9.4 Intrusion Detection Sensors & Alarm Communications

Intrusion Detection as a Layer, Not a Siren

An intrusion detection system (IDS) announces that a barrier was crossed or a space was occupied when it should have been empty. PSP-level design matches sensor physics to the environment, then matches communications to a supervising station that can still receive the signal after copper phone lines are gone. A loud siren with no path and a 40% nuisance rate trains everyone to ignore the system.

North American listing practice for detection units includes UL 639 (intrusion-detection units). Central-station and supervising-station service concepts live in families such as UL 827. European and international alarm families commonly cited on specifications include EN 50131 and related IEC alarm-system standards. Name them when you read a cut sheet. Do not treat a listing as a substitute for aiming a PIR off an HVAC diffuser.

Interior Motion: PIR, Microwave, Dual-Tech

Passive infrared (PIR) sensors look for changes in thermal radiation across detection zones. They do not "see motion" the way a camera does; they see heat contrast moving between segments. HVAC registers, space heaters, sunlight walking across a floor, and pets in the beam are classic nuisance sources. Pet-immune patterns and pet alleys (aiming over the animal) reduce residential-style problems; they also create crawl-under paths if you aim too high in a warehouse office.

Microwave sensors (usually Doppler) detect motion of objects that reflect RF. They can see through some lightweight materials and around some obstructions, which is useful and dangerous. Microwave energy that leaves the room through a window can detect rain or hallway traffic. They are less fooled by heat and more fooled by vibration and moving metal.

Dual-technology detectors typically combine PIR and microwave in AND logic: both must trip. A heater that fools only the PIR, or a vibrating duct that fools only the microwave, is less likely to alarm. That is why dual-tech appears in warehouses with space heaters. The trade: if one technology is blocked, failed, or masked, AND logic can miss a real intruder. Supervised dual-tech and anti-mask features exist because of that. OR logic (either technology) is noisier. Read the spec; do not assume "dual" always means AND.

Walk-test every motion sensor at commissioning, at the actual mounting height, with the HVAC on. A sensor that was walk-tested in a quiet unfinished space will nuisance-alarm on opening day.

Openings: Balanced Magnetic Switches and Glass-Break

Magnetic contacts on doors and windows are the workhorse. A cheap reed switch can be defeated with an external magnet that holds the reed closed while the door opens. A balanced magnetic switch (BMS) uses a bias magnet and a supervised loop (often with end-of-line resistance) so an external magnet disturbs the balance and trips the loop. Use BMS on high-value openings. Use listed contacts on fire doors only in ways the fire-door listing still allows.

Glass-break sensors come in two physics families. Acoustic detectors listen for the frequency signature of breaking glass; range claims assume line-of-sight sound, not heavy drapes, white-noise machines, or a sensor in a soffit around a corner. Shock / inertia sensors mount on the pane or frame and feel the impact. Dual acoustic-plus-shock units reduce some nuisance (thunder, trucks) at the cost of complexity. Tempered, laminated, and filmed glass sound different; match the sensor to the glazing. A warehouse polycarbonate panel is not a showroom window.

Perimeter: Fence, Buried, and Line Sensors

Fence sensors include taut-wire, microphonic cable, fiber-optic cable, and infrared or microwave line beams. They give earlier warning than an interior PIR—and they live outdoors. Wind, ice, wildlife, loose fabric, and poorly tensioned fence fabric are the nuisance budget. Fiber that detects cutting and climbing is not "nuisance-free"; it is a different nuisance (thermal expansion, poor clamping). Always pair fence detection with assessment (video, patrol, lighting). A fence alarm with no way to see the zone becomes a 02:00 guessing game.

Buried line sensors (ported coaxial, some fiber, seismic cables) follow the soil. They can be covert. They also follow standing water, frost heave, and lawn maintenance. Line-of-sight microwave or IR beams across a clear zone need graded ground and a vegetation program; tall grass is an alarm generator.

Duress devices are a different IDS function: the occupant is already inside and under threat. Hold-up buttons, under-desk rails, and duress PINs (a duress variant of the disarm code, sometimes taught as last-digit-plus-one—confirm the vendor's actual behavior) send a silent message. Train for accidental presses. A duress that also sounds a local siren can escalate a robbery. Document whether duress is silent, who is notified, and whether video is automatically marked.

Partitions, Arming, and Nuisance Alarm Rate

Partitions let you arm the warehouse while the offices stay open, or arm the jewelry vault while the sales floor is occupied. Without partitions, night cleaners become a chronic nuisance source—or staff never arms the system. Each partition needs its own keypad or credential arming path, its own delay times, and a clear map of which sensors belong to it. Shared motion in a corridor that serves two partitions is a programming error waiting to happen.

Nuisance alarm rate (unwanted alarms from environment, users, or poor application) is a performance metric. False alarms from failed hardware are a smaller slice in many sites; pets, HVAC, unlocked doors, and "test" trips dominate. Police response policies increasingly penalize chronic nuisance. Design responses are mechanical: relocate the PIR, add a BMS instead of motion on a loading dock with plastic strip doors, put a pet alley where a cat lives, lock the HVAC diffuser away from the sensor, and train closing procedures. Faster dispatch does not fix a sensor looking at a heater.

Pet and HVAC false alarms deserve a dedicated sentence on the exam. A residential-style PIR in a lobby with a floor register will alarm all winter. A dual-tech unit still needs to be aimed. A glass-break over a door closer that slams will teach the night operator to hit ignore.

Alarm Communications: POTS Decline, Cellular, IP, AES Radio

POTS (analog copper telephone) seizure and listen-in were the historical path. Carriers are retiring analog lines, voltage is no longer guaranteed, and a communicator that "has a dial tone in 2014" is not a 2026 design. Treat leftover POTS as a dying backup at best, and test it if it still exists.

IP communicators send signals over customer or managed networks. They are fast and cheap and they die with the router, the firewall rule, or the ISP. Encrypt the path (AES as in the Advanced Encryption Standard cipher is common in product literature). Network design belongs to the security specification: VLAN, failover, and what happens in a power cut.

Cellular communicators (LTE and successor networks, often with dual SIMs or roaming) provide a path that does not depend on the site's LAN. They depend on coverage, antennas, carrier sunsets, and power. Dual-path (IP plus cellular, or cellular plus radio) is the current listed-system pattern for many supervising-station accounts.

AES-IntelliNet and similar mesh radio networks are a North American alarm-industry radio path (the letters AES here are a company/network name, not the cipher). They appear in specifications as a listed wireless alternative where cellular is weak or as an additional path. If a stem says "AES radio to the central station," think alarm mesh radio, then confirm whether encryption (AES the cipher) is also required on an IP path. Do not conflate the two on a design drawing.

A UL supervising / central station concept is a listed facility with trained operators, documented procedures, supervised communications, and a contractual response—not a receptionist who happens to have a keypad app. UL 827-style listing is about the station and the service. Your panel can be UL 639-listed and still report to an unlisted answering service; that is a different (usually weaker) operational model. Know which model the owner bought.

Sequence of operations at 02:00 should be boring: sensor, partition, communicator path A, path B on failure, station operator, call list, video assessment if available, then dispatch policy. If path A is a single ISP with no backup, the sequence ends at the router.

Name UL 639, UL 827, EN 50131, and related IEC alarm families when a specification claims "listed." Then walk the warehouse with the HVAC on.