End-of-Line Resistor Supervision: Open, Short and Normal

Why this matters

An end-of-line resistor is the only thing that lets a hardwired panel tell a closed door from a cable that has been jumpered or crushed. Put it where it belongs and a staple through the cable, a pinched wire behind a baseboard or a deliberate jumper at a window all show up at the keypad. Twist it under the panel terminal, as plenty of installers did to save a trip to the far end, and the panel reads "normal" no matter what happens to the cable. The system looks finished, passes a casual test, and has every device past the first short silently out of service.

What the panel is actually measuring

A supervised zone input is a resistance meter with three bands. The panel puts a small voltage on the zone terminal, reads what comes back, and sorts it:

• Normal: the loop reads at or near the EOL resistor value the panel's documentation calls for, plus a few ohms of wire.
• Open: the loop reads very high or infinite. A contact has opened, or a conductor is cut.
• Short: the loop reads near zero. The two conductors are touching somewhere between the panel and the resistor.

The resistor value is whatever the panel's documentation specifies, and makers differ, so there is no universal number to carry from one panel to another. Each panel also accepts a band around the nominal value before it calls the zone faulted; that band is in the documentation too.

What the panel DOES with open and short depends on the zone type and arm state (see related: Zone Types and What Each One Does When Tripped). On a burglary zone, open while armed is an alarm and open while disarmed is "not ready". A short is reported as an alarm on some panels and as a trouble or tamper on others, per the documentation. The point of the resistor is that the panel can tell the three apart at all.

Where the resistor goes, and why

The rule: the resistor sits at the last device on the loop, not at the panel. Normally closed contacts are wired in series on one conductor; the resistor closes the loop at the far end; the other conductor comes straight back.

PANEL      ONE CABLE, TWO CONDUCTORS USED
Z3  o---[C1 NC]-----[C2 NC]-----[C3 NC]---+
        front dr    side win    back win  |
        30 ft       +25 ft      +25 ft  [EOL]
                                          |  in C3 box
COM o-------------------------------------+
        return conductor, no devices on it

WRONG: RESISTOR AT THE PANEL
Z3  o-[EOL]-[C1 NC]-----[C2 NC]-----[C3 NC]---+
COM o-----------------------------------------+
  any short from C1 onward still reads EOL value

Trace the correct loop for each fault. A contact opens: the series path breaks, the panel reads open. The cable is cut anywhere: open. The two conductors are shorted together anywhere along the run: the current takes the short and never reaches the resistor, so the panel reads near zero. Every point on the cable is inside the supervised path.

Now trace the wrong one. The resistor is permanently in series right at the terminal. Short the two conductors at the side window and the panel still sees the resistor, because the resistor is on the panel side of the short. It reads normal. C2 and C3 can now open all night and nothing happens, because the short bypasses them. The open-circuit protection survives (cut the cable and you still get open), so a tech who tests by opening each door sees every zone work, leaves, and never learns the cable is unsupervised for shorts.

That last point is why the wrong layout lasts for years. A walk test opens contacts; it does not short cables. Only a meter or a deliberate short at the far end tells the two layouts apart.

The wire is a rounding error, until it is not

Twenty-two gauge copper runs about 16.1 ohms per 1,000 ft of single conductor. The loop above is 30 + 25 + 25 = 80 ft of cable, so 160 ft of conductor round trip:

• 160 ft x 16.1 ohms / 1,000 ft = about 2.6 ohms of wire.

Against an EOL resistor in the kilohm range, 2.6 ohms is noise. That has two consequences:

• A reading well off the resistor value is not "long wire". Ordinary residential runs cannot add enough copper resistance to move a kilohm-range reading meaningfully. If the loop reads hundreds of ohms high, look for a corroded splice, a crimp on varnished wire, or a resistor lead broken inside its sleeve.
• A short reads like a few ohms, not zero. A staple 45 ft out reads about 2 x 45 x 16.1 / 1,000 = 1.45 ohms. On the meter that looks like zero, and that is correct.

A splice that drifts is the one wiring fault that hides between bands. A connection that adds a few hundred ohms one humid week and less the next can wander in and out of the panel's acceptance window, which shows up as an intermittent fault with no open device. If a zone faults and restores with nobody near it, meter the loop at the panel and then at each splice.

A takeover, metered

A house comes over from another dealer. Zone 3 (front door C1, side window C2, back window C3, laid out as in the diagram) "always works" per the customer. The new panel is going in, so every loop is getting metered.

Account on test first. Lifting zone wires faults the zone and can send a signal; put the account on test with the central station before touching the terminals (see related: How to Put an Alarm Account on Test and Take It Off). Zone wiring is low voltage, but the panel enclosure carries the transformer leads; leave the transformer unplugged or, if it is hardwired to 120 V, leave that side to an electrician where local rules require.

1. Meter at the panel with the zone disconnected. Lift both zone conductors so the panel's own sensing voltage does not corrupt the reading. With all three openings closed, the meter shows the documented resistor value. Looks fine.

2. Find the resistor. Under the old zone terminal there is a resistor twisted onto the conductor. That is the wrong layout. Remove it and meter the bare loop: about 2.6 ohms with everything closed, open-circuit with any one device open. The contacts and the cable continuity are good.

3. Find the last device. Opening contacts cannot tell you their order, because any one of them breaks a series loop the same way. Look instead for the device box with only ONE cable entering it. Mid-run devices have two cables (in and out); the end of the run has one. Here that is C3, the back window.

4. Move the resistor. Land it across the two conductors inside the C3 contact, insulated, with the splice made the way the contact maker specifies. The panel end now gets the bare conductors.

5. Prove it is downstream. At the panel, with the zone still lifted, the meter reads the resistor value plus 2.6 ohms. Have a helper jumper the two conductors inside the C2 box for a moment: the reading drops to about 2 x 55 x 16.1 / 1,000 = 1.8 ohms, because the jumper now bypasses C3 and the resistor. With the old panel-end resistor, that same jumper would have left the reading at the resistor value. That drop is the single test that proves supervision.

6. Land, then confirm with the station. Reconnect the zone and plug the transformer back in, and confirm the keypad shows AC power with no trouble. Confirm the zone reads normal at the keypad, open each device for an open, and short the pair at a device for the short response the documentation says to expect. Confirm the central station received the zone with the right zone number, then take the account off test.

What the takeover found matters for the customer conversation. Their system "always worked" because every walk test only opened doors. A wire fault anywhere between C1 and C3 would have taken out two windows and they would never have known.

What changes the answer

• Normally open devices (some panic buttons, some floor mats) close to alarm, so they sit across the pair in parallel and the resistor still terminates the far end. Mixing them on an NC loop has its own rules; see related: Normally Closed vs Normally Open Devices on One Zone.
• Double end-of-line adds a second resistor so one pair reports tamper separately from alarm; see related: Double End-of-Line and Tamper on One Zone.
• Fire initiating circuits are supervised for open and short too, but the devices, the resistor and the response belong to NFPA 72 in the edition your jurisdiction has adopted, and fire work is often separately licensed. Do not move an EOL on a fire circuit under a burglary job.
• Wireless sensors have no loop and no resistor; their supervision is a timed check-in. See related: Wireless Sensor Supervision, Signal and Battery Life.
• Panels with a "no EOL" option accept a bare NC loop. That option exists for compatibility, not as a choice: it reads open but cannot detect a short anywhere. Use it only where the documentation requires it for a device, and note it on the zone sheet.

How to verify you got this right

At the panel, with the zone disconnected: the reading equals the documented resistor value plus the wire you calculated, and a jumper across the pair at the next-to-last device drops it to a few ohms. Reconnected, each device opens to a fault, a short at the far device produces the documented short response, and the central station logs every event under the right zone. Write the resistor location on the zone sheet ("EOL in C3, back window") so the next tech does not have to find it again.

References

• Panel manufacturer's installation manual (resistor value, acceptance band, open and short response by zone type)
• NFPA 72, National Fire Alarm and Signaling Code, in the edition adopted by your jurisdiction (fire initiating circuits)
• See related: Security + Alarm Systems Reference; Zone Types and What Each One Does When Tripped; How to Wire a Hardwired Zone With the Resistor at the Last Device