I am installing a security system (Honeywell/Ademco Vista 15p), and trying to figure out an installation of a 4-wire smoke detector.

According to the installation manual (http://www.nationwidesecuritycorp.com/wp-content/uploads/FA160C-Install-Manual.pdf, section 2-6, diagram in paragraph 3). It is necessary to install an end-of-line power supervision relay. My question is what's the purpose of that relay?

Does control panel uses that relay to verify the presence of power supplied to the 4-wire smoke detector? Or is it used to verify that power to the smoke detector is removed when smoke detector is being reset?

Also, I am little confused as to how do I connect multiple smoke detectors? Do I connect them in series or in parallel?

And one more - whit's the "heat detector", and do I connect it just like other 2-wire smoke detectors?

1 Answer 1


The end of line relay is intended to make sure all the detectors have power and all detectors are present.

The wiring for each detector is such that all wires are connected IN and OUT. At the end of the line a final device, the end of line relay, ensures the the entire circuit is powered. Note that the IN and OUT wiring to each detector provides the supervision to the entire string. If you were to say pigtail the wires at each detector and then only use IN terminals then each detector is NOT supervised because that wiring error allows the circuit to function with a detector removed.

For each 4-wire smoke detector, you must actually connect 8-wires to each detector. To maintain supervision you cannot shortcut this with pigtail style wiring.

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Heat detectors are just simple 2 wire devices that don't require power. While they technically can be mixed with 4 wire smoke detectors on the same zone, for the system to remain listed you would need to use a separate zone for 2 wire heat detectors.

  • Thank you. It answers my question. Bonus question: when panel resets a smoke alarm by driving #17 high - that will open the EOL relay, won't that trigger fault (trouble, open) condition on the zone? Or is panel smart enough to know that it caused the open condition, and will ignore it?
    – THX-1138
    Feb 1, 2017 at 16:04
  • The panel is smart enough to ignore the trouble condition during a reset cycle.
    – Tyson
    Feb 1, 2017 at 16:07
  • Remember that this is technology that was invented before addressable devices existed.
    – Tyson
    Feb 1, 2017 at 16:09
  • I just noticed that detectors are to be connected in series on the power line. I would expect them to be connected in parallel. What's the idea behind that? Is the current draw so low that there isn't significant voltage drop?
    – THX-1138
    Feb 5, 2017 at 15:41
  • 1
    Yes. That prevents a detector from being removed from the circuit. If wiring was pigtailed and made a single connection to a pair of + and - terminals the circuit could not be considered "supervised" since a single detector could become missing by loose connection. Remember "supervised" means making sure all detectors are present. Also remember this technology pre-dates addressable devices. The wiring methods and EOL relay together create supervision.
    – Tyson
    Feb 5, 2017 at 15:57

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