Douglas Krantz - Technical Writer - Describing How It Works
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Why All the Contacts on a 4 Wire Duct Detector?

Two wires are needed to send an alarm, two wires are needed to supply power. Added to these four wires are three wires for trouble supervision, and 3 wires for fan or damper shut down. Sometimes, there are even more wires.

Air Duct Smoke Detector on the side of an Air Handling Unit
OooooooooooooK, There's a lot more than 4 wires, but only 4 wires are really needed.

2 wires are needed for sending an alarm to the panel and 2 wires are needed to power the duct detector assembly.

Also, 2 wires can be used to send a trouble signal to the panel, and 2 wires can be used to shut down the air handler, close a smoke damper in an air duct, or perform some other building function when there's smoke detected.


By Douglas Krantz

Conventional Air Duct Smoke Detectors are assemblies that include a smoke detector, support electronics, and an internal power supply. The minimum requirements for a conventional duct smoke detector are four wires.

Four Wires

By shorting the IDC (Initiating Device Circuit), two of the wires are for sending the alarm to the fire control panel on the conventional input zone.

To power the detector and the support electronics, using the other two wires, the duct detector receives power from the HVAC air handling unit it's attached to, or from the fire alarm control panel.

Air Duct Smoke Detectors are not Life Safety

Installed to sense smoke, a duct detector is a smoke detector in an air duct. Once smoke is sensed, to prevent the air handlers from spreading the smoke throughout the building, the smoke dampers in the ductwork are closed and the air handling fans are shut down.

Because the air handers aren't always on and circulating air, duct detectors are not considered to be life safety devices; if the air handler is off, no smoke will be sensed.

Power Hungry Support Electronics

In the assembly, the duct detector itself is just regular smoke detector (sometimes with modifications), and doesn't use much electricity.

Compared to the smoke detector, however, the assembly's support electronics are a huge consumer of electricity. It's the extra electricity used for the support electronics that changes this from a two wire smoke detector to a four wire assembly.

Support Relays

The support electronics has at least two relays. One of the relays, the alarm relay, is rather hefty. It's needed shut down the air handling fans and dampers.

It also sends the alarm to the fire alarm panel using an extra set of contacts. These are connected to the wires of the fire alarm input zone, making these two wires of the four wire duct smoke detector.

Indicating that something is wrong with the duct detector, that the power is lost, or that the cover is off, a second relay sends a supervision signal to the fire panel.

Internal Testing and Reset Circuit

Besides relays, the electronics usually include test and reset circuits for the assembly.

Included can be:

Support Power Supply

Because of the added current needed to run the support electronics and relays, a conventional duct detector assembly is power hungry.

To power everything, a duct detector assembly often has choices of:
These are the other two wires on a four wire duct smoke detector.

Resetting the Duct Detector

Like any conventional two wire smoke detector, by cutting power to the assembly and powering it up again, the duct detector assembly is reset. Unlike a conventional two wire smoke detector, cutting power to the zone power doesn't do anything; it's the power wires that have their electricity cut.

If the duct detector receives power from the air handling unit, the air handling unit has to be powered down to reset the duct detector. If the duct detector receives power from the fire alarm panel, either the auxiliary resettable power or smoke power has to be used, or some other means of cutting the power temporarily, like a push button switch, has to be used.

Four Wire Duct Detector Assembly

The air duct smoke detector is a four wire smoke detector, with extra contacts. So the detector can send in its alarm, two wires are for the input loop of the fire alarm panel. Providing electricity to the assembly's electronics the extra two wires provide power.

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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