CO Detector Symbol
Definition: The CO Detector symbol represents a carbon monoxide detection and alarm device — required by NFPA 720, UL 2034, and IEC 60335-2-90 — depicted in residential and commercial electrical wiring diagrams with a Power pin (line voltage supply) and an Interconnect pin (alarm bus), used to detect dangerous concentrations of odourless CO gas and trigger an audible/visual alarm.
Also known as: CO detector, carbon monoxide detector, CO alarm, CO monitor, carbon monoxide alarm, CO sensor.
What the CO Detector symbol means
The CO Detector symbol marks the location of a carbon monoxide detector in an electrical floor plan or wiring diagram. The Power pin (top) connects to the line voltage supply (120 V in North America, 230 V internationally) or to a low-voltage DC supply (9 V battery or 24 VDC supervised circuit). The Interconnect pin (bottom) connects to an alarm interconnect bus that allows multiple CO detectors (and smoke detectors) to trigger simultaneously when any one unit detects CO — so an alarm in the basement wakes occupants in the bedroom.
CO detectors continuously measure the carbon monoxide concentration in air (in parts per million, ppm) using an electrochemical sensor or metal oxide semiconductor (MOS) sensor. When CO concentration exceeds the alarm threshold (typically 70 ppm for a sustained period per UL 2034), the detector sounds an 85+ dB alarm and, if interconnected, signals all other detectors on the bus to alarm as well.
How to identify the CO Detector symbol
The CO Detector symbol is typically drawn as a circle (representing the detector housing) with a small 'CO' text label or a symbol indicating gas detection, and two pins: Power at the top and Interconnect at the bottom. In some drawing libraries it appears as a circle with an arrow or sensor glyph. The symbol is distinguished from the smoke detector symbol by the 'CO' annotation rather than a smoke-wave symbol.
Function and usage boundary
A CO detector continuously monitors the ambient carbon monoxide concentration. Electrochemical CO sensors use an electrolyte and electrode pair to oxidise CO, generating a current proportional to CO concentration. Metal oxide semiconductor (MOS) sensors change resistance in the presence of CO. When the measured CO exceeds the alarm threshold (per UL 2034 or EN 50291), the detector activates its onboard buzzer and indicator LED, and pulls the interconnect line low (or pulses it) to trigger all interconnected alarms throughout the building.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60335-2-90 covers household CO detector safety requirements. EN 50291-1 (residential) and EN 50291-2 (commercial) are the European standards for CO detector performance, alarm thresholds, and testing. EN 50292 provides application guidance. The symbol follows IEC 60617 safety device conventions. |
|---|---|
| ANSI/IEEE 315 | UL 2034 is the North American product safety and performance standard for single and multiple-station carbon monoxide alarms. NFPA 720 (Standard for the Installation of Carbon Monoxide (CO) Detection and Warning Equipment) governs installation requirements in the US, including locations and interconnection. ANSI/NEMA ICS 1 covers control and signalling wiring conventions. |
| Key difference | UL 2034 (US) and EN 50291 (EU/IEC-aligned) have similar alarm thresholds but differ in test methodology and timing requirements. UL 2034 requires alarm at 70 ppm within 60–240 minutes or at 150 ppm within 10–50 minutes; EN 50291 uses concentration × time (COHb) exposure limits. The schematic symbol is the same in both jurisdictions. |
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Logical terminals
| Logical ID | Editor terminal name |
|---|---|
| power | Power |
| interconnect | Interconnect |
Where the CO Detector symbol is used
- Residential homes — bedroom and hallway locations per NFPA 720 and IRC R315 to protect sleeping occupants
- Apartments and multi-family dwellings — interconnected CO detectors on each floor and near sleeping areas
- Hotels and commercial buildings — hard-wired networked CO detection with central fire alarm panel integration
- Garages and utility rooms — CO detection near gas appliances (furnaces, water heaters, boilers)
- Boat and marine cabins — portable or 12 V DC CO detectors where carbon monoxide poisoning from generator exhaust is a risk
- RVs and caravans — CO and combination smoke/CO detectors powered from the 12 V RV battery system
- Commercial kitchens and parking garages — high-sensitivity CO monitoring with building management system integration
Key facts
- A CO detector continuously monitors carbon monoxide (CO) concentration in parts per million (ppm) and alarms when concentration exceeds the threshold defined by UL 2034 or EN 50291 (typically 70 ppm sustained).
- The symbol has two pins: Power (line voltage or battery supply, top) and Interconnect (alarm bus connecting to other detectors, bottom).
- Carbon monoxide is colourless, odourless, and tasteless — it cannot be detected by human senses; CO detectors are the only reliable warning system.
- NFPA 720 (US) requires CO detectors outside each sleeping area and on each level of a dwelling; many US states and the IRC R315 codify this requirement into law.
- Electrochemical CO sensors are the most accurate but have a limited life (5–10 years); MOS sensors are lower cost but less accurate at low concentrations.
- The interconnect feature (two-wire or three-wire) allows all CO and smoke alarms in a building to sound simultaneously when any one is triggered, regardless of where occupants are sleeping.
- CO at 400 ppm is immediately dangerous to life and health (IDLH) with life-threatening symptoms within 15 minutes; at 70 ppm occupants may experience headache and nausea after several hours of exposure.
Frequently asked questions
What does the CO detector symbol mean in a wiring diagram?
The CO detector symbol marks the location of a carbon monoxide detector in an electrical plan. The Power pin shows the electrical supply connection (120 VAC hard-wired or battery), and the Interconnect pin shows the connection to the alarm bus that links multiple CO and smoke detectors together. The symbol tells the electrician where to rough-in the wiring and box for the detector.
What does the CO detector symbol look like?
The CO detector symbol is typically a circle annotated with 'CO' or a gas sensor indicator, with a Power pin at the top and an Interconnect pin at the bottom. It is similar to the smoke detector symbol but distinguished by the 'CO' label. In some drawing conventions it appears as a circle with a sensor or alarm icon.
Where should CO detectors be installed?
NFPA 720 requires CO detectors outside each separate sleeping area (adjacent to bedrooms), on each level of the dwelling including the basement, and in any room containing a fuel-burning appliance. They should be mounted approximately 1–1.5 m above the floor on the wall or on the ceiling, but not directly above fuel-burning appliances or in dead air corners.
What is the CO alarm threshold per UL 2034?
UL 2034 requires a CO detector to alarm at 70 ppm within 60–240 minutes, at 150 ppm within 10–50 minutes, and at 400 ppm within 4–15 minutes. These thresholds are based on COHb (carboxyhaemaglobin) saturation levels in the blood that cause adverse health effects. The detector must not alarm below 30 ppm to prevent nuisance alarms.
What is the difference between CO detector interconnect and non-interconnect models?
A non-interconnected CO detector (typically battery-only) only sounds its own alarm when it detects CO, alerting only occupants in the immediate vicinity. An interconnected CO detector (hard-wired with an interconnect wire or using wireless interconnect) sends a signal that causes all interconnected alarms in the building to sound simultaneously, ensuring occupants in remote parts of the building (e.g. asleep upstairs) are alerted.
How long does a CO detector last?
The electrochemical CO sensor has a service life of 5–10 years; after this the sensor reagent is depleted and the detector must be replaced. Most hard-wired CO detectors have an end-of-life indicator (chirping pattern or fault LED) that activates when the sensor approaches its end of life. UL 2034 and EN 50291 require detectors to have a clearly audible end-of-life signal.
What standard governs CO detector installation?
In the US, NFPA 720 is the primary installation standard for CO detection equipment in buildings; the International Residential Code (IRC) R315 codifies CO alarm requirements into building codes. Product performance is governed by UL 2034. In Europe, EN 50291-1 covers residential CO alarms and EN 50292 provides installation guidance.
Related symbols
- Buzzer / Audible Alarm symbol
- Horn / Siren symbol
- Piezo Buzzer symbol
- RTD Temperature Sensor symbol
- Smoke Detector symbol
- Zero-Crossing Detector symbol
Physical breadboard, PCB, and panel mapping
No universal physical profile is published for this symbol yet. Create a versioned custom part from the exact manufacturer dimensions and terminal map before fabrication.
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