End of Line Resistor: Role in Conventional Circuits

End of line resistor is a resistor of defined value fitted at the far end of a conventional fire alarm circuit, allowing the panel to verify that the circuit is intact by measuring the current that flows through it under quiescent conditions.

On a conventional zone, the panel applies a small DC voltage across the circuit and reads the resulting current. With the end-of-line resistor in place, a known steady current flows; an open-circuit fault (broken cable) drops that current to zero, and a short-circuit fault (cable damage or a device in alarm) raises it sharply. By comparing the measured current against pre-set thresholds, the panel distinguishes healthy, alarm, and fault states on the same pair of conductors. The resistor is typically 4.7 kilohms but values from 1 kilohm to 10 kilohms appear depending on manufacturer; the panel must be configured to match.

End-of-line resistors are not used on addressable loops, where supervision happens via the digital protocol on every device. Their persistence in conventional systems is one reason conventional installations remain attractive for small projects: a multimeter and a known resistor value are enough to fault-find the entire zone. Lose the resistor (or fit the wrong value) and the panel reports a permanent fault until corrected.

For the wider context, see conventional fire alarm systems. For a longer technical treatment, see end of line resistors explained.