2 Wire Smoke Detector Wiring

Smoke detectors for security and fire alarm systems connect to a control panel in one of three manners. 

1.    2-wire conventional 
2.    4-wire conventional 
3.    Polling Loop (aka addressable, Vplex or other brands specific protocol. Used mostly in commercial applications)

2-Wire conventional smoke circuits use a pair of wires for both power and signally functions. On 2-wire circuits, the detectors are wired in parallel along a single path with an EOL resistor at the end of the circuit. When any detector goes into alarm, the circuit is brought to a near short.  The panel senses the change and goes into alarm.

Three 2-wire smoke detectors wired in parallel with an EOL resistor
Smoke detector loop in an EOL state resulting in a Normal Condition.

 Any detector which goes into alarm condition will “latch”.  The term latch means the detector stays in alarm condition even when the smoke event is over. When latched, a detector will show a red light. This aids the user in locating which detector caused the alarm.  

2-wire Smoke detector in alarm with red light
Detector latches and displays red light.
Smoke detector in latch with no smoke.
After smoke clears the detector stays latched and red light stays on.

To release the latch thereby resetting the system, power must be removed from the 2-wire circuit for a few seconds. When power is restored the smoke detectors return to normal operation. If the smoke circuit is reset before the smoke is cleared or if the detector is damaged from the event, the fire alarm will trip again. 

Smoke detectors reset when power is removed for a few seconds
During a Fire Reset, power is removed from the circuit for a few seconds.

Heat detectors can be installed on the same circuit as smoke detectors. They wire and function in essentially the same manner. The only difference is the type of conditions that trigger an alarm. While heat detectors can be added to a smoke detector circuit, it is often better to add the heat detectors to their own zone. This will provide more detail location information to the monitoring station if dispatch is required. 

It is important to realize that the images above are shown in a schematic format and do not reflect the precise method of wiring a 2-wire smoke detector. The circuit must have the ability to open if a device is removed. The open circuit is the same as a cut wire. The panel goes into trouble. Below is a graphic of a commonly used base plate terminal. The smoke detector clips into the base. If the detector head is removed, the circuit needs to reflect the open circuit so the panel can go into a trouble state.  

2-wire smoke detector terminals
The + power leads are NEVER under the same terminal  in a two piece detector


When a smoke detector is installed it bridges the gap between the two + terminals thereby completing the circuit for the rest of the detectors. 

2-wire smoke detector terminals showing circuit bridge when detector is installed.
Notice the wires are NEVER twisted together. If a wire disconnects from the base it MUST open the circuit.

 

If you have any questions when installing devices ask your installation lead technician. 

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