Smoke Detector Symbol
Definition: The Smoke Detector symbol represents a life-safety sensing device in electrical and fire-alarm wiring diagrams that detects airborne combustion particles and sounds an alarm, drawn as a labelled circular or rectangular symbol with a Power pin for supply voltage and an Interconnect pin for multi-detector linking, as standardised under NFPA 72 (National Fire Alarm and Signalling Code) and IEC 60268 / EN 54-7.
Also known as: smoke alarm, smoke sensor, fire detector, ionisation smoke detector, photoelectric smoke detector, SD.
What the Smoke Detector symbol means
The Smoke Detector symbol marks the location of a smoke-sensing device in a wiring diagram. Residential detectors are typically self-contained units powered from 120 V AC mains (with battery backup) or from a 9 V battery alone. Commercial and industrial smoke detectors are addressable or conventional devices wired to a fire-alarm control panel (FACP) over a 2-wire or 4-wire supervised loop.
The Power pin represents the supply connection (mains AC, 12 V DC from a panel, or battery) and the Interconnect pin represents the hardwire interconnect bus that links multiple detectors so that any single alarm triggers all detectors in the building to sound simultaneously, as required by NFPA 72 and IEC 60268 for residential and light-commercial installations.
How to identify the Smoke Detector symbol
The Smoke Detector symbol is drawn as a circle (representing the round detector housing) with a cross or dot at the centre (indicating the sensing chamber), and two external pins: Power at the top (supply connection) and Interconnect at the side or bottom (alarm linking bus). In fire-alarm system drawings the symbol may include a letter designation such as SD or S inside the circle. In simplified wiring diagrams it appears as a labelled rectangular block.
Function in a circuit
A smoke detector continuously monitors air samples for either ionised combustion products (ionisation type) or scattered light from smoke particles (photoelectric type). When smoke concentration exceeds the alarm threshold, the detector energises its internal sounder and activates the Interconnect line, which signals all connected detectors to sound simultaneously. Photoelectric detectors respond faster to slow smouldering fires; ionisation detectors respond faster to fast-flaming fires.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60268 covers sound system equipment; EN 54-7 (Fire detection and fire alarm systems — Part 7: Smoke detectors) is the European product standard for smoke detectors. IEC 60617 does not define a dedicated symbol; fire-alarm drawings typically use circular symbols per EN 54 graphical conventions. |
|---|---|
| ANSI/IEEE 315 | NFPA 72 is the National Fire Alarm and Signalling Code governing smoke detector installation in the USA. UL 217 is the product standard for single- and multiple-station smoke alarms. ANSI/NFPA 170 defines graphical symbols for fire protection drawings, using a circle with 'S' or 'SD' for a smoke detector. |
| Key difference | EN 54-7 (European/IEC) and UL 217 / NFPA 72 (North American) govern the same functional device but differ in sensitivity calibration methods and installation spacings. EN 54 drawings use a circle with a dot; NFPA 170 uses a circle with 'S'. Both use circle-based symbols. |
Terminals / pins
| Pin | Name |
|---|---|
| power | Power |
| interconnect | Interconnect |
Typical values
Supply voltage: 120 V AC mains (US residential) or 9 V battery or 12 V DC / 24 V DC from FACP (commercial). Quiescent current: 5 µA–50 µA (battery); 50 µA–100 µA (4-wire hardwired). Alarm current: 30 mA–100 mA. Sound output: ≥85 dB(A) at 3 m (UL 217 / EN 54-3). Alarm threshold: optical obscuration 1%–4% per metre (photoelectric) or ionisation chamber current change (ionisation). Maximum coverage area per detector: 93 m² (1 000 ft²) for spot-type detectors per NFPA 72.
Where the Smoke Detector symbol is used
- Residential bedroom and hallway wiring: mains-powered smoke detectors with battery backup interconnected for simultaneous alarm throughout the dwelling
- Commercial office and retail fire-alarm systems: addressable smoke detectors wired to a 2-wire Class A or Class B supervised loop connecting to a FACP
- Industrial facilities: duct smoke detectors installed in HVAC duct work to detect smoke in air-handling systems and shut down fans
- Hotel and dormitory life-safety systems: smoke detectors integrated with door-closing relays, stairwell pressurisation, and elevator recall systems
- Kitchen and utility areas: heat detectors (not smoke detectors) are used near cooking equipment; smoke detectors are placed outside kitchen areas per NFPA 72 to avoid nuisance alarms
- Garage and attached dwelling structures: combination CO and smoke detectors where both combustion gases and smoke are present hazards
Example
In a residential floor-plan wiring diagram, three Smoke Detector symbols are placed in the master bedroom, hallway, and living room. The Power pin of each connects to the 120 V AC electrical circuit dedicated to smoke detectors, and the Interconnect pins are all wired to a common three-wire interconnect bus. When the hallway detector senses smoke and enters alarm, the Interconnect line goes high, causing all three detectors to sound their alarms simultaneously.
Key facts
- The Smoke Detector symbol's Power pin represents the mains (120 V AC) or DC supply connection; the Interconnect pin represents the alarm bus that links multiple detectors to sound simultaneously when any single unit alarms.
- NFPA 72 requires smoke detectors in every bedroom, outside each sleeping area, and on each level of a dwelling; EN 54 and national building codes specify similar minimum coverage requirements.
- Two sensing technologies exist: ionisation (uses a small radioactive source Am-241 to detect fast-flaming fires) and photoelectric (uses an LED and light sensor to detect slow-smouldering fires); dual-technology detectors combine both principles.
- The hardwire interconnect bus allows all detectors to sound simultaneously when any detector enters alarm; this is required by NFPA 72 for new construction and is a critical life-safety feature that battery-only detectors cannot provide.
- Maximum detector spacing per NFPA 72: smoke detectors must be placed no more than 9.1 m (30 ft) apart on a flat ceiling, with the first detector no more than 4.6 m (15 ft) from any wall or partition.
- Addressable smoke detectors used in commercial fire-alarm systems communicate their individual device address and alarm/fault status to the FACP over a two-wire Class A or B loop, allowing the fire-alarm panel to identify the exact location of the alarm.
- UL 217 (single-station smoke alarms) and UL 268 (smoke detectors for fire-alarm systems) are the two primary North American product standards; residential smoke alarms are UL 217 listed, while panel-connected detectors are UL 268 listed.
Diagrams that use this symbol
Frequently asked questions
What does the smoke detector symbol mean in a wiring diagram?
The smoke detector symbol represents a life-safety sensing device that detects combustion particles and sounds an alarm. The Power pin shows the supply connection (mains AC or DC from a fire panel) and the Interconnect pin shows the bus that links multiple detectors to alarm simultaneously when any single unit detects smoke.
What does the smoke detector symbol look like?
The smoke detector symbol is typically a circle (representing the round detector housing) with a dot or cross at the centre, and two external pins: Power (supply) at the top and Interconnect (alarm bus) at the bottom or side. In NFPA 170 fire-protection drawings it is a circle with 'S' or 'SD' inside.
What standard defines the smoke detector symbol and installation rules?
In North America, NFPA 72 (National Fire Alarm and Signalling Code) governs installation locations, spacing, and interconnection. UL 217 is the product standard for residential smoke alarms. In Europe, EN 54-7 is the product standard and BS 5839 or EN 54 Part 14 covers installation. ANSI/NFPA 170 defines the graphical symbol for fire-protection drawings.
What is the difference between ionisation and photoelectric smoke detectors?
Ionisation detectors use a small radioactive source (Am-241) to ionise air; fast-flaming fires disturb the ion current and trigger the alarm quickly for flaming fires. Photoelectric detectors use an LED and light sensor; smoke particles from slow-smouldering fires scatter the light beam and trigger the alarm, making them faster for smouldering fires. Dual-technology detectors combine both sensing methods.
Why does a smoke detector need a neutral or interconnect wire in addition to power?
The power wire provides the live (hot) supply to the detector. The neutral wire completes the AC circuit for mains-powered units. The interconnect wire is a separate signal bus: when any detector alarms, it drives the interconnect line to a set voltage, causing all other connected detectors on the same bus to sound their alarms simultaneously, as required by NFPA 72.
How many smoke detectors are required per floor?
NFPA 72 requires at least one smoke detector outside each sleeping area (hallway), one inside each bedroom, and one on every level including the basement. Maximum horizontal spacing between detectors on a smooth ceiling is 9.1 m (30 ft), with the first detector within 4.6 m (15 ft) of any wall.
What is the difference between a residential smoke alarm and a commercial smoke detector?
A residential smoke alarm (UL 217) is a self-contained unit with a built-in sounder, powered from 120 V AC mains or a 9 V battery. A commercial smoke detector (UL 268) is a separate sensing head wired to a fire-alarm control panel (FACP), which provides power, monitors detector status, and activates separate notification appliances (horns, strobes). Commercial detectors can be addressable (each reports its exact location to the FACP) or conventional (zone-based).
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