I’ve received a few calls on how to connect 4-wire smoke detectors so I thought it might be a good idea to share what I know about these smoke sensors.

Why is it referred to a 4-wire detector? As with just about all powered sensors, it requires four wire connections, two of the wires are used for power, typically 12 VDC, and the other two are used for signal. The signal comes from a switch within the smoke sensors that closes its contacts when smoke is detected. The internal switch is open when the situation is normal, and closes when it detects smoke.

Power going to the smoke detector is supervised, meaning that if power is lost to the detector for whatever reason, the security or automation system that the smoke sensors is connected to will know about that loss of power. The supervision is accomplished by the use of a relay and a resistor which is installed at the end of the line, or after, the smoke detector. If there are multiple smoke sensors connected to a zone input of the system, the end-of-line (EOL) relay and resistor are installed after the last smoke sensor in the chain. The combination of the EOL relay, resistor, and wiring scheme allows for the supervision status to be in one of three possible conditions: System Ready, Alarm or Trouble.

You might want to take a look at the provided wiring diagram to fully grasp my next statement. 

Though power applied to the smoke detector(s) and the coil of the EOL relay are electrically in parallel, the physical wiring is in series.

Huh?!

You cannot use “home-runned” wiring to supply power to a smoke detector, or use a 12 VDC power distribution module with multiple outputs each connected to a smoke detector. You cannot get supervision with these wiring schemes.

Instead, you need to daisy-chain the wiring such that a single wire from the 12 VDC output of the system goes to the first smoke sensor’s “+” terminal, and then a second wire from the same “+” terminal goes to the next smoke sensor in the chain’s “+” terminal, or to the EOL relay. Use the same daisy-chain wiring from the system’s 12 VDC Return to go to the first smoke sensor’s “-” terminal, and a second wire from the “-” terminal going to the next smoke sensor in the chain’s “-” terminal, or to the EOL relay. Wiring in this fashion ensures that if any power wire is compromised, the chain is broken which causes the EOL relay to open its contacts.

Enter the signal wiring and EOL resistor. The signal wiring is connected from the system’s zone “+” and “-” input to the smoke sensor’s Common (COM) and normally open (NO) terminal, respectively, in the same daisy-chain fashion as the power wiring. Once the wiring has passed the last smoke detector, one lead of the resistor is connected in series with either one of the signal wires (in the diagram the resistor is on the NO side) and the other lead from the resistor continues on to one side of the EOL relay’s contacts.

The remaining signal wire is connected directly to the EOL relay’s other contact. Note that when power is applied and the EOL relay is energized, the EOL resistor is placed in a series loop as seen by the zone input. The value of the resistor varies with the panel used and in the diagram a resistance of 1000 Ohms is used, but the key point to remember is that when the situation is normal, the zone input “sees” the resistor. Electrically speaking, a specific amount of current flows through the wiring and EOL resistor which the systems acknowledges as a normal condition. This can be described as “System Ready.” System Ready means just that, the system is OK, there is no smoke detection, all is working normally, the system can be armed if needed, all green and good to go, the system is ready. “We are ready for launch, Houston.”

Now when a smoke sensor detects the presence of smoke, the internal normally open switch closes. This ties the COM and NO terminal together. This switch closure effectively shorts out the resistor that the zone input normally sees. Instead of having the normal amount of current flowing through the resistor, the short causes an over-current condition. The short circuit causes an “Alarm” condition and the system responds accordingly by sounding an alarm or siren and initiating a dial-out call.

In the event of a compromised wire, be it for power or signal, a “Trouble” condition will exist. Trouble occurs when the zone input no longer senses any current flow at all. Any break in the power wires causes the relay contacts to open, and a break in the signal wires will open the circuit to the zone input.

You can easily test your security or automation system’s zone input to see if it has been properly configured to work with a smoke detector without actually having a smoke sensor installed. “Spoof” the system by placing the appropriate value of resistor across the zone input. If configured properly, you’ll get a System Ready. Next, take a short piece of wire and connect it across the zone input shorting out the resistor and you’ll get an Alarm condition. And lastly, lose the short piece of wire and remove one lead of the resistor from the input to simulate a Trouble condition.

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