Intruder Alarm Systems: Principles, Components, and Standards

Comprehensive vocabulary flashcards covering the components, wiring topologies, detection technologies, control zones, signaling devices, and Irish regulatory standards (IS 199) 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.

Control Panel

The central processing hub or brain of an intruder alarm system, comprising a printed circuit board with a microprocessor, a standby battery power supply, and terminals for connecting field wiring, sensors, keypads, and sounders.

Remote Keypad (RKP)

A wall-mounted interface device used to program the alarm panel, set exit and entry delay times, arm and disarm the system with user codes, and access installer diagnostic functions via an engineer code.

Normally Open (NO) Detection Circuit

An unsupervised detector wiring loop where sensor contacts remain open in standby mode and close to sound an alarm; if the cabling is cut or disconnected, the panel cannot detect a fault and detection capability is completely bypassed.

Normally Closed (NC) Detection Circuit

A supervised detector circuit where contacts are wired in series and remain closed in standby mode to keep a relay coil continuously energized; if any sensor contact opens or if the wiring is cut, the relay de-energizes and triggers an alarm.

Tamper Loop (24-Hour Monitoring)

A continuously supervised circuit that remains active at all times ($24\,\text{hours}$, set or unset) to detect deliberate interference, cable cutting, or the opening of device enclosures via internal microswitches.

End-of-Line (EOL) Resistor

A terminating resistor of a specific ohmic value fitted at the end of a detection circuit, allowing the control panel to analyze loop impedance and detect open circuits, normal standby state, or short-circuit tamper attempts.

Diode Line Monitoring (Polarity Reversal)

A line-supervision method where continuity diodes allow current to bypass contacts randomly opening during unarmed (day) mode, but an armed (night) polarity reversal forces current through series contacts so any opened switch triggers an alarm.

Double-Pole 4-Wire System

A standard alarm wiring layout using four cores within a single cable: two cores carry the positive alarm loop through series detector contacts, and two cores carry the negative tamper loop through detector cover microswitches.

Internal Shunt Resistor

A specific resistor wired in parallel with a sensor contact in a two-wire monitored loop; when the contact opens, its resistance value is added to the loop, allowing the panel to identify the exact detector type that tripped.

Perimeter Protection

The outer layer of security applied to the external property boundary fence or gates, designed to detect unauthorized persons entering onto a site before they reach any buildings.

Shell Protection

The security layer protecting the external envelope, walls, doors, and windows of a building using devices such as reed switches, inertia shock sensors, and glass break detectors to detect penetration attempts.

Trap Protection

Internal volumetric security deployed within hallways, staircases, and passageways (such as PIRs and pressure mats) to detect an intruder moving inside a building after the shell has been penetrated.

Magnetic Reed Contact

A perimeter sensor consisting of a reed switch mounted on a static frame and a permanent magnet mounted on the moving door or window sash, which opens to trigger an alarm when the magnet is pulled away.

Operating Gap

The specified maximum distance permissible between a magnetic reed switch and its magnet during installation to ensure proper circuit closure and prevent false alarms or commissioning failures.

Pressure Mat

A trap detector consisting of two thin conductive plates separated by high-resistance globules placed under carpets or on stairs; stepping on it forces the plates together to close a normally open contact, requiring an interface relay for normally closed circuits.

Inertia Sensor (Shock Sensor)

A shell detection device containing a gold-plated ball resting on contacts that momentarily dislodges and breaks an electrical circuit when subjected to physical shock waves or vibrations from a forced entry attempt.

Pulse Count

A programmable sensitivity setting on an alarm panel (typically $1\text{ to }9$) determining how many times an inertia sensor's contacts must open before an alarm activates, preventing false alarms from wind or environmental flexing.

Vibration Detector (Leaf Spring)

A mechanical sensor consisting of a suspended vertical leaf spring with a weight and moving contact at its bottom that parts from a fixed contact on the device casing when agitated by vibration.

Passive Infra-Red (PIR) Sensor

An optical space detector that senses rapid changes in ambient infrared radiation emitted by moving warm bodies across its field of view without generating or transmitting any radiation itself.

Fresnel Lens

A faceted plastic lens in a PIR detector that sections the coverage area into multiple optical fingers or detection zones and focuses incoming thermal infrared energy directly onto the pyroelectric sensor element.

Pet Immunity

A specialized signal-processing feature in motion detectors configured to ignore thermal and volumetric signatures typical of domestic animals below a designated size or weight.

Active Infra-Red Beam

A point-to-point barrier detector consisting of an infrared transmitter aligned directly with an opposing receiver across open spaces, triggering an alarm when an intruder breaks the continuous modulated light beam.

Ultrasonic Detector

A volumetric motion detector that emits inaudible high-frequency sound waves (typically above $30\,\text{kHz}$) into an enclosed space and triggers an alarm when movement alters the reflected sound wave pattern via frequency shifts.

Acoustic Glass Break Detector

A shell detection device utilizing a microphone and microprocessor to detect airborne sound frequencies in the range of $3\,\text{kHz}\text{ to }5\,\text{kHz}$, matching the characteristic acoustic profile of breaking glass.

Dual-Technology Glass Break Detector

A glass break detector that requires both an airborne acoustic frequency and a simultaneous structural mechanical vibration through the frame or wall before initiating an alarm signal.

Microwave Intrusion Detector

A motion sensor that floods an area with microwave radio waves and utilizes the Doppler Effect to detect intruders by measuring frequency shifts in the reflected radio waves caused by movement.

Doppler Effect

The physical shift in the perceived frequency of sound, radio, or light waves caused by relative motion between the transmitting or reflecting object and the receiving sensor.

Entry/Exit Zone

A programmable timed alarm zone providing a preset countdown delay that permits authorized users to leave after arming or enter to disarm at a keypad without triggering an instant intrusion alarm.

Personal Attack (PA) Circuit

A manually operated emergency circuit ($24\,\text{hour}$ monitored) triggered via hand or foot panic buttons to generate an immediate local alarm or transmit a quiet panic signal to an alarm receiving centre.

Duress Code

A covert secondary passcode entered into a control keypad under coercion that appears to disarm the alarm locally while silently transmitting a distress alert to the central monitoring station.

Full Set vs. Part Set

Operating states of an alarm system where 'Full Set' protects the entire premises, while 'Part Set' arms only selected zones (such as downstairs perimeters while occupants sleep upstairs).

Walk Test

A diagnostic routine selected at the control panel allowing an installer or user to walk past and activate all connected detectors to confirm operation via internal chime tones without sounding external sirens.

Self-Activating Bell (SAB)

An external sounder unit containing an internal rechargeable backup battery and a hold-off relay; if the external low-voltage supply from the panel is cut or lost, the relay de-energizes and sounds the bell using its internal battery.

Xenon Strobe Light

A visual beacon integrated into an external bell housing that discharges high voltage across a xenon-filled glass tube to produce brilliant flashes at $60\,\text{flashes/minute}$, continuing for at least $60\,\text{minutes}$ until manually reset.

Digital Communicator

A remote signaling device integrated into an alarm panel connected to the telephone network that dials pre-programmed numbers (typically at least three) or a speech dialler to transmit automated alerts during an alarm event.

Central Monitoring Station (ARC)

A secure, continuously manned ($24\,\text{hour}$) facility that receives transmitted digital signals from alarm communicators and coordinates emergency responses with keyholders or the Gardaí.

Keyholding Service

A contracted service where an alarm installation company holds building keys and dispatches an engineer or security officer to the premises to investigate when notified of an alarm trip by the monitoring station.

Shunt Lock

A key-operated mechanical switch lock fitted to an entry door that bypasses a specific alarm contact or zone, allowing authorized access without setting off the alarm system.

IS 199 Sounder Limits

Irish standards mandating that external sounders produce a sound pressure level of at least $70\,\text{dB}$ at $3\,\text{m}$, run for a minimum of $15\,\text{minutes}$, and automatically shut off within a maximum of $30\,\text{minutes}$.

IS 199 Power and Standby Rules

Irish standards requiring an alarm backup battery to maintain full protection for at least $12\,\text{hours}$ during a mains failure and the panel charging circuit to restore $80\%$ of nominal capacity within $24\,\text{hours}$.

IS 199 Zone Allocation Limits

Standard limits restricting any single alarm zone to a maximum of $12$ detection devices (contacts or pressure mats), a maximum of $3$ devices on an entry/exit zone, and a maximum of $1$ PIR detector per zone.

Four Stages of the Alarm Process

The sequential operational lifecycle of an alarm activation: the Input Stage (detection), the Analysis Stage (panel signal verification), the Response Stage (local sounders/strobes), and the Monitoring Stage (remote alert transmission).

Domestic Exemption (GDPR / Data Protection)

The legal provision under Irish privacy law allowing homeowners to operate security cameras without data controller obligations, provided cameras do not capture public spaces or neighbouring private property.