What is the primary purpose of an end of line resistor (EOLR)?

• A. Provide power to the control panel.
• B. Detect fire alarms.
• C. Terminate protective loops or zones.
• D. Connect mechanical wires.

Answer: (C)

An end of line resistor provides a defined impedance at the far end of a supervised circuit so the control panel can continuously monitor the integrity of the entire loop. By placing a known resistance at the end, the panel expects a specific total resistance when the loop is intact. If the loop opens or shorts, the total resistance changes, and the panel can detect the fault and trigger an alert rather than misinterpreting the condition. This is why the resistor is placed at the end of the line—to help the panel distinguish normal operation from faults. It’s a passive component, not a power source, and it isn’t used to detect fire directly or to simply connect wires, but to supervise the loop for reliable security sensing.