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Wiring An End-Of-Line Power Supervision Relay on 4-Wire Smoke Detector

 What is a 4-Wire Smoke Detector and why do I need a Relay?


A 4-wire smoke detector uses 4 wires to operate. 2 wires are utilized for power and 2 wires are used as initiation circuit. Most modern 4-wire smoke detectors use a wiring base and a smoke head that is installed on the wiring / mounting base. 

Just as on a 2-wire smoke detector, it is critical that in the event a smoke head is removed, the panel creates a trouble condition. On a 2-wire system (as seen below), when the smoke head is removed the positive leg is broken and the End-Of-Line resistor is no longer present on the circuit. More about installing 2-wire smoke detectors.


A 4-wire smoke detector system uses the same wiring scheme for the power, but the initiation or alarm circuit wires straight through in a ladder design. Any short along the ladder will result in an alarm condition. Any open will create a trouble.




Improper 4-Wire Smoke Detector Wiring

The image below shows 2 4-wire smoke detectors wired together. With the heads removed the power circuit is not complete. You can see that the initiation circuit would provide a NORMAL condition even though the smoke heads are removed. To supervise the status of a 4-wire smoke detector system you need to install an End-Of-Line Power Supervision Relay to break open the initiation circuit, creating a trouble. 

4-Wire smoke detectors wired but without heads installed



The End-Of-Line Power Supervision Relay is used to switch the End-Of-Line Resistor in and out of the initiation circuit. When energized by the smoke power circuit, the relay and resistor work in series to provide proper end-of-line resistance. 

Proper 4-Wire Smoke Installation with End-of-Line Power Supervision Relay
4-wire smoke detector wiring shown with end-of-line power supervision relay


Shows 4-wire smoke circuit with head removed.

Another wiring option would be to extend the 4-wire circuit back to the Alarm Panel and install the relay at the panel location. This provides a simpler design for servicing. It is important to verify that the relay you are installing is rated for the voltage of your system. Residential smoke detectors often operate at 12 VDC while a commercial system is powered with 24 VDC.