A stop from a presence sensing device should always be reset manually by an operator.

• A. True
• B. It depends on PLC model
• C. It can be reset automatically by PLC
• D. Only after a fault

Answer: (D)

A key idea here is the difference between a normal protective stop and a fault condition in safety circuits. A stop caused by a presence sensing device is a protective action to prevent injury when someone enters a hazardous area. After the hazard is cleared, the machine can be restarted through the normal start controls without forcing a manual reset each time. Manual reset should not be required for every protective stop.

Manual reset is appropriate only when the safety circuit has detected a fault and has latched the stop to prevent immediate re-energization until someone acknowledges and clears the fault. In that fault scenario, the operator verifies the issue and then resets the system before restarting, ensuring awareness of the fault condition.

So, the reset by an operator is appropriate after a fault, not after every stop triggered by a presence sensing device. That’s why the best understanding is that reset is required only after a fault.