Intrusion alarms: detect the right event, not every event

An intrusion system that cries wolf is worse than none at all, because it teaches the people receiving its alarms to ignore them. The goal is not to detect everything that moves; it is to detect the things that should not be happening, in the places where they would matter, at the times when they would be unexpected.

Define protected zones

That makes the environment the starting point. A perimeter with vegetation and passing traffic, an entrance used by deliveries, an after-hours office and a server room each have a different definition of normal, and each needs a detector type and a sensitivity chosen for it. Zoning should follow how the building is actually used, and arming schedules should follow when people are genuinely meant to be absent.

Match sensors to the environment

Then design the response, because detection without it is just noise. Decide who is notified, in what order, how they verify the alarm — usually against a camera covering that zone — and what they do next. Walk-test every zone at commissioning, and retest periodically: a detector aimed past a shelf that was moved six months ago stops protecting anything.