When would you use a normally closed safety interlock in a machine control system, and what happens on door open?

• A. A safety interlock is used to guarantee a safe state when access is opened.
• B. Opening the door energizes all actuators for rapid operation.
• C. It is used to monitor, not affect power.
• D. Opening the door interrupts power or de-energizes actuators to stop the machine.

Answer: (D)

Using a normally closed safety interlock in a machine control system provides a fail-safe way to stop hazardous motion whenever a guard or door is opened. When everything is closed, the interlock keeps the control circuit complete and the machine can operate. If the door is opened, the interlock interrupts the circuit, which cuts power to the machine or de-energizes the actuators. This immediate loss of power stops all movement, preventing exposure to danger. The normally closed configuration also means a fault that breaks the circuit will naturally result in a stop, reinforcing safety.