4-Wire Smoke Detector Wiring

Whereas 2-Wire smoke detectors use a single pair of wires for both power and sensor circuits, 4-Wire smoke detectors separate the power and the sensor circuit into 2 pairs.   

The main downside to 4-wire systems is the ability to monitor system integrity. It is critical that any device removal, wiring damage or power loss result in a fault or trouble condition at the panel. The image below illustrates typical 4-wire detector wiring on a two-piece detector. A two piece detector uses a base with a removable head. The power side wires the same as a 2-wire system. If a head is removed, the power opens.

Not all 4-wire system use a separate base and head. Both positive wires are placed under the same terminal. Removing the head does not open a circuit, removing the wires from the terminals opens the circuit, creating a trouble condition.

4-Wire smoke detector wiring
Typical 2 piece 4-Wire smoke detector wiring. Shown with smoke head removed.

 
The image below shows  the wiring for the last detector  in a 4-Wire system. However, there is a problem. A 4-wire system is wired Normally Open (NO) on the sensor circuit, no power is required to maintain the NO state. The state of the circuit would remain unchanged from a power loss, cut wire or device removal.  If power was required to maintain the NO state then  a power loss or cut wire would close the sensor relay and trip a false fire alarm.  You don't want a fire alarm every time the power goes out!

End Of Line 4-Wire smoke detector terminals
The last smoke detector requires an EOL resistor

In any 4-wire smoke detector system you must install an End-Of-Line power supervision relay. The relay uses the power side of the circuit to maintain a complete sensor circuit. The image below shows the proper wiring for installing an End-Of-Line power supervision relay.

4-Wire smoke detector with relay open
With smoke head removed or power wire failure the relay opens resulting in a trouble condition.

The image below shows a complete circuit after the smoke head is installed. It is important that the resistor is placed in-line of the sensor circuit.

4-Wire smoke detector wiring with end of line power supervision relay
With smoke head installed the circuit is complete and the relay closes.

In some systems you might find that the installation technician extended the four wires so that the End-Of-Line power supervision relay is actually located at the panel or a more easily accessible location. There is nothing wrong with this method and in most cases it is better for future service and troubleshooting.  

4-Wire smoke detector wiring with power supervision relay remotely located
Locating the power supervision relay in the alarm panel makes troubleshooting a lot easier. 

 

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