Got a callback once on a strip mall unit that had been quiet for two years, panel showing a trouble on a NAC circuit that made no sense. Techs before me had chased it three times and each time it "fixed itself" after a visit. Turned out someone had moved a resistor from the last horn strobe on the run to a junction box halfway down the line, probably during a tenant buildout, and never told anyone. Circuit still worked most days. It just wasn't actually supervised anymore. Nobody would have known until a real fault happened and the panel had no way to see it.
That's the whole point of an EOL resistor, and it's worth being precise about it because a lot of guys can tell you it goes at the end of the circuit without being able to say why that matters.
What supervision actually means
A fire alarm circuit isn't just wire carrying voltage to devices. The panel wants to know the wire itself is intact, at all times, whether or not an alarm ever happens. It does that by pushing a small constant current through the entire loop and measuring it. The end of line resistor is what completes that loop and gives the panel a known value to check against. No resistor, no closed circuit, no number to measure. The panel is just guessing.
Think of it like a doctor checking a pulse instead of waiting for you to say you feel sick. The resistor is the pulse. Cut the wire anywhere along the run and that current disappears, panel calls it an open. Cross the two conductors together anywhere, current spikes toward a dead short, panel calls that too. Either way you get a trouble before anyone's relying on a device that can't report back.
Why it has to be at the last device, not the panel
I've had apprentices ask why we don't just stick the resistor in the panel cabinet where it's easy to get to. Makes sense on paper. Here's the problem: if the resistor lived at the panel, the panel would only ever be checking the two feed conductors right at its own terminals. Everything downstream of that point, every splice, every device, every foot of wire out to the far end of the building, would be completely unsupervised. A wire nutted joint could fall apart in the wall and the panel would never know, because current would still be flowing fine through the short loop back at the cabinet.
Putting the resistor at the physical end of the circuit forces the supervising current to travel through the entire wire run and every device on it before it can complete the loop. That's the whole trick. The resistor location IS the supervised boundary. Move it in from the end and you shrink what's actually being watched, even though the panel might look happy.
Reading the trouble before you drive out
This is where a lot of time gets wasted. A tech gets a call, "panel's got a trouble," and drives out blind. If you can get someone on site to read you a voltage at the panel's zone or NAC terminals first, you can usually tell open from short before you even grab your bag.
| Condition at the circuit | Typical reading at panel terminals (24V nominal Class B) | What's actually happening |
|---|---|---|
| Normal, EOL intact | around 24V DC, small steady current | loop closed, panel sees expected standby current through the resistor |
| Open circuit somewhere in the run | voltage present but current drops to near zero | break in a conductor, loose splice, or resistor knocked loose, current path interrupted |
| Dead short across the pair | reading drops toward 0V | line to line contact, wire nut failure, chafed insulation against a box, current spikes and panel pulls it as a fault |
I'd hedge on the exact numbers, panels differ and some techs will tell you their brand reads a little different at rest, so check your install manual and don't treat 24V as gospel on every board. But the shape of it, open equals current disappearing, short equals voltage collapsing, holds pretty universal across Fire-Lite, Notifier, Potter and Silent Knight Class B loops.
Once you know which one you're dealing with, the field work changes. An open sends you looking for a bad splice, a device that got backed out of its base halfway, or a resistor that walked off its terminals like the one at the strip mall. A short sends you looking for pinched wire, water intrusion in a box, or someone landing two conductors under one screw. Different search, different day.
Worth a mention while we're on wiring topology: this whole conversation only applies to Class B. Class A rings the circuit back to the panel on a second path, so a single break doesn't kill supervision the same way, though it costs you a lot more wire and conduit runs to get there. If you're deciding which one to spec on a job, that trade off is really its own article, we've got one on Class A vs Class B wiring if you want the full comparison. And if the trouble you're chasing is actually a NAC that's tripping on current draw rather than supervision, that's a different animal, worth checking our NAC circuit current limits piece too.
Anyway. None of this is exotic, it's just easy to treat as a black box until the day a resistor goes missing and costs you an afternoon. Full disclosure, the paperwork side of this exact job, matching device counts and circuit assignments back to what actually got installed, is the itch that got me building FireDeck. Hope this saves someone a truck roll.