What is the purpose of End of Line resistors (EOLR) in an alarm loop?

• A. To power the control panel during outages
• B. To speed up signal transmission
• C. To supervise field wiring for open or short circuit conditions and differentiate them by a known resistance
• D. To provide surge suppression at the last device

Answer: (C)

End of line resistors serve to supervise the alarm loop by establishing a known resistance at the far end of the circuit so the control panel can continuously verify wiring integrity. By placing a resistor with a specific value at the end of the loop, the panel expects a consistent resistance when everything is normal. If the wiring is opened, the loop resistance shoots up toward infinity, signaling an open condition. If a short occurs, the resistance drops to a much lower value, signaling a short condition. Because the panel knows the exact end-of-line resistance to expect, it can distinguish between these states and reliably detect faults anywhere along the loop.

This concept is distinct from powering the panel during outages, which is a function of the power supply and backup battery, not wiring supervision. It also isn’t about speeding up signal transmission or providing surge suppression at the last device. The End of Line resistor’s purpose is specifically to supervise the loop and differentiate open versus short conditions by a known resistance value.