Space or Perimeter?

There are generally two types of intruder alarm systems, space or perimeter, although some use a combination of both for maximum protection.

This article will help you to decide which system is best suited to your needs.

Introduction

As you should know, all intruder alarm systems comprise:

Control Panel
Detectors
Signalling devices (usually an external sounder)

Space Protection

A space detection system relies upon detecting intruders as they move around within the protected premises, after they have forced entry, whilst some may consider this is closing the barn door after the horse has bolted, these systems are very popular.

Advantages of a space protection intruder alarm:-

Ease of installation - cables to windows are difficult to install neatly in most premises
Reliability - if a window or door is left open and no forced entry occurs, the system still detects the intruder
Simplicity - most people can understand and install space protection devices
Cost - only one detector per area is required regardless of the number of doors/windows in the area

Disadvantages of a space protection intruder alarm:-

Pets - space detection systems can be triggered by pets, it is always best to lock them out of any detection areas
False alarms - persons living within the premises may stray into a protected area when part set

Detectors designed to cause an alarm condition when an intruder is within the premises (protected area) these devices include:-

Magnetic Contacts

Magnetic contacts are used on doors and windows. They are a Normally Closed (NC) detector and comprise two parts - a reed switch and a magnet. The reed switch is closed when in close proximity to the magnet. The magnet is fitted to the door or window and the switch to the door or window frame such that when the door or window is closed the two parts are adjacent and the switch is closed.

Passive Infra-red (PIR) Detectors

Also referred to as space or motion detectors. They are used to protect large areas of space (typically 10-15m range x 90°). They work by detecting heat (infra-red) emitted by objects within their range. The detector is covered by a lens which partitions the seen area into several zones. Their internal processor monitors the temperature of the zones and if any of the zones sees a temperature rise or fall the detector generates an alarm output. The rate of temperature rise or fall is pre-set to certain limits such that, for example,a central heating radiator will not create an alarm condition.

Pet Immune versions of PIR are available. These are generally simply less sensitive although some are only less sensitive below a certain height.

Combined PIR and Microwave Detectors (Dual Tech)

These are similar to PIRs but have a Microwave Detector fitted in the same enclosure. The microwave detector sees space (volume). A microwave frequency radio signal is emitted from the detector which is reflected back to the detector at a constant frequency. If this constant alters (e.g. by a door opening or something in the seen area moving) then the detector generates an alarm condition. However for an alarm condition to be generated both the PIR and the Microwave detectors must go open circuit.

Active Infra-red Detectors

These comprise an Infra-red transmitter and a receiver in two separate enclosures, the transmitter sends a beam of focussed infra red light to the receiver which collects the transmission through a lens array, if blocked an alarm condition is generated, often used as dual or quad beam devices requiring both or all four beams to be interrupted simultaneously. They are used in situations requiring a long curtain of protection e.g. inside a factory fence or factory corridor.

Perimeter Protection

A perimeter detection system relies upon detecting intruders as they force an entry, whilst more complex to install and more expensive, these systems are very popular.

Advantages of a perimeter protection intruder alarm:-

Pets - perimeter protection has the advantage that pets can roam, beware of dogs jumping up at windows though
Less False alarms - persons living within the premises can not stray into a protected area when part set

Disadvantages of a perimeter protection intruder alarm:-

Ease of installation - cables to windows are difficult to install neatly in most premises
Reliability - if a window or door is left open and no forced entry occurs, the system will not detect the intruder
Simplicity - it is inherently more difficult to specify and commission perimeter protection devices
Cost - usually one inertia detector and contact per door / window is required for all accessible areas

Detectors designed to cause an alarm condition when an intruder is within the premises (protected area) these devices include:-

Magnetic Contacts

Inertia Detectors

These are usually fitted to door and window frames. They contain an internal processor which is tuned to detect the frequencies generated by someone attempting to break in through the door or window. They can also be used on walls.

Breakglass Detectors

These are used to protect large areas of glass (typical detection range is ~ 7 metres radius). They work by detecting the sound of breaking glass through one or more microphones. They contain an internal processor which is tuned to detect the frequencies generated by the sound of breaking glass.