System Sensor Fire Alarm Detectors: 2-Wire, 4-Wire and Addressable Interfaces
System Sensor manufactures conventional and addressable smoke, heat, carbon-monoxide and duct-detection products used with compatible fire alarm and security control equipment. Selecting a detector requires more than matching a voltage or number of wires—the device must be listed as compatible with the control panel, circuit and application.
This guide explains the difference between conventional two-wire and four-wire detectors and how conventional devices may interface with an addressable fire alarm system.
Two-wire conventional smoke detectors
A two-wire smoke detector receives power and reports alarm over the same initiating-device circuit. Compatible detectors are connected to the circuit according to the control-unit and detector installation instructions, with the required end-of-line component located at the end of the circuit.
The detectors should not be described simply as “wired in series.” Their electrical connection, quantity, alarm current, reset operation and end-of-line supervision must match the specific panel’s published compatibility information.
Two-wire smoke compatibility is especially important because the control panel supplies and resets the detector power through the initiating circuit. A detector that operates at the correct nominal voltage is not automatically listed for use with the panel.
Four-wire conventional smoke detectors
A four-wire detector uses one pair of conductors for operating power and a separate alarm relay circuit to report to the control panel. Because detector power is separate from the initiating circuit, the installation normally requires listed power supervision at the end of the power run and a method to interrupt power for detector reset.
Four-wire arrangements can provide flexibility, but they require additional conductors, power calculations, supervision and reset coordination. The exact circuit must follow the detector, power-supply and control-unit instructions.
Conventional versus addressable detection
A conventional initiating circuit identifies the affected zone or circuit. An addressable system communicates with individually addressed detectors and modules over a signaling line circuit, allowing the control unit to identify the programmed device or point.
A conventional detector does not connect directly to an addressable SLC merely because both operate at low voltage. A listed compatible monitor module, conventional-zone module or other approved interface is required. Detector power, end-of-line supervision and reset operation must also be provided as specified.
Read our addressable SLC loop guide for more about detectors, modules and circuit troubleshooting.
Examples from the System Sensor i3 family
System Sensor has offered i3-series conventional detectors in two-wire and four-wire configurations, including photoelectric and photoelectric-with-fixed-temperature models. Model availability, listings and panel compatibility can change, so the exact part number should be verified before replacement or installation.
Commonly encountered model designations include:
- 2W-B: two-wire photoelectric smoke detector.
- 2WT-B: two-wire photoelectric detector with fixed-temperature thermal detection.
- 4W-B: four-wire photoelectric smoke detector.
- 4WT-B: four-wire photoelectric detector with fixed-temperature thermal detection.
These examples should not be used as a substitute for the current manufacturer data sheet, control-panel compatibility list or approved fire alarm drawings.
Smoke, heat and carbon-monoxide detection are different
Smoke, heat and carbon-monoxide detectors respond to different hazards and are selected according to the environment and required protection. A heat detector is not a direct replacement for required smoke detection, and a smoke detector does not provide carbon-monoxide detection unless the listed device specifically includes that function.
Carbon-monoxide sensing elements can have defined service-life and replacement requirements. Smoke and heat devices also require maintenance and testing according to their listing, manufacturer instructions and applicable inspection program.
Residential security panels versus commercial fire alarms
Selected conventional smoke or carbon-monoxide devices may be listed for use with compatible residential security panels. That does not make the intrusion panel a substitute for a separately required commercial fire alarm system.
Commercial fire alarm work must follow the approved design, applicable licensing requirements, adopted code and authority having jurisdiction. Devices should not be moved between systems or substituted without confirming listing and compatibility.
Replacement-in-kind and compatibility
When replacing an existing detector, verify the manufacturer, model, base, circuit type, voltage, compatibility listing and initiating function. A replacement that looks identical may use a different base, protocol or electrical characteristics.
Where replacement-in-kind is permitted, the replacement still must be compatible with the existing control equipment and installed according to its listing. Changes to device type, circuit operation or system design may require additional documentation or approval.
Testing and sensitivity
Detector testing should verify alarm operation at the control panel and the associated system response. Smoke-detector sensitivity must be evaluated using the methods permitted by the detector and applicable inspection requirements; an alarm response alone does not necessarily document sensitivity.
Testing should also confirm the correct device or zone description, central-station signal when monitored, annunciation, reset and restoration to normal condition.
Professional fire alarm service
Avenger Security provides commercial fire alarm inspection, monitoring, troubleshooting and device replacement throughout Austin and Central Texas. Review our commercial fire alarm monitoring and maintenance guide for service information.