Fire Alarm Systems

Comprehensive vocabulary flashcards covering fire alarm systems, operational principles, device specifications, system categories, circuit supervision, installation standards, and routine maintenance protocols.

Normally Open Alarm Circuit

An early fire alarm circuit wiring detector contacts in parallel; closing any contact completes the circuit to sound the alarm, but a broken conductor disables all downstream devices without triggering a fault warning.

Normally Closed Alarm Circuit

An early fire alarm circuit wiring contacts in series to continuously energise a line relay; opening any contact drops out the relay to trigger sounders, but a short circuit prevents downstream call points from operating and an open circuit cannot be distinguished from a genuine fire alarm.

Conventional Fire Alarm System

A fire detection system wired as radial circuits into discrete zones with end-of-line supervisory resistors at the final devices, where the control panel indicates the collective zone in which an alarm or fault occurs rather than pinpointing an individual device.

End-of-Line (EOL) Resistor

A resistor (typically $6800\,\Omega$) installed across the terminals of the final device on a conventional detection or sounder radial circuit to permit a continuous supervisory monitoring current for open-circuit and short-circuit fault detection.

Call Point Series Resistor

A series resistor (typically $470\,\Omega$ to $680\,\Omega$) built into a manual call point that produces a partial short circuit of approximately $500\,\Omega$ upon actuation, enabling the control panel to distinguish between an alarm condition and a dead short-circuit fault.

Addressable Fire Alarm System

A system where all automatic detectors and manual call points are wired on a supervised loop and identified by individual digital addresses (such as via a 7-position DIP switch addressing numbers $1$ to $127$), allowing the panel to pinpoint the exact device and its programmed physical location.

Multiplexing in Addressable Systems

The digital communication technique where the central control panel sequentially interrogates each device with a brief signal lasting approximately $0.02\,\text{s}$ to assess its individual operating condition over shared loop wiring.

Short-Circuit Isolator Module

A protective device connected along an addressable wiring loop that automatically isolates a faulted or short-circuited cable segment, allowing the healthy remainder of the loop to maintain two-way communication with the control panel.

Category M System

A purely manual fire alarm classification under I.S. 3218 with no subdivisions, relying solely on occupants discovering a fire and manually activating call points to initiate an alarm.

Category L1 System

An automatic fire detection system designed for the total protection of life, installed throughout all areas of the protected building.

Category L2 System

An automatic life safety detection system installed only in defined parts of a building, which incorporates the escape route coverage of either a Category L3 or Category L4 system augmented by detection in specified high-risk areas.

Category L3 System

An automatic life safety detection system installed specifically to protect escape routes and all rooms or spaces opening directly onto those escape routes.

Category L4 System

An automatic life safety detection system installed within escape routes only, permitted solely where structurally protected alternative escape routes are available and occupants are ambulant, wakeful, and trained.

Category P1 System

An automatic fire detection system intended for property protection and business continuity installed throughout the entirety of the protected premises.

Category P2 System

An automatic property protection fire detection system installed only in specified or defined high-risk areas of the building where fire probability or consequence is severe.

Break-Glass Manual Call Point

A manually operated initiating unit containing a spring-loaded switch held depressed by a frangible glass element; breaking the glass releases the switch contacts through an internal series resistor to trigger an alarm.

Manual Call Point Siting and Mounting Limits

Installation guidelines requiring manual call points to be situated on staircase floor landings, exit routes, and exits to open air, mounted at a height of $900\,\text{mm}$ to $1200\,\text{mm}$ above floor level, such that travel distance to a unit does not exceed $30\,\text{m}$.

Optical (Photoelectric Light Scatter) Smoke Detector

A point detector operating on the light-scatter principle where an internal light sensor is shielded from an LED until smoke scatters light onto the sensor, lowering circuit resistance from standby ($0.1\,\text{mA}$ current draw) to approximately $500\,\Omega$; best suited for slow, smouldering fires.

Ionisation Smoke Detector

A point detector containing twin chambers and a radioactive source that ionises air to support a monitoring current ($0.03\,\text{mA}$); smoke particles enter the chamber and attach to ions, reducing chamber current to signal an alarm; best suited for fast-burning flaming fires.

Fixed Temperature Heat Detector

A thermal detector utilizing a bimetallic strip or thermistor calibrated to close an alarm contact when an ambient set point (such as $60^\circ\text{C}$, $65^\circ\text{C}$, or $90^\circ\text{C}$) is reached; suitable for smoky, dusty, or steamy environments such as kitchens and boiler rooms.

Rate of Rise Heat Detector

A thermal detector combining an exposed sensor and a thermally shielded reference sensor to detect rapid temperature increases, incorporating a fixed upper-temperature stop; ideal for drying rooms or laundries with wide ambient swings but not kitchens where sudden heat surges naturally occur.

Photoelectric Optical Beam (Line) Detector

A high-level line detector consisting of an optical transmitter and receiver spaced up to $40\,\text{m}$ apart (or up to $25\,\text{m}$ with a reflector unit) that triggers an alarm when smoke particles obscure or diffuse the light path across large open areas.

Pulsating Infra-Red Line Detector

An optical line detector spanning distances up to $100\,\text{m}$ that emits a pulsed infrared beam; it triggers an alarm when flame flicker within $6\,\text{m}$ on either side of the beam modulates the received signal at a frequency between $4\,\text{Hz}$ and $15\,\text{Hz}$.

Aspirating Smoke Detection (ASD) System

A high-sensitivity detection system compliant with I.S. EN 54-20 that continuously draws air samples through a network of sampling tubes via a fan or pump into a central chamber using optical or ionisation sensors, where each sampling hole is treated as an individual point detector.

Ultraviolet (UV) and Infra-Red (IR) Flame Detectors

Radiation-sensitive optical detectors that respond directly to flame flicker: UV types sense wavelengths between $200\,\text{nm}$ and $270\,\text{nm}$ (unaffected by sunlight due to high-altitude ozone absorption), while IR types use flicker-frequency analysis or narrow emission bands to reject solar interference.

Smoke Detector Spacing on Flat Ceilings

Spacing criteria establishing an individual coverage radius of $7.5\,\text{m}$ ($112\,\text{m}^2$ area), with practical simplified design limits of $100\,\text{m}^2$ maximum coverage per detector, $5\,\text{m}$ maximum distance to any wall, and $10\,\text{m}$ maximum spacing between adjacent detectors.

Heat Detector Spacing on Flat Ceilings

Spacing criteria establishing an individual coverage radius of $5.3\,\text{m}$ ($56.3\,\text{m}^2$ area), with practical simplified design limits of $50\,\text{m}^2$ maximum coverage per detector, $3.5\,\text{m}$ maximum distance to any wall, and $7\,\text{m}$ maximum spacing between adjacent detectors.

Ceiling Depth Limits for Detector Sensing Elements

The vertical mounting depth beneath a ceiling or roof apex: heat detector sensing elements must be positioned between $25\,\text{mm}$ and $150\,\text{mm}$ below the surface, while smoke detector sensing elements must be positioned between $25\,\text{mm}$ and $600\,\text{mm}$ below.

Corridor Smoke Detector Spacing Limits

Installation guidelines for corridors less than $2\,\text{m}$ wide permitting maximum detector spacing along the centreline to increase up to $15\,\text{m}$ between detectors and $7.5\,\text{m}$ to an end wall (noting that heat detectors are not recommended for escape corridors).

Ceiling Obstruction Rules for Detectors

Installation rules stating that ceiling downstands or beams with a depth exceeding $10\%$ of floor-to-ceiling height must be treated as walls, and detectors must maintain a minimum clearance of $500\,\text{mm}$ from any such obstruction.

Centrifugal Fire Alarm Bell

A modern polarised audible device consisting of a miniature D.C. motor driving a rotating striker through a reduction gearbox to strike an enamelled steel gong, consuming approximately $25\,\text{mA}$ to produce an audio output of roughly $90\,\text{dB}$.

Electromagnetic Fire Alarm Bell

A traditional sounder where current energises a solenoid coil to magnetically pull a steel plunger against a strike dome, simultaneously opening a set of contacts to interrupt current and spring-return the plunger.

Fire Alarm Siren

A polarised audible device producing a high audio output between $100\,\text{dB}$ and $105\,\text{dB}$ while drawing a relatively low current of approximately $15\,\text{mA}$, making it suited for noisy factories and industrial works.

Electronic Sounder

A solid-state polarised acoustic warning device consuming around $20\,\text{mA}$ to generate roughly $100\,\text{dB}$ of sound, configurable to emit continuous, intermittent, or warble (variable pitch) alarm patterns.

Reverse Polarity Sounder Supervision

A monitoring method where the control panel maintains a reverse DC voltage on the sounder circuit during normal standby; series diodes inside each sounder block current while allowing supervisory current through a $6800\,\Omega$ EOL resistor to detect open and short circuits.

General Alarm Audibility Requirement

The requirement that sounders produce a minimum sound level of either $65\,\text{dB}$ or $5\,\text{dB}$ above any persistent background noise exceeding $30\,\text{s}$ (whichever is greater) across $500\,\text{Hz}$ to $1000\,\text{Hz}$, without exceeding a maximum level of $118\,\text{dB}$.

Bedhead Audibility Requirement

The mandatory minimum sound pressure level of $75\,\text{dB}$ required at the bedhead with all intervening doors closed in premises where an audible alarm is designed to awaken sleeping occupants.

Door Acoustic Attenuation

The reduction in fire alarm sound intensity caused by building partitions, rated as a loss of at least $20\,\text{dB}$ through standard single doors and at least $30\,\text{dB}$ through solid fire-rated doors.

Sounder Circuit Redundancy Rules

Mandatory installation criteria requiring that a fire alarm system provide at least two sounders wired across at least two separate circuits, ensuring audible coverage remains operative if one circuit fails and that at least one sounder near the panel stays operational.

Standby Power Supply Battery Duration

Secondary power capacity requiring rechargeable cells (lead-acid or nickel-cadmium, minimum $4\,\text{year}$ design life) to power the full alarm load for at least $30\,\text{minutes}$ following a minimum standby period of $24\,\text{hours}$.

Mains Electrical Supply for Fire Alarm Panels

A dedicated $230\,\text{V}$, $50\,\text{Hz}$ AC supply connected via an exclusive switch-fuse with a red cover labelled 'FIRE ALARM: DO NOT SWITCH OFF', transformed and rectified to provide a safe operating DC voltage and maintain battery float charge.

Detection Zone Boundaries and Limits

A physical subdivision within a protected building restricted to a maximum floor area of $2000\,\text{m}^2$, a maximum search distance of $30\,\text{m}$, and a single storey (unless the entire building floor area is $\le 300\,\text{m}^2$).

Alarm Zone Criteria

An area bounded by fire-resisting construction configured so that occupants receive an unambiguous, uniform alert or evacuation warning without acoustic confusion from adjacent zones, coinciding with one or more detection zones.

Main Emergency Isolating Switch

A dedicated, clearly marked switch positioned at the fire brigade entrance that directly or remotely disconnects the entire electrical supply to an installation during a fire event.

Firefighter's Switch Requirements

A red-painted switch complying with I.S. EN 60669-2-6 or I.S. EN 60947-3, labelled 'Fireman's Switch' with clear ON and OFF positions and an anti-accidental-reset latch, mounted no higher than $2.75\,\text{m}$ above standing level to isolate discharge lighting.

Wiring Penetration Fire Stopping (I.S. 10101 Clause 527.2)

The requirement that any opening remaining around cable or containment penetrations through fire-rated elements must be sealed internally and externally to match the element's rated fire resistance, backed by cable supports within $750\,\text{mm}$ to resist mechanical collapse.

Flammable Liquid Equipment Precaution (I.S. 10101 Clause 421.5)

The safety rule mandating containment precautions, such as a retention pit or ventilated fire-resistant chamber with sills, whenever electrical equipment in a single location contains flammable liquid in excess of $25\,\text{L}$.

Standard PH 30 vs Enhanced PH 120 Fire-Resistant Cables

Cable classifications under I.S. EN 50200 where Standard PH 30 provides $30\,\text{minutes}$ of fire survival, whereas Enhanced PH 120 (meeting BS 8434-2) provides $120\,\text{minutes}$, mandatory in buildings over $30\,\text{m}$ high, phased evacuation, or critical routes through occupied zones.

Fire Alarm Cable Segregation Rules

The requirement that fire alarm circuits be mechanically separated from other services using dedicated conduit/trunking, metallic sheathing (e.g., MICC), or an air spacing of at least $300\,\text{mm}$ along parallel runs ($15\,\text{mm}$ at $90^\circ$ crossovers).

Fire Alarm Cable Support Integrity

The installation rule requiring that all cable supports and fixings be non-combustible and capable of withstanding the same fire exposure as the cable, strictly prohibiting plastic clips, plastic ties, or plastic trunking as the sole means of support.

Fire Alarm Loop Route Physical Separation

The mandatory installation principle under I.S. 3218 requiring that the send and return legs of an addressable loop follow separate physical routes, strictly prohibiting four-core or multi-core cables from sharing both loop legs.

Fire Alarm Conductor Sizing and Colour Coding

Installation standards requiring conductors to have a cross-sectional area of at least $1\,\text{mm}^2$ (except for data cables) and all system cables to feature a single, uniform distinguishing colour, with red or orange preferred.

Cable Reaction to Fire Rating (Class Dca-s2,d2,a2)

The mandatory cable performance standard under I.S. 10101 / EN 50575, specifying flame propagation and heat release ($\text{D}_{ca}$), smoke production ($s2$), burning droplets ($d2$), and combustion gas acidity ($a2$).

Routine Testing Schedule: Daily and Weekly Checks

The maintenance protocol requiring daily checks that the panel indicates normal operation without unaddressed faults, followed by weekly operation of a different initiating device, verification of sounder operation, battery connection checks, and event log entries.

Routine Testing Schedule: Quarterly and Annual Checks

The maintenance protocol requiring quarterly logbook reviews, simulated panel faults, battery inspections, and structural alteration reviews, followed by annual in situ testing of every detector and physical inspection of all fittings and cable integrity.

Fire Alarm Control Panel Auxiliary Circuits

Ancillary control interfaces actuated by the alarm system upon confirmation of fire, including electromagnetic fire door releases, escape door lock releases, ventilation and damper shutdown, extinguishing system activation, and off-site monitoring transmission links.