PIR Detector Siting: Coverage Angles, Pet Immunity, Anti-Masking and Avoiding False Alarms

Summary

PIR (Passive Infrared) detectors are the workhorses of the UK intruder alarm industry. They detect the change in infrared energy caused by a warm body moving across their detection curtains. When sited correctly they are extremely reliable; when sited poorly they generate false alarms, nuisance activations, and gaps in coverage that a burglar can exploit.

The detection pattern of a PIR is not a solid cone — it is a series of discrete detection curtains arranged in a fan. Movement must cross these curtains to be detected. This means siting matters: a detector aimed straight at an approach path (longitudinal movement) will detect an intruder far later, and with less reliability, than one positioned so the intruder walks across the curtains.

False alarms cost the industry money, erode customer confidence, and — most importantly — risk cancellation of police response. NPCC policy allows police forces to withdraw response from systems that generate a defined number of false alarms per year. Correct siting at installation is the most effective way to prevent this.

Key Facts

• Mounting height — standard domestic PIR: 2.0–2.4 m; long-range detectors: up to 2.5 m; always check manufacturer data sheet for optimum height
• Horizontal coverage — standard: 90°; wide-angle: 120°; some corridor detectors: 15° × 80° vertical (curtain pattern)
• Range — standard: 10–15 m; long-range: 20–25 m; pet-immune: typically 10–12 m
• Detection works on cross-axis movement — walk parallel to the detector face for best detection; walking directly towards the detector gives worst detection
• Corner mounting — most effective siting; detector covers two approach routes from one device
• Pet immunity — vertical masking of lower detection zones; animals up to 25 kg (BS EN 50131-2-2 test standard) not detected if they remain below approximately 50–60 cm
• Anti-masking — Grade 2 and above requirement (BS EN 50131-2-2:2017); detects foam spray, tape, or close-proximity obstruction of the detector lens
• Anti-masking activation — Grade 2: can generate fault; Grade 3: must generate alarm condition
• RF immunity — all modern detectors should meet EN 50130-4 for electromagnetic immunity; check spec sheet
• Temperature range — UK indoor installation: Class I environmental class (0°C to +40°C) is standard
• Creep zones — area immediately below the detector that may not be covered; some detectors have a down-look zone to fill this gap
• Dual-technology detectors — combine PIR with microwave (Doppler); both must trigger simultaneously for alarm; dramatically reduces false alarm rate in difficult environments
• Staircase detectors — special wide-angle unit sited at top of stairs; curtains cover both treads and landing
• BS EN 50131-2-2:2017 — the specific standard for passive infrared detectors in intruder alarm systems

Quick Reference Table

Detailed Guidance

Detection Pattern Fundamentals

Understanding how a PIR generates its pattern is essential for correct siting. The detector uses a segmented Fresnel lens that focuses IR energy from different zones of the room onto the pyroelectric sensor. Movement is detected when the sensor sees a rapid change in IR level as a body moves from one lens segment to the next.

This creates discrete "fingers" or "curtains" of detection, not a solid detection zone. The gaps between curtains are dead zones. This is why:

1. Cross-axis detection is superior — the intruder crosses multiple curtain boundaries, generating multiple pulses
2. End-on detection is poor — walking directly at the detector, the body remains in one curtain for longer and the signal is weaker
3. Corner mounting maximises cross-axis coverage for the most likely approach routes

Practical layout:

• Mount in a corner, angled to cover the main approach routes into the room
• Avoid siting where the primary direction of movement will be towards/away from the detector
• In rooms with a single door, site the detector so the curtains are perpendicular to the door's swing

Pet-Immune Siting Requirements

Pet-immune detectors work by masking out the lower detection zones so that small animals moving below approximately 50–60 cm are not detected. This creates two constraints:

1. Mounting height is critical — the detector must be mounted at the correct height stated in the data sheet (typically 2.0–2.1 m); too low and the pet immune zone drops too close to the floor; too high and the zone rises above medium-sized pets
2. Climbing is the enemy — a 15 kg springer spaniel that can climb onto a sofa, counter, or staircase landing will be above the masking zone and will trigger the detector

Practical guidance for pet-immune installations:

• Walk the site and identify where pets typically move and sleep
• Note any furniture the pet can climb on (sofas, beds, counters)
• If a pet routinely goes to height, pet-immune PIR may not be suitable — discuss with customer and document the limitation
• Do not install pet-immune detectors in rooms where cats are kept (cats climb)
• Re-site or add a passive beam break at height rather than relying on pet-immunity in high-climb-risk rooms

The BS EN 50131-2-2:2017 test methodology for pet immunity uses a 25 kg dog-shaped thermal mass moved across the detector field — detectors that pass this test must not generate an alarm for the test mass. This does not guarantee immunity to all real-world pets; manufacturer field testing data is more informative.

Anti-Masking Technology

Anti-masking detects attempts to defeat the PIR by placing an obstruction over the lens. Common attack methods include:

• Aerosol foam sprayed onto lens
• Tape applied over lens
• Close-proximity card or board placed in front of lens

Anti-masking is implemented in one of three ways:

1. IR proximity sensor — a secondary IR emitter/receiver in the detector detects close proximity of an object; effective against foam and tape
2. Optical masking detection — monitors the received IR signal pattern for a sudden drop (blockage)
3. Combination — both methods simultaneously for higher grade compliance

Under BS EN 50131-2-2:2017:

• Grade 2 — anti-masking must be fitted; triggering anti-masking may generate a fault (not necessarily an alarm)
• Grade 3 — anti-masking trigger must generate an alarm condition and be reported to the ARC

Installer note: anti-masking will be triggered by installation errors such as:

• Detector mounted too close to a wall (masking zone too small)
• Object placed on shelf immediately below detector
• Loose tamper cover

Always test anti-masking function during commissioning — the system must generate the correct response (fault or alarm) when the lens is covered.

Common Causes of False Alarms and Prevention

FALSE ALARM CAUSE FINDER
═══════════════════════════════════════════════════════
Alarm activation in empty property?
├── Has anyone entered recently? → Check log; may be genuine
└── No entry → Continue:
    │
    ├─ IS IT DAY OR NIGHT?
    │   ├── Day → Likely thermal: check for direct sunlight
    │   │         on lens or nearby heat source
    │   └── Night → More likely: draught, vibration, RF
    │
    ├─ CHECK ZONE TIME:
    │   ├── Short burst (< 5 sec) → Draught or vibration
    │   └── Sustained → Thermal soak or genuine intruder
    │
    ├─ THERMAL CAUSES:
    │   ├── South-facing window? → Relocate or add blind/curtain
    │   ├── Radiator directly below? → Raise bracket, move detector
    │   └── Air con duct nearby? → Move to avoid airflow
    │
    ├─ VIBRATION CAUSES:
    │   ├── Railway/road traffic? → Add anti-vibration mounts
    │   ├── Washing machine/dishwasher in room? → Dual-tech
    │   └── Loose fitting/conduit vibrating? → Secure mechanically
    │
    └─ RF INTERFERENCE:
        ├── Near DECT phone base? → Move detector > 1m
        ├── Near WiFi router? → Move; check EN 50130-4 compliance
        └── Near neon light starter? → Replace lamp or relocate

Corridor and Staircase Siting

Long corridors need curtain-pattern detectors, not standard wide-angle units. A wide-angle PIR at the end of a 20 m corridor will have poor detection in the first 10 m due to end-on approach.

Use corridor (curtain) detectors — typically 15° horizontal × 80° vertical — positioned at ceiling height (2.4–3.0 m) and aimed lengthways. The intruder crosses multiple vertical curtains as they walk along the corridor.

For staircases:

• Mount a staircase detector (wide-angle, ceiling or near-ceiling siting) at the top of the stairs
• Ensure coverage extends to the bottom tread and the landing
• Pet-immune staircase detectors are not effective if pets can access stairs

Frequently Asked Questions

How close to the ceiling should a PIR be?

Leave at least 150–200 mm between the top of the detector and the ceiling. The thin warm air layer at the ceiling can cause thermal drift. Mounting flush to the ceiling also restricts the downward detection angle, leaving a potential "creep zone" immediately below the detector.

Can a PIR detect movement through glass?

No. Standard PIR detectors cannot detect movement through glass — glass is opaque to the mid-wave infrared that PIRs operate at (8–14 μm). A person outside cannot trigger an interior PIR. However, if a window is open, warm body heat can reach the detector and cause spurious activations — ensure detection zones do not point at open windows.

What is the difference between Grade 2 and Grade 3 in terms of PIR specification?

Grade 2 requires anti-masking but allows it to generate a fault rather than an alarm. Grade 3 requires anti-masking to generate an alarm condition reported to the ARC. Grade 3 detectors also have more stringent environmental testing and may require a higher tamper protection level. Always verify the CE/UKCA marking on the detector shows the grade and environmental class.

How many PIR detectors should be fitted per room?

Typically one per room is sufficient if sited correctly (corner mounting, cross-axis coverage). Large rooms (over 15 m × 10 m), L-shaped rooms, or rooms with areas that cannot be covered from a single corner may require two detectors. Avoid overlapping coverage zones at close range — dual-detection with immediate alarm can mask test problems.

Should I use a dual-technology detector?

Dual-tech (PIR + microwave) detectors require both technologies to trigger simultaneously before generating an alarm. This dramatically reduces false alarms in challenging environments (retail stores, industrial units, garages). The trade-off is slightly reduced detection probability — if the microwave element is in a dead zone, the detector won't alarm even if PIR sees movement. Use dual-tech where false alarms are the primary concern; use standard PIR where detection reliability is paramount.

Regulations & Standards

• BS EN 50131-2-2:2017 — Alarm systems; intruder and hold-up systems; passive infrared detectors; test and performance requirements by grade

• BS EN 50131-1:2006+A3:2022 — General requirements; defines grades and environmental classes for all intruder alarm components

• BS EN 50130-4:2011+A1:2014 — Alarm systems; electromagnetic compatibility; product standard

• PD 6662:2017 — UK application document; references detector grade and anti-masking requirements

• NPCC Security Systems Policy — false alarm thresholds that trigger withdrawal of police response

• BS EN 50131-2-2:2017 — BSI standard for passive infrared detectors

• NSI False Alarm Management Guidance — NSI guidance on reducing false alarms

• BSIA False Alarm Reduction Guidelines — BSIA Form 175, managing false alarms in intruder alarm systems

• Pyronix PIR Technical Guide — Example manufacturer technical installation guidance

• Ajax Systems Detector Handbook — Example modern wireless detector documentation

• intruder alarm grades — Grade requirements that determine anti-masking specification

• wireless alarm systems — Wireless PIR detectors and their grade compliance

• security system commissioning — Walk-test procedure for verifying PIR coverage

• security system maintenance contracts — Annual PIR test and inspection requirements