Intruder Alarm Systems: Principles, Detection, Signalling, and Standards

Vocabulary flashcards covering key definitions, detection devices, circuit topologies, signalling equipment, and IS 199 compliance requirements for intruder alarm systems.

Intruder Alarm System

An electrical system designed to detect and signal the presence, entry, or attempted entry of an intruder into protected premises.

Open Loop Alarm System

A detector circuit where sensors with normally open ($\text{NO}$) contacts are wired in parallel; closing a contact energises a relay to sound the alarm, but disconnected or damaged wiring cannot be distinguished from normal conditions.

Closed Loop Alarm System

A detector circuit where sensors with normally closed ($\text{NC}$) contacts are wired in series; current continuously maintains a relay in an energised state, and breaking any contact de-energises the relay to trigger the alarm sounder.

Tamper Loop (24-Hour Circuit)

A continuously monitored circuit that operates in both set and unset states to trigger an alarm if cables are cut, disconnected, or if detector enclosure microswitches open.

End-of-Line (EOL) Resistor

A terminating resistor installed at the end of a detection loop that allows the control panel to constantly measure and analyze circuit impedance, detecting both open-circuit disconnections and short-circuit bypasses.

Dual Operation Two-Wire System

A wiring method utilizing specific internal shunt resistor values inside individual devices (such as contacts, sensors, and panic buttons) across a two-core cable, allowing the panel to detect the specific detector type tripped while maintaining full tamper protection.

Zone

A designated area of a building (such as a room, floor, or set of windows) protected by one or more detectors connected via an alarm cable looped back to the control panel.

Magnetic Switch

A perimeter detection device consisting of a frame-mounted reed switch and a door- or window-mounted magnet that opens the electrical circuit when the moving component is displaced.

Operating Gap

The specified installation distance between a reed switch and its corresponding magnet; exceeding this gap causes commissioning failure or subsequent false alarms.

Pressure Mat

A secondary backup detector designed for staircases or under rugs, composed of two thin conductive plates separated by globules of high resistance that close an electrical circuit when stepped on.

Inertia Detector (Shock Sensor)

A vibration detector mounted on doors or windows consisting of a gold-plated ball seated in contacts that open upon structural shock waves or sudden strikes.

Pulse Count

A programmable panel setting (typically $1\text{ to }9$) specifying how many times an inertia sensor switch must operate before an alarm activates, filtering out environmental disturbances like stormy winds.

Leaf Spring Vibration Detector

A mechanical sensor consisting of a suspended leaf spring with a weighted lower end and a moving contact that parts from a fixed casing contact during vibration or movement.

Passive Infra-Red (PIR) Detector

A space detector containing a pyroelectric sensor and Fresnel lens that detects rapid fluctuations in emitted infrared thermal energy without generating any radiation itself.

Fresnel Lens

An optical component in a PIR detector that focuses radiated infrared energy from designated coverage zones onto the face of the pyroelectric sensor.

Pet Immunity

A motion sensor signal processing feature that ignores the specific infrared thermal signatures typical of small animals to prevent false alarms.

Infra-Red Beam Detector

An active barrier detector consisting of an infrared transmitter and an optical receiver mounted directly opposite each other, triggering an alarm when the modulated or pulsed light beam is broken.

Ultrasonic Detector

A detector that emits high-frequency sound waves (usually above $30\,\text{kHz}$) to establish a stabilized room frequency and triggers when physical movement causes Doppler frequency shifts.

Acoustic Glass Break Detector

A shell protection device using a microphone and processing circuitry to detect high-amplitude shock frequencies produced during window breakage, typically peaking around $3\,\text{kHz to }5\,\text{kHz}$.

Dual-Technology Sensor

A sensor that combines two distinct detection methods in a single unit—such as receiving an airborne acoustic wave and simultaneously sensing structural mechanical vibrations—to prevent false alarms.

Piezoelectric Glass Break Detector

A sensor mounted on or near window structures that detects mechanical shock frequencies traveling through breaking glass and surrounding frames.

Exterior Microwave Detection Device

A point or beam detector that emits radio frequency microwaves across an area and analyzes reflected wave frequency changes via the Doppler Effect to identify moving intruders.

Doppler Effect

The physical shift in frequency or wavelength that occurs when moving objects compress or expand reflected waves relative to a stationary sensor.

Perimeter Protection

Security detection focused on the boundary fence, gates, and property line to identify unauthorized entry before a person approaches physical structures.

Shell Protection

Security detection applied directly to the exterior walls, windows, and doors of a building using reed switches, shock sensors, and glass break detectors.

Trap Protection

Internal volumetric security protecting hallways, stairways, and passageways using devices like PIRs and pressure mats to detect intruders moving inside a building.

Remote Keypad (RKP)

A user and technician interface utilized to arm and disarm the alarm, set entry/exit codes, and access engineer mode for parameter configuration and hardware diagnostics.

Entry/Exit Zone

A timed control panel circuit programmed to give occupants adequate countdown time to exit after arming or reach the keypad to disarm upon entry without sounding an alarm.

Personal Attack (PA) Zone

A continuously monitored ($24\text{-hour}$) input triggered by manual hand or foot switches, capable of sounding an audible alert or sending a silent emergency signal to a central monitoring station.

Duress Signalling

The transmission of a silent panic alarm to a central monitoring station caused by an authorized user entering a specific forced-entry code at the keypad under coercion.

Walk Test

A testing routine activated at the control panel that produces an audible tone each time a detection device is triggered, verifying functionality without activating external sounders.

Self-Activating Bell (SAB)

An external alarm housing containing an internal rechargeable battery, sounder, and hold-off relay that sounds automatically if its power feed or tamper loop from the panel is severed.

Internal Sounder

An indoor sounder that provides audible entry/exit countdown beeps and activates immediately during tamper faults or full alarm conditions.

Xenon Strobe

A visual warning device that discharges high voltage across xenon gas to produce brilliant flashes (at $60\text{ flashes per minute}$), designed to continue flashing after audible sounders time out.

Digital Communicator

A dialing device located in the panel that connects to the telephone network to transmit data or recorded speech messages to pre-programmed telephone numbers or a monitoring station upon alarm activation.

Central Monitoring Station

A secure, $24\text{-hour}$ manned monitoring facility that receives incoming alarm and tamper transmissions and coordinates emergency action with property keyholders or the Gardaí.

Shunt Lock

A specialized key-operated exterior door lock containing internal switching contacts that bypasses an alarm zone to permit authorized entry without sounding the alarm.

Ionisation Smoke Detector

A smoke detection device powered at $12\,\text{V DC}$ that is suited for detecting fast, clean-burning hydrocarbon fires such as burning wood or paper.

Optical Smoke Detector

A smoke detector powered at $12\,\text{V DC}$ suited for identifying slow-burning, smouldering fires characterized by heavy smoke from bedding, plastics, or upholstery.

IS 199 Standby Battery Capacity Requirement

The regulatory requirement that an alarm back-up battery must provide a minimum of $12\text{ hours}$ of full protection during a mains failure and be capable of recharging to $80\%$ capacity within $24\text{ hours}$.

IS 199 Sounder Audible and Duration Limits

The standard mandating that external sounders produce at least $70\,\text{dB}$ at $3\,\text{metres}$, run for a minimum duration of $15\text{ minutes}$, and automatically cut off at a maximum of $30\text{ minutes}$.

IS 199 Routine Maintenance Interval

The mandatory inspection and testing interval for intruder alarm wiring, detection zones, power supplies, batteries, communicators, and sounders, not to exceed $6\text{ months}$.