Douglas Krantz - Technical Writer - Describing How It Works
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What is a 4 Wire Smoke Detector?

A 4 wire smoke detector works the same way a 2 wire smoke detector works, except it gets its operating power from a separate power source, rather than from the zone circuit.

A 4 Wire Smoke Detector is a conventional smoke detector with a current limiting resistor and a contact closure
Inside a 4 wire smoke detector is a standard conventional smoke detector, a current limiting resistor to keep the smoke detector from burning up when it goes into alarm, and relay contacts that close in alarm.


By Douglas Krantz

Even though it uses an internal protection resistor and internal alarm relay contacts, a 4 wire smoke detector is basically a 2 wire smoke detector; it detects smoke and sends an alarm using the Class A or B zone wiring of the fire alarm system.

Internal Resistor

The only purpose of this internal protection resistor is to limit the amount of current that the detector can use from the wires providing power to the detector. In essence, it keeps the detector from burning up.

But by protecting the smoke detector, the internal resistor does make it different from just a regular conventional smoke detector; the detector cannot short out the Class A or B zone wires properly and can't directly send an alarm signal to the Fire Alarm Control Panel (FACP).

Instead, it's the internal relay contacts of the smoke detector that send the alarm.

Zone Wiring

The alarm relay contacts of the 4 wire smoke detector are connected to the Class B or Class A zone wiring of the fire alarm system. When alarmed, just like a conventional heat detector, pull station, or waterflow switch, the relay contacts short out the zone wires.

The zone wires, of course, are a standard conventional Class A or B wiring system complete with the Class A or B method of supervision of the wires, and no t-taps.

Power Wiring

Instead of connecting the normal detector power wires to the conventional Class A or B zone wires of the fire alarm system, the detector input wires are connected to a resettable 24 volt power source.

Many fire alarm panels provide this resettable power. To differentiate this power from the uninterruptible auxiliary 24 volt power supply, rather than just calling it an auxiliary power supply, the 24 volt power source is called the resettable power supply, the smoke power supply, the interruptible power supply, or something similar.

The detector power, through the internal resistor, has to be interruptible because this is how the detector is reset --- after the detector has gone into alarm, during the reset process, the 24 volt power to the detector is momentarily turned off and then on again. The detector then starts over and powers up normal.

24 Volt Class B Type Zone Power Wiring

The wiring for this 24 volt smoke power is similar to Class B zone wiring in that it is daisy-chained from detector to detector, doesn't have any t-taps, and using an end-of-line device like a relay or power monitor the wires themselves are supervised.

If a wire is broken, in order to send a trouble back to the FACP, the relay or power monitor loses its 24 volt power and then opens the wiring to the end-of-line resistor on the conventional Class B fire alarm zone.

Opening up the wire to the end-of-line resistor causes the panel to show trouble on the zone.

4 Wire Smoke Detector

For smoke detection, the 4 wire smoke detector is just about the same as a 2 wire smoke detector. The difference is that it has an internal resistor so it uses 24 volts to operate, and sends the alarm using relay contacts.
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.

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

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