End of Line (EOL) resistor used for what?

• A. A power supply for the alarm panel.
• B. A wireless module for remote access.
• C. A tamper detector.
• D. An end of line resistor used to terminate protective loops or zones. The purpose is to allow the control panel to supervise the field wiring for open or short circuit conditions.

Answer: (D)

An End of Line resistor is used to terminate a protected loop so the control panel can continuously supervise the wiring for opens or shorts. By placing a known resistance at the far end of the zone, the panel expects that exact value during normal operation. If the circuit is opened or shorted anywhere along the run, the total resistance changes, and the panel detects a fault. This arrangement helps prevent false alarms and ensures reliable detection of wiring problems. It’s not a power supply, wireless module, or tamper detector—the resistor provides the defined impedance the panel needs to monitor the loop.