A production line uses an automatic conveying system. The conveyor motor control circuit has a normally closed (NC) limit switch installed at the end of the conveyor belt, intended to stop the motor if an item reaches the end. The motor starts and runs correctly, but does not stop when the limit switch is actuated. What is the most likely fault?

Question: A production line uses an automatic conveying system. The conveyor motor control circuit has a normally closed (NC) limit switch installed at the end of the conveyor belt, intended to stop the motor if an item reaches the end. The motor starts and runs correctly, but does not stop when the limit switch is actuated. What is the most likely fault?

Answer options:

• The limit switch contacts are wired in series with the start button
• The limit switch contacts are wired in parallel with the stop button
• The limit switch has an internal short circuit ✅ The limit switch contacts are wired in parallel with the motor starter coil holding contacts

Correct answer: The limit switch contacts are wired in parallel with the motor starter coil holding contacts

Explanation: For a normally closed limit switch to stop a motor, it must break the control circuit. If it's wired in parallel with the motor starter coil's holding contacts (seal-in contacts), then actuating the limit switch will not open the circuit to the coil because the holding contacts will continue to provide the power path. To stop the motor, NC control devices must be wired in series with the motor starter coil and the stop button.