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Fire Alarm -- Description

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






What is a 4 Wire Smoke Detector?

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.

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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.







Douglas Krantz

Describing How It Works
writer@douglaskrantz.com
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612/986-4210

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Electrical Flow


On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at www.douglaskrantz.com/
ElecElectricalFlow.html
.