Designing an intrusion alarm system: zones, partitions and sensor choice
Before anyone specs a single sensor, a proper alarm design starts on paper: what needs protecting, in what groups, and armed under what conditions. Get that right and the hardware choice becomes straightforward.
The design sequence, in order
Map zones
Walk the property and group it into perimeter (fence line, gates, outbuildings), interior (rooms and corridors once the perimeter is breached) and 24h zones such as a safe or server room that stay armed even when the rest of the system is disarmed for daily living.
Define partitions
Split the system into independently armable sections -- arm a garage or office wing overnight while family stays active in living areas, or let a shop floor arm while a caretaker's flat stays live.
Match sensor type to location
PIR motion for open interiors, magnetic contacts for perimeter doors and windows, glass-break for rooms with large glazing, vibration/shock for doors and safes, outdoor beams or curtain PIRs for approach paths -- chosen per zone, not from a generic kit.
Place for false-alarm immunity
Keep PIRs off pet routes and heating vents, avoid mounting near draughty windows or doors that move in the wind, and away from direct sunlight or heat sources -- placement discipline, not a different sensor model.
What a proper design gets right
Protection driven by the property, not the box
When zones and partitions are defined before any sensor is chosen, the resulting sensor count and placement is driven by what actually needs protecting -- not by whatever a generic package happens to include.
Sensor types matched to purpose
PIR, magnetic contacts, glass-break, vibration/shock and outdoor beams each answer a different threat -- movement, opening, breaking glass, physical attack, or approach before the building line.
Placement discipline
Correct siting -- away from pets, draughts and heat sources -- matters as much as the sensor type for keeping false alarms down.
Panel, keypads and sirens as roles
The panel reads every zone and manages the alarm event; keypads at every regular entry point avoid walking through an active zone in the dark; indoor and outdoor sirens are usually wanted together, not as substitutes.
Wired vs wireless, an honest tradeoff
Wired suits new-build or open-wall renovation with no batteries to manage; wireless suits a finished retrofit but needs supervision and periodic battery checks -- a mixed design is common and legitimate.
Monitoring path decided deliberately
Self-monitoring via an app keeps cost low but depends on someone seeing the notification; a central monitoring station (ARC) gives a defined, staffed response procedure -- many projects run both.
What this page covers
A vendor-neutral methodology for the design stage, before any brand or model is on the table.
- {'t': 'Zone and partition planning', 'd': 'How to walk a property and translate it into perimeter, interior and 24h zones, plus independently armable partitions.'}
- {'t': 'Sensor type selection logic', 'd': 'Why PIR, magnetic contacts, glass-break, vibration/shock and outdoor beams each suit different locations -- covered conceptually, without naming a model or a range in metres.'}
- {'t': 'False-alarm placement discipline', 'd': 'The common siting mistakes -- pets, heating vents, draughts, direct sun -- and how to avoid them.'}
- {'t': 'Panel, keypad and siren roles', 'd': 'What each component is responsible for, and how many keypads and siren types a design typically needs.'}
- {'t': 'Power, monitoring and standards orientation', 'd': 'Backup battery as a project-specific decision, self-monitoring vs ARC monitoring as a deliberate tradeoff, and EN 50131 grades as a concept your installer and insurer confirm together.'}
Frequently asked questions
How many zones does a typical home or business need?
It depends entirely on the property's layout and what needs independent protection -- there is no universal number. The right approach is to walk the site and group it into perimeter, interior and any 24h areas before counting anything.
Which sensor detects intruders at the longest range?
Detection range always depends on the specific sensor model, its mounting height and the scene it covers -- check the manufacturer's datasheet rather than relying on a generic figure, and confirm the choice with a certified installer.
Should I choose wired or wireless sensors?
Both are legitimate and the right choice depends on the building: wired suits new-build or open-wall renovation, wireless suits a finished retrofit. Many designs mix a wired perimeter with wireless additions -- an installer can size this to your project.
How long does the backup battery last during a power cut?
This is scope-dependent and calculated per project, not a fixed number of hours -- it depends on the panel load, sensor count and how quickly a fault would realistically be noticed and fixed.
Which EN 50131 grade do I need?
We deliberately don't state a grade here -- grade selection depends on a proper risk assessment and, very often, your insurer's own requirements. Get the assessment from a certified installer and confirm any grade requirement with the insurer before designing to it.
Get your zone and partition plan reviewed
Send us the building layout and what needs independent protection -- our engineers will help you turn it into a zoning and sensor plan before any hardware is specified.
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