End-of-line (EOL) device

An end-of-line device, historically a resistor, sits at the far end of a supervised fire alarm circuit on a Canadian fire alarm system and gives the panel a known electrical value to measure against, so a broken conductor reads differently from a healthy, quiet circuit. Without it an open circuit and a normal standby circuit look identical, because neither is drawing device current. The resistance value is set by the control unit manufacturer, not by code: Mircom's FX-3500, for example, ships a 3.9 kilohm end-of-line resistor plate. It has to be installed at the electrical end of the circuit, after the last device, or the wiring beyond it is unsupervised.

What it is and why the value matters

On a conventional two wire circuit the panel cannot tell "nothing is happening" from "the wire is cut" unless something at the far end keeps a small current flowing. The end-of-line device provides that. It is usually a resistor, sometimes a diode or a small module on a notification circuit, and its value is dictated by the control unit, not by any Canadian standard.

That last point catches people. There is no universal Canadian end-of-line resistance. Mircom's FX-3500 installation manual lists an MP-300 end-of-line resistor plate at 3.9 kilohms. Another manufacturer will specify something else entirely. Fitting the wrong value gives you a circuit that reads as trouble, or worse, one that reads healthy while sitting outside its supervisory window and will not detect a real change. Always take the value from the manual for the panel actually installed, and record it, because the next technician will need it.

Where the Canadian requirement chain lands

No Canadian code says "install a 3.9 kilohm resistor." The chain is indirect and worth understanding. The Building Code requires that a fire alarm system be electrically supervised, placed at Article 3.2.4.9(1) in the Ontario Building Code. CAN/ULC-S524 sets the installation rules that make supervision work, including how far a single fault may spread. CAN/ULC-S527 governs the control unit, which is what defines the supervisory current and therefore the end-of-line value. The resistor is simply the cheapest way to satisfy all three.

S524 is paywalled, so we do not reproduce its circuit class definitions or its exact fault tolerances here. Where the end-of-line device physically goes on a Class A return loop, and whether one is needed at all, is governed by that text and by the panel listing. Read both.

How it behaves in the field

The device belongs at the electrical end of the circuit, which is the last device in wiring order, not the device that is furthest away in the building. Those are frequently different points, and a riser that reads left to right on paper often runs a different way in the ceiling. Reading the riser diagram before you pull is the difference between supervising the whole circuit and supervising most of it.

The classic failure is an end-of-line resistor left in the panel, or clipped across the terminals of the first device, because it made the trouble light go out during rough-in and nobody moved it. Everything downstream of that point is then invisible to the panel. A cut conductor in that section produces no trouble and no alarm. It is a silent, total loss of protection on that leg, and it will pass a casual visual inspection because the panel is normal.

The second common one is a resistor installed under the terminal screw alongside the outgoing conductors, so the circuit is looped through rather than terminated. Lift the device and the supervision stays intact through the resistor, hiding the missing device.

How it relates to the neighbouring terms

An open circuit fault is exactly what the end-of-line device exists to reveal. A short circuit fault shows up as loop resistance collapsing below the expected value, and a ground fault is invisible to it, which is why the control unit carries a separate earth leakage detector. On an addressable system the SLC supervises itself through device polling, so end-of-line devices largely disappear, though monitor and control modules driving conventional legs still need them.

For the wire itself, see what Section 32 requires of fire alarm cable in Canada.

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