The only accurate schematics for a particular fire alarm system is in the fire alarm system's installation manual or in the installation sheets that come with the four-wire smoke detectors. However, when you know the reasoning behind the very limited schematics, you have a better idea of how to expand the schematics to include more smoke detectors.
Conventional Fire Alarm System Four-Wire Smoke Detector Wiring
A four-wire smoke detector fire alarm system is controlled by almost any conventional fire alarm panel. The panels and their input zones are the same whether it's a two-wire smoke detector circuit, or a four-wire smoke detector circuit.
What makes a four-wire circuit different from the common two-wire circuit is the addition of a second "Power" circuit added to the zone input circuit.
Both circuits, the zone input circuit and the power circuit are individual Class B circuits.
Class B Circuits
According to the NFPA, with a Class B Initiating Device Circuits (IDCs):
- There is no redundant path
- Any device beyond a break won't work
- A wire-to-wire short will send an alarm
- The panel shows a trouble signal when the supervision current stops
There are lots of wires, but rather than looking at four wires for the smoke detectors, first, look at what the smoke detector does.
Two-Circuit - Four-Wire Smoke Detector
Inside the smoke detector housing is a two-wire smoke detector, and a relay. The smoke detector gets its power to operate from the power circuit, and the relay is connected across the zone input circuit.
One way of looking at a four-wire smoke detector is the detector detects smoke, the relay announces that the smoke detector is in alarm. The relay, of course, does the announcing the alarm by closing its contacts, which, just like a pull station switch, shorts the zone-input wires to the fire alarm control panel.
Zone Input Circuit
A four-wire smoke detector requires power to operate the smoke detector. A second circuit carries this power from the fire alarm control panel to all the four-wire smoke detectors.
The second circuit, like the first circuit, is outside the box of the fire alarm control panel. It's the power circuit needed to operate the smoke detectors; it's on a Class B Power Circuit.
End-of-Line and Supervision
To have confidence that the smoke detectors are always connected to the control panel, the control panel "supervises" the zone wiring.
However, the power circuit can't be directly supervised by the panel, so, at the end of the power circuit is an end-of-line device, which uses the zone input's end-of-line resistor for the supervision.
Zone End-of-Line
For supervision of the zone input circuit, the panel constantly runs an electrical current through all the wires and the end-of-line resistor.
- If a wire comes loose from a device, the supervision current stops, and the panel turns on its trouble light and buzzer
- If a wire breaks, the supervision current stops, and the panel turns on its trouble light and buzzer
- If the end-of-line resistor is disconnected, the supervision current stops, and the panel turns on its trouble light and buzzer
Interconnecting Both Ends-of-Line
The power circuit for the four-wire detector can't be supervised by the panel. Instead of an end-of-line resistor for the power circuit, there is an End-of-Line Device (EOL Device) - commonly a relay. When there's trouble on the power circuit, the end-of-line device opens up its contacts. This action creates a trouble on the zone input circuit, by disconnecting the end-of-line resistor.
The trouble on the zone input circuit (IDC) is the only trouble the panel sees. A trouble on the zone input circuit could actually a trouble on the power circuit.
Because each manufacturer's fire alarm system is different from any other system, sometimes the end-of-line device has to be something very specific. To learn what the manufacturer specifies, read the installation manual for the fire alarm control system, the installation sheets that come with the smoke detectors, or talk to the technical support team of the fire alarm manufacturer.
Voltage Sensor
The end-of-line device, itself, is a voltage sensor. It senses the voltage at the end of the circuit. If the voltage doesn't get to the end of the circuit, the EOL Device quits working because something is wrong.
- If the power supply quits working, the voltage is lost so the EOL Device stops working and sends a trouble signal to the panel
- If a wire comes loose from a smoke detector, the voltage is lost so the EOL Device stops working and sends a trouble signal to the panel
- If a wire breaks along the way so power doesn't reach the last device, the voltage is lost so the EOL Device stops working and sends a trouble signal to the panel
- If a wire comes loose from the EOL Device, the voltage is lost so the EOL Device stops working and sends a trouble signal to the panel
One of the zone input end-of-line resistor wires goes through the Common and Normally Open contacts of the power's EOL device. As long as the power keeps the EOL Device active, the contacts are connected together, and the zone input circuit does not show trouble.
As soon as power does not reach the EOL Device, the EOL Device contacts open-up, stopping the zone input supervision current and the trouble is sent to the panel.
Resetting the Smoke Detectors
Conventional smoke detectors can only be reset by turning off the smoke detector's power, and turning on the power. Most conventional fire alarm panels, it turns out, have two power supply outputs. One labeled "Auxiliary Power Supply", and the other labeled something like "Smoke Power Supply", or "S Power", or "Reset Power", "R Power", or some such words.
The smoke power supply provides the full voltage needed to power the four-wire smoke detectors, except during the "Reset-the-Panel" process. Then the voltage is turned off to reset any four-wire smoke detectors on the circuit. At the end of the panel's reset cycle, the smoke power is turned on again, the smoke detectors turn-on normally, ready to detect smoke again.
Four-Wire Addressable Smoke Detectors
Four-wire addressable smoke detectors don't use the four-wire - two-circuit systems that are described above. Each fire alarm system is different. The only place to get the schematic for the building wiring is from the manufacturer, either in their official documents and diagrams, or from their technical support teams.
Douglas Krantz
