A basic introduction to intruder alarms

Intruder alarm systems fall into two types – domestic and commercial, and can be as simple or as complex as the security risk requires. A security risk assessment should be carried out at the time of survey in order to determine the requirements.

A security risk assessment for a domestic alarm system is not a complicated matter. The object is to identify which areas are vulnerable to access from outside and protect these areas with contacts or sensors.

The system will have a control panel at the heart of the system. This panel will provide the power for the system and the controls for programming and operation. The system will have at least one external sounder, generally located on the visible fascia of the building.

Many systems will be provided with a communication transmitter which is connected via a telephone line to an Alarm Receiving Centre (ARC). This will relay information to the police in the event of a verified intrusion. Systems utilising ARC connections need to be provided with a Unique Reference Number (URN) by the police. This number is used as a limiter of false alarms by ensuring that the installer is an approved organisation. Systems that are not connected to an ARC do not need a URN.

Domestic intruder alarm systems are generally limited to contacts, passive infra-red sensors, and in certain situations, beam detectors or break-glass detectors may be applicable.

Commercial systems have different requirements, since the security risk may be greater depending on the value of the property being protected.  Often the insurance company will have an influence on the level of security required.

The range of sensors which may be applicable could be door or shutter contacts, passive infra-red sensors, or various ranges of multi-sensors using a combination of passive infra-red and microwave; internal and external beams and microwave sensors, vibration and break-glass sensors; window tape and batterns. However, this would be determined at the site survey, normally with the customer and his insurer.

In certain situations, ancillary equipment may be specified such as smoke machines. These flood specific areas with dense smoke in order to make it difficult or impossible for intruders to operate. Commercial intruder alarm systems often operate in conjunction with CCTV systems, access control systems, and even as part of a major integrated system.

These types of systems are generally designed by specialist consultants in close liaison with the end user's security management team. Drawings and specifications would normally be provided in the event that you are asked to tender for such a system.

Detection devices

These are the devices which recognise an unauthorised attempt to enter a building once the system has been set.

They are placed around the building in strategic locations in order to form zones and areas, allowing the zones to be set or unset depending on how the user wishes the system to function. For instance, the house could be divided into separate zones; i.e. bedrooms, living room, kitchen, bathroom, utility room, outbuildings such as garages, shets, etc.  These would normally be installed as a separate zone. Installing a system in this manner allows certain zones to be isolated whilst the remainder of the system remains in operation.

There are two types of protection; these are referred to as parameter detection and movement detection.

Parameter detection senses attempted intrusion and utilises:

• magnetic reed switches
• window foil
• window battens
• break-glass detectors
• inertia detectors
• external beams.

Movement detection senses movement (sometimes referred to as trap detection) and utilises:

• passive infra-red (PIR)
• ultrasonic detectors
• combined PIR/ultrasonic detectors
• microwave detectors
• dual technology detectors
• combined acoustic PIR detectors
• active infra-red detectors
• capacitance detectors
• ceiling mounted PIR detectors.

Manufacturers of these products provide comprehensive information related to the performance and application of these detectors. Personal attack or panic buttons are deliberately activated devices and are used to activate an alarm by pressing a button. This type of device is active 24 hours a day and is generally located in the main bedroom or to suit specific needs.

Warning Devices

In the UK, most systems will be provided with an outside sounder (SAB). This is a sounder and strobe encased in a strong weatherproof box, and has its own separate battery supply.  This battery allows the SAB to function in the event of the cable between the SAB and the control panel being cut.

Zones may well be provided with an internal sounder depending on the size or location of the building and a final exit and entry buzzer may be needed if the keypad is to be located in a cupboard or under the stairs.

General References