Intruder Alarm Systems

Flashcards covering detection devices, circuits, signalling, zones, and IS 199 standards from Intruder Alarm Systems Unit 6.4.5.

Intruder Alarm System

An electrical installation designed to detect and signal the presence, entry, or attempted entry of an unauthorized person into protected premises.

Five Main Components of an Intruder Alarm System

The control panel, detection devices, signalling devices, remote keypad (RKP), and interlinking cabling.

Open Loop Alarm Circuit

An unsupervised detector circuit employing normally open (NO) contacts wired in parallel, where closing any contact completes the circuit to energise an alarm relay.

Primary Vulnerability of an Open Loop Circuit

If an interconnecting cable is cut or broken (open circuit), the system cannot detect the break and will fail to trigger when an alarm contact closes.

Closed Loop Alarm Circuit

A supervised circuit employing normally closed (NC) contacts wired in series, through which a continuous monitoring current passes to hold a sensing relay energized.

Primary Vulnerability of a Simple Closed Loop Circuit

If an intruder places a short circuit across the detection loop conductors, all downstream contacts are bypassed without triggering an alarm condition.

Tamper Circuit (Anti-Tamper Loop)

A continuously active ($24\,\text{hour}$) closed loop running through device microswitches and cables that initiates an alarm if an equipment cover is removed or wiring is severed.

End of Line (EOL) Resistor

A precision terminating resistor placed at the end of a detection zone circuit, enabling the control panel to monitor total line impedance for both open circuits and short circuits.

Four-Wire Wiring Method

A wiring arrangement that utilizes two separate conductors for the normally closed alarm detection loop and two dedicated conductors for the continuous tamper loop.

Two-Wire Line Monitoring System

A circuit technique using internal shunt resistors across individual contacts combined with an EOL resistor, allowing the panel to identify specific sensor activations and tamper states over a single pair of cores.

Polarity Reversal Line Supervision

A monitoring configuration using continuity diodes in parallel with contacts; when disarmed (day mode), current flows freely through the diodes, but when armed (night mode), polarity reverses and forces current through the contacts.

Double-Pole Detection Circuit

A closed monitoring circuit arranged so that two adjacent conductors are maintained at significantly different electrical potentials throughout their length.

Standard Alarm Cable Core Sizes

Multi-core cabling manufactured in $4$-, $6$-, and $8$-core variants, with $8$-core commonly standardized across commercial installations to provide spare cores.

Perimeter Protection

The outer security layer applied to property boundary lines and access gates (such as active infrared or microwave beam barriers) to detect intruders before they reach a building.

Shell Protection

The structural security layer guarding the external boundary of a building (doors, windows, and outer walls) using reed contacts, shock sensors, and glass break detectors.

Trap Protection

The internal volumetric detection layer positioned in hallways, staircases, and corridors (using PIRs or pressure mats) to intercept an intruder moving through the building.

Magnetic Reed Switch

A perimeter sensor consisting of sealed ferromagnetic reeds inside a frame-mounted tube that remain closed in proximity to a sash-mounted magnet, opening when the door or window opens.

Operating Gap Consideration (Magnetic Switch)

The maximum permissible alignment distance between the magnet and reed switch, which if exceeded during installation causes commissioning failure or intermittent false alarms.

Inertia Detector (Shock Sensor)

A perimeter sensor consisting of a gold-plated ball seated on contacts that momentarily breaks an electrical circuit when mechanical impact vibrations travel through the frame.

Pulse Count

A programmable panel setting ($1\text{ to }9$) that requires an inertia sensor to open a preset number of times within a time window before sounding an alarm, filtering out wind gusts.

Vibration Detector (Leaf Spring)

An electromechanical sensor composed of a vertically suspended leaf spring with a weighted bottom end and moving contact that separates from a fixed casing contact during physical movement.

Piezoelectric Impact Detector

An electronic sensor containing a supported quartz crystal rod that oscillates under mechanical shock to generate an EMF, allowing operation in any physical plane without relying on gravity.

Pressure Mat

A normally open secondary detection device placed under carpets consisting of conductive foil plates separated by perforated foam or resistive globules that closes upon foot pressure.

Window Foil Strip Specifications (IS 199)

Continuous metallic strips adhering to glass surfaces with a maximum thickness of $0.04\,\text{mm}$ for aluminium or $0.08\,\text{mm}$ for lead, and a maximum width of $12.5\,\text{mm}$.

Passive Infra-Red (PIR) Detector

A volumetric motion sensor that emits no energy but utilizes a pyroelectric sensor and segmented lens to detect sudden shifts in ambient thermal infrared radiation across its optical zones.

Six Functional Parts of a PIR Detector

The protective case, window with optical filter, focusing device (Fresnel lens or mirror), pyroelectric sensing element, amplifying circuit, and signal comparator/processor.

Pyroelectric Sensor Operation

A ceramic crystalline element that temporarily alters its internal electrical polarity and produces a microvolt charge in response to minute temperature variations.

Optimal PIR Siting Criterion

Positioning the sensor so an approaching intruder cuts across the optical detection zones at a right angle, which maximizes the rate of thermal change compared to a direct head-on approach.

Pet Immunity (PIR)

A signal processing algorithm that evaluates thermal target amplitude and profile to ignore heat signatures typical of domestic animals below specific body weights or heights.

Anti-Masking Protection

A detector security feature that triggers a tamper/alarm signal if an unauthorized person attempts to blind, spray, or cover the sensor lens while the system is disarmed.

Active Infra-Red Beam Detector

A point-to-point barrier device comprising an infrared LED transmitter and an aligned photoelectric receiver that activates an alarm when an intruder interrupts the invisible modulated light beam.

Ultrasonic Motion Detector

An active space sensor that radiates inaudible acoustic waves (typically above $30\,\text{kHz}$) and monitors reflections for Doppler frequency shifts caused by intruder movement.

Beat Frequency

The differential frequency generated when a microphone mixes the stable transmitted acoustic frequency with the Doppler-shifted reflected wave coming off a moving target.

Microwave Point Detector

An active volumetric detector that radiates ultra-high frequency electromagnetic waves ($10.7\,\text{GHz}$ or $1.48\,\text{GHz}$) to register intruder motion using the Doppler effect.

Doppler Effect

The physical shift in wave frequency occurring when source and reflector move relative to one another, compressing waves to increase frequency on approach and elongating them on retreat.

Dual Technology Motion Detector

A sensor integrating passive infrared (PIR) and microwave detection within a single housing, requiring both channels to trigger simultaneously before signaling an alarm.

Acoustic Glass Break Detector

A shell sensor utilizing a specialized microphone and filter circuitry to detect the distinct high-amplitude airborne acoustic frequencies of shattering glass ($3\text{ to }5\,\text{kHz}$).

Piezoelectric Glass Break Detector

A contact transducer bonded directly to a glass pane that converts structural flexural shock waves into a proportional electrical signal upon impact or breakage.

Seismic Detector

A high-security contact sensor calibrated to detect characteristic structural acoustic frequencies generated by thermal lances, diamond-core drills, or explosives attacking bank vaults.

Control Panel (CP)

The central processing hub that supervises circuit loops, processes detector triggers, executes timing sequences, controls sounders, and interfaces with external communicators.

Non-Volatile Memory (NVM)

An onboard memory chip within the control panel that permanently stores programmed system parameters, zone configurations, and access codes during total electrical blackout.

Remote Keypad (RKP)

A peripheral interface keypad allowing the user to arm, disarm, review system status, reset alarm conditions, and configure engineering parameters via unique numeric codes.

Detection Zone

A distinct electrical input circuit to which one or more detection devices are connected, allowing the control panel to locate activations and isolate faults.

Entry/Exit Zone

A designated path programmed with a countdown delay that provides occupants sufficient time to vacate after setting or disarm the panel upon entering before triggering an alarm.

Intermediate (Inhibited) Zone

A zone along the entry route that permits delayed transit without sounding an alarm during an active countdown, but functions as an immediate alarm zone if entered unannounced.

Personal Attack (PA) Circuit

A permanently supervised ($24\,\text{hour}$) unlatched circuit wired to manual panic buttons that signals an immediate audible or silent distress condition regardless of system state.

Duress Signalling

The covert generation of a silent alarm transmission to a central monitoring station accomplished by entering a specific distress PIN code at the keypad under coercion.

Full Set vs. Part Set

Full Set arms every programmed detection zone across the entire facility, whereas Part Set arms only a pre-selected subset of zones to permit occupant movement in unprotected areas.

Walk Test Facility

A diagnostic mode in which an installer or occupant triggers each sensor sequentially, receiving confirmation via internal chime and visual LEDs without sounding the main siren.

Chime Facility

A convenience setting that prompts the control panel to sound a gentle internal tone whenever designated perimeter contacts are opened while the system is unset.

Shunt Lock

An exterior key-operated mechanical lock incorporating an integrated electrical switch that temporarily bypasses a zone to permit authorized physical entry without alarming.

Self-Activating Bell (SAB)

An external sounder unit containing an internal rechargeable battery and hold-off relay that sounds automatically if external supply cables are severed or an alarm trigger is applied.

SAB Hold-Off Supply Mechanism

A continuous $12\,\text{V DC}$ line from the panel that energizes an SAB internal relay to keep the sounder disconnected from the internal battery while maintaining battery trickle charge.

External Sounder Cut-Off Duration (IS 199)

The statutory noise pollution interval requiring an external sounder to shut off automatically after sounding for a duration between $15\text{ and }30\,\text{minutes}$.

External Strobe Flashing Duration (IS 199)

The requirement that the xenon visual beacon continue flashing for at least $60\,\text{minutes}$ following sounder timeout until manually reset at the control panel.

Minimum Audible Alarm Output (IS 199)

The statutory minimum sound output from an external sounder module, which must not fall below $70\,\text{dB(A)}$ measured at a distance of $3\,\text{metres}$.

Standby Battery Quiescent Capacity (IS 199)

The requirement that the control panel standby battery maintain full system protection in an armed, quiescent state for at least $12\,\text{hours}$ during a total mains failure.

Battery Recharge Capacity Specification (IS 199)

The standard requiring the panel charging circuit to restore an exhausted standby battery to at least $80\%$ of its nominal rating within $24\,\text{hours}$.

Mains Connection Standard (IS 199)

The requirement that intruder alarm panels connect to the building mains supply via a solid, unswitched fused neon spur rather than a standard plug and socket.

Zone Device Quantity Limitations (IS 199)

A maximum of $12$ contact switches or pressure mats per zone, a maximum of $3$ devices on an entry/exit circuit, and a maximum of $1$ movement detector per zone unless latching.

Mandatory Maintenance Frequency (IS 199)

The requirement that the complete intruder alarm installation undergo professional inspection, testing, and servicing at routine intervals not exceeding $6\,\text{months}$.

Security Grades Classification (EN 50131)

A standardized European system classifying alarm systems into four grades ($1\text{ to }4$) matching system resilience against intruder knowledge, skill, and equipment availability.

Lowest Grade Component Principle

The compliance rule mandating that an alarm system's overall security grade matches the grade of its lowest-rated individual component.

Digital Communicator and Speech Dialler

Signalling modules that transmit digitized emergency codes to a central monitoring station or dial pre-programmed phone numbers (minimum 3) to deliver recorded verbal messages.

Ionisation vs. Optical Smoke Detectors

Ionisation units specialize in fast-flaming, clean-burning fires (wood, paper), whereas optical units excel in detecting slow, smouldering fires (foam, plastics), both operating at $12\,\text{V DC}$.