload
• D. A pressure sensor

Answer: (B)

To ensure a machine won’t start unless its guard is closed, you need a sensor that confirms the guard’s position and feeds that status to the control circuit. A limit switch serves this role as a guard interlock: it is placed so that when the guard is closed, the switch is actuated and completes the circuit to allow starting. If the guard opens, the switch changes state and prevents the machine from starting or immediately stops it, providing a reliable guard status check. This method is quick, durable, and directly tied to the physical guard. A pushbutton is a manual start/stop control and doesn’t verify guard status. A thermal overload protects the motor, not the guarding condition. A pressure sensor could detect contact, but it isn’t the standard, straightforward guard interlock used to authorize starting.