An intrusion alarm system detects unauthorized entry into a building or an area within it and raises an alarm, locally and usually at a monitoring centre. It does this with sensors on doors, windows and interior spaces, grouped into zones on a control panel that is armed when the area should be empty. Most of what makes a system good or bad comes down to choosing the right detector for each place, dividing the building sensibly, and making sure every alarm reaches someone who will act.
Three kinds of protection
Designers think about intrusion protection in three layers, and most buildings use some of each.
- Perimeter protection detects someone opening or breaking through the building envelope: doors, windows, overhead doors, roof hatches. Its advantage is that it can detect an intruder before they are inside, and it can stay armed while people are in the building.
- Interior protection, sometimes called volumetric or space protection, detects movement inside a room or corridor. It catches intruders who get past the perimeter, including through walls or roofs, and people who hid inside before the building was locked.
- Point protection covers a specific object or small area: a safe, a cabinet of controlled drugs, a server room, a display case.
Common detectors and where they fit
| Detector | How it detects | Watch out for |
|---|---|---|
| Magnetic door and window contacts | A magnet on the moving part and a switch on the frame; opening separates them | Doors that do not latch, worn hardware, surface contacts that are easy to defeat |
| Passive infrared (PIR) motion | Changes in infrared energy as a warm body moves across its field of view | Heaters, sunlight, air from vents, and racking or partitions that block the view |
| Dual-technology motion | PIR combined with microwave; both must agree before alarming | Microwave seeing through thin walls if poorly aimed |
| Glass-break detectors | Listen for the sound pattern of breaking glass | Range limits, heavy curtains, glass types the detector is not rated for |
| Shock or vibration sensors | Detect attack on a wall, safe, window or ATM | Sensitivity set too high near traffic or machinery |
| Photoelectric beams | An infrared beam across a corridor or opening; breaking it alarms | Alignment, and objects placed in the beam path |
Hold-up and panic buttons are also connected to the intrusion panel in many buildings. Unlike other devices, they are active all the time, armed or not. Our guide to duress and panic alarms covers them in detail.
Zones, partitions and arming
Each detector, or small group of detectors, is a zone. Zones tell responders where an alarm came from, so a detector per door or per room is far more useful than one zone for a whole floor. Zones also have types: an entry door zone allows a delay so a user can reach the keypad; an interior zone may be ignored when the system is armed in a "stay" mode while people are inside; a 24-hour zone such as a panic button or a tamper switch is always active.
Partitions divide the building into areas that are armed and disarmed separately. A building with an office, a warehouse that runs a night shift and a server room can keep each partition in the right state, with each user allowed to control only the areas they need.
What happens when someone enters
Entry and exit delays are the most common source of false alarms, so it helps to understand the sequence. When an armed entry door opens, the panel starts a countdown and the keypad sounds. A valid code, card or credential within the delay disarms the system and logs who did it. If the delay runs out, or if an interior detector trips before the door does, the panel goes into alarm.
Getting the alarm out
An alarm that stays inside the building only works if someone hears it. Most commercial systems send alarms to a monitoring centre through a communicator. Older systems used telephone lines, which are disappearing, and many panels have quietly lost their only path when lines were removed. Current practice is to use IP over the building's internet connection as one path and a cellular path as the other, with the monitoring centre checking regularly that both paths are working. Our guide on alarm monitoring and response explains what happens after the signal arrives.
The panel also needs its own standby battery so it keeps working and signalling during a power cut, and tamper switches on the panel, keypads and detectors so that opening or disconnecting them raises an alarm.
Wired or wireless
Wired detectors are the usual choice for new commercial buildings, where cabling is installed with everything else. Wireless detectors are useful in finished spaces, heritage buildings and temporary sites. They need batteries that must be replaced, and their signals must reach the receiver reliably through the building, so a wireless design should include a signal check at every device location.
Why false alarms happen
False alarms cost money, strain relationships with police and monitoring centres, and teach staff to ignore the system. Many police services have false alarm policies that can include fees or a lower response priority for sites with repeated false alarms, so they are worth taking seriously. The usual causes are:
- User error: forgotten codes, staff arriving early or leaving late without knowing how to disarm, cleaners who were never trained.
- Doors and windows: doors that do not latch, overhead doors that move in the wind, windows left open.
- Detector placement: motion detectors aimed at heaters, fans, sunlit windows, hanging signs or areas where animals get in.
- Changes to the space: new racking, partitions or stored goods that block or confuse detectors.
- Equipment: low batteries in wireless devices, failing detectors and loose wiring.
Looking at the alarm history by zone and time of day usually points to the cause within minutes.
Testing and upkeep
- Walk-test every detector and contact, and confirm each zone reports correctly at the panel and the monitoring centre.
- Test both communication paths and the standby battery.
- Check that doors close and latch properly, since a door that does not close will not arm.
- Review detector coverage after layout changes, new racking or renovations.
- Remove user codes and credentials for people who have left, and update the call list.
- Follow the manufacturer's schedule for battery replacement and the testing interval your insurer or monitoring contract requires.
Bypassed zones are unprotected zones. When a zone is bypassed to stop nuisance alarms, find and fix the cause and restore the zone. A zone bypassed for months is a gap that an intruder may find before you do.
How ehZee can help
Our intrusion and security systems service designs new systems and reviews existing ones, from detector selection and zoning to monitoring paths and response plans. For how intrusion fits with access control and video, see layered security design.
Common questions
Should the alarm sound a siren on site?
Often yes, because it tells an intruder they have been detected and alerts anyone nearby. Some sites use silent alarms instead, especially for hold-up devices, where a siren could put staff at risk.
Can access control arm and disarm the alarm?
Many systems can be integrated so that presenting an authorized card at an entry door disarms the right partition, and the last person out arms it. It simplifies use and reduces false alarms caused by forgotten codes.
How often should the system be tested?
Your insurer, monitoring contract or the manufacturer may set an interval. Commonly, systems are inspected and fully tested at least once a year, with communication paths checked automatically and far more often.