In protective loops, the end-of-line resistor is used to monitor for which conditions?

• A. Open or short circuit conditions.
• B. Voltage fluctuations.
• C. Power outages.
• D. Wiring color coding.

Answer: (A)

The end-of-line resistor provides a known impedance at the end of the alarm loop so the supervising panel can detect wiring problems. When the loop is intact, the panel sees this expected resistance. If a wire opens, the circuit becomes open and the resistance spikes toward infinity, triggering a fault. If a wire shorts, the resistance drops toward zero, also triggering a fault. This is why it’s used to monitor for open or short circuit conditions.

Voltage fluctuations aren’t reliably tracked by the end-of-line resistor, since they affect power or signal levels rather than the continuity of the loop. Power outages involve the system’s power supply, not the wiring continuity the resistor is guarding. Wiring color coding is simply a wiring convention and not a monitoring function.