How Do Burglar Alarms Work? A Plain-English Guide

What are the parts of a burglar alarm system?

• Control panel: the brain — usually a metal box out of sight, with keypads and the app talking to it
• PIR motion sensors: detect body heat moving across a room; pet-immune versions ignore animals up to ~25kg
• Door and window contacts: magnetic switches that trigger when an opening moves
• External sounder (bellbox): siren and strobe on the front of the building — also the most visible deterrent
• Keypads, fobs and apps: how you arm, disarm and check status
• Backup battery: keeps the system running through power cuts (12+ hours on Grade 2 systems)

What happens when the alarm is triggered?

On a bells-only system: the siren sounds (UK law limits external sounders to 20 minutes), the strobe keeps flashing, and the app notifies you — the response is whoever hears it or sees the notification. On a monitored system, the panel also signals an Alarm Receiving Centre within seconds. The ARC follows your agreed plan: call keyholders in order, and on police-response systems with a confirmed activation (two detectors triggered), pass it to police with your URN.

What do zones, part-set and entry delay mean?

Each detector is a zone the panel treats individually. Part-set arms the perimeter and downstairs while you sleep upstairs — contacts and downstairs PIRs live, landing PIR ignored. Entry/exit zones give a programmed delay (typically 30–45 seconds) on the route to the keypad so you can disarm after opening the front door; everything else triggers instantly. Good system design is mostly getting zones and part-set patterns right for how you actually live or work in the building.