Douglas Krantz - Technical Writer - Describing How It Works
Go to the Writer Home Page of Douglas Krantz -- Describing How It Works
Go to the Fire Alarm Operation Map Page of Douglas Krantz - Technical Writer
Go to the General Electrical Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Description Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Installing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Maintaining Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Testing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Suppression Map Page of Douglas Krantz - Technical Writer
Offers from Technician's Corner

The 4 wires are for the connections on the detector and there are 8 wires to the building.
Even though the description for the detector says "4-wire", there are 8 wires that go to the detector from the building wiring.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

What is a Four Wire Smoke Detector?

By Douglas Krantz

A Smoke Detector is a Smoke Detector

A 4-wire smoke detector is just like a 2-wire smoke detector, except that it receives its power from an auxiliary power supply rather than the conventional Initiating Device Circuit. Both the 4-wire and the 2-wire smoke detector sense particles in the air and call the particles smoke.

When you get down to it, the 4-wire smoke detector is a lot like a conventional air duct smoke detector. Just like a conventional air duct smoke detector, 2 wires are used to get power for the smoke detector; just like a duct detector 2 wires are normally open alarm relay contacts that connect to the zone circuit (Initiating Line Circuit or IDC).

A 4-wire smoke detector itself has 4 connections - two to power the smoke detector and two to send the alarm on the fire alarm zone.
There are 4 terminals - two terminals are to send alarms in on the zone and two of the terminals are to power the detector.

4 Wires or 4 Pairs of Wires

To Describe a 4-wire smoke detector, four wires is really a misnomer. Most 4-wire smoke detectors have 4 screw-terminal contacts, but if instead of the 4 screw-terminals there are wires, then there are 8 wires. That's 4 pairs of wires.

The four contacts or four pairs of wires are:
  • Plus 24 volt power in; Minus 24 volt power out (1 contact, 2 wires)
  • Minus 24 volt power in; Minus 24 volt power out (1 contact, 2 wires)
  • Plus Zone in; Plus Zone out (1 contact, 2 wires)
  • Minus Zone in; Minus Zone out (1 contact, 2 wires)

Zone Wiring

To send the alarm to the fire alarm panel, the 4-wire smoke detector uses normally open relay contacts. Like a pull station or a waterflow switch, the relay contacts (the output of the detector) are connected to the building's Class B or Class A input zone wiring (Initiating Device Circuit or IDC). Just like a pull station or waterflow switch going into alarm, the relay contacts "short out" the zone wires.

Resettable Power Wiring

A 4-wire smoke detector, like a 2-wire smoke detector, has to be resettable. The fire alarm code helps in the resetting.

By code, all fire alarm devices are required to power up in the normal or stand-by condition - not in alarm condition. Designers of fire alarm systems use this feature to reset 2-wire smoke detectors. To reset a detector, power to the zone (IDC) is turned off for a few seconds, and then power is turned back on. The detector then powers up in the non-alarm condition.

Relay Contacts Don't Pass On the Reset

Unlike a 2-wire smoke detector, which gets its power (and therefore its reset signal reset) through its output contacts. A 4-wire smoke detector, though, can't be reset through its alarm output contacts. Basically, the alarm output relay contacts of the 4-wire smoke detector electrically isolate the smoke detector from the zone wiring (IDC).

Instead, the 4-wire smoke detector's reset is sent through the auxiliary power source.

Basically, to reset a 4-wire smoke detector, the auxiliary 24 volt power supply has to turn off and on. This means that the steady output of the 24 volt auxiliary power supply on the fire alarm panel can't be used.

Instead, an interruptible 24 volt power supply, the turn-off being triggered by the reset button, has to be used.

Often times this interruptible power supply is part of the fire alarm panel itself. The label on the power supply might be S Power (Smoke Power), R Power (Resettable Power), or some other label. Read the installation instructions for the panel to see which terminals to use for this.


Walk-testing is a special function of the fire alarm control panel. It is not a life safety alarm function, and often it only tests the Initiating Line Circuit and the detectors to see if everything is connected.

To reset the detectors as they are being walk-tested, fire alarm panels internally send a reset signal to the zone inputs (the IDCs). In reality, this extra reset signal momentarily turns the zone power off and on.

However, some of the fire alarm panels don't also internally send the walk-test reset signal to the panel's resettable auxiliary power supply. If during walk-test the panel doesn't send a signal to turn off the internal resettable power supply, any 4-wire smoke detector on the fire alarm system can't be walk-tested.

To find this out if the resettable power supply resets during walk-testing, you'll have to read the panel's instruction manual and maybe talk to tech support.

A 4-wire smoke detector with an auxiliary relay is still a 4-wire smoke detector. It only has the auxiliary relay contacts that activate when the detector goes into alarm.
The extra auxiliary relay contacts are part of the alarm relay of the 4-wire smoke detector. They are electrically isolated so other electrical or electronic systems can be used with it.

4-Wire Detector with an Auxiliary Relay

Sometimes, because there is already an alarm relay inside the 4-wire smoke detector, the detector will have an extra set of Form-C contacts on the relay. This extra set of contacts is added so the 4-wire smoke detector can be used to shut down fans, close fire doors, capture an elevator, etc.

For those of you who can count, this would be 7 contacts, or 11 wires, or some other combination on the smoke detector. The 4 wire smoke detector (2 wires for the zone alarms and 2 wires for the power) is still describing the type of detector but the extra Form-C relay contacts are an added "with an auxiliary relay" to the description of the 4-wire smoke detector.

Power Supervision

Unlike duct smoke detectors, the 4-wire smoke detectors don't supervise the power circuit; the 4-wire smoke detectors can't inform the control panel when there's power trouble. Because of the electrical isolation provided relay contacts of the detectors connected on the zone (IDC), if the power is lost, the detectors just sit there and do nothing (as if they had become ornaments on the ceiling).

There has to be a way to tell the fire alarm control panel (FACP) that there's power trouble.

The wiring for detector power is basically Class B wiring (classified as a Class B Signal Path in the NFPA 72). Conventional Class B wiring cannot have any T-Taps because, to help the panel perform a continuity check of the wiring (supervise the wires), the conventional Class B wiring completes the circuit using a single end-of-line resistor.

For power supervision, a continuity check of the wiring can't be performed on a power supply like the panel does with an Initiating Device Circuit (IDC) or a Notification Appliance Circuit (NAC).

Remember, the real purpose of power supervision is to make sure power gets to every device. If power gets all the way through to the end, it is assumed that power has gotten to all the devices in-between. Instead of using an end-of-line resistor, an end-of-line device (usually a relay) is used.

End-of-Line Device

To confirm that the 24 volts goes through reaching the end of the circuit, a 24 volt relay is used; the relay coil is energized by 24 volts getting to the end of the line.

If the 24 volts doesn't make it to the end of the circuit where the relay coil is located, the relay relaxes and opens up the wiring (using its Normally Open Contacts) to the end-of-line resistor for the detector zone. This insures that the panel will show trouble on the zone if the power doesn't reach the end of the circuit.

More Articles

Can a Magnet Really be Used to Test a Smoke Detector? -- Smoke detectors usually have two ways of being tested. Smoke (smoke particles in the air, or some sort of canned smoke), and magnets (the activation of an internal magnetic... Read More

Why Doesn't a Smoke Detector Always Detect? -- The NFPA wants smoke detectors tested in place to make sure smoke enters the smoke chamber. Keep in mind that a smoke detector can't detect smoke that doesn't get to the detector. When a smoke detector doesn't... Read More

Should Canned Smoke be Used to Test Smoke Detectors? -- There have been many discussions about the best way to test smoke detectors --- canned smoke or... Read More

Are All Smoke Detectors Polarity Sensitive? -- I have a question... Is it important for smoke detectors to be polarity correct? Are all smoke detectors polarity sensitive? Read More

 Get your free diagram showing supervision for Class B wiring

Get the book Make It Work - Conventional Fire Alarm Systems

This website uses cookies. See Privacy for details.

Get the book Make It Work - Conventional Fire Alarm Systems

Get help finding ground faults

I fund this website on the commissions received for purchases made through links in this post