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Intrusion Detection

Protecting your buildings

Last Updated 27 February 2023


The primary purpose of an intrusion detection system, IDS, is to detect and signal the presence of an intruder, or an intrusion attempt, into a secured area. A secured area can be a selected room, an entire building, or group of buildings.

A basic intruder detection system will comprise of a number of elements. There will be a detector located in the protected area. There will often be a means to interface with the system user(s), e.g. a keypad with alpha-numeric display permitting authorised persons to interact with the system by setting and un-setting the IDS and viewing status indications. Control electronics connected to the user interface will perform the set and unset functions and will have provision to receive inputs from detection devices distributed throughout the supervised area/premises in strategically located positions. The action of signalling the intrusion event (or 'to notify') is also made by the control electronics.

There are numerous IDS on the market. Deciding which detector is best for you is achieved by carrying out a comprehensive Operational Requirement, OR. Depending on what you are trying to protect there are many options:



These are examples only, there are other products which can be explored in the following videos. Each will offer its own benefits and limitations. Selecting the correct detector is important since, as with all detectors, false activations can occur which, at best, waste time, cost money and reduce confidence in the system. At worst, these can lead to security compromises as security staff will respond slower to, or even ignore, repeat activations. High false alarm rates may even result in control room staff switching off the system!

The Catalogue of Security Equipment contains IDS equipment evaluated by NPSA.

View Video Transcript

The primary purpose of an Intrusion Detection System or IDS is to detect and signal the presence of an intruder or an intrusion attempt into a secure area.  

There are several types of IDS on the market, and it's important to match the suitability of the technology with the environment you are securing.  

Intruder detection systems will always consist of at least two separate components: the control panel and the detectors. The control panel is the brains for the system. It receives signals from its detectors and feeds these into its indicating equipment, which may be audio or visual alarms. This is the user interface for the IDS and allows for the user to monitor all detections, alarms and false alarms.  

Detectors are the individual components that sense an intruder. Detectors are split into several different categories.  

Protective switches. these are commonly known as door contacts and are traditionally used on doors and windows. A magnet is housed in one side of the contact, a magnetic switch or reed switch in the other. When they're close together, they create a closed loop. Breaking this loop alerts the control panel that the window or door has been opened. The distance that the contacts can be away from each other while still forming a loop is called gap spacing. This is a security concern as the door could register as closed whilst actually being open. 

Single or multiple reeds switches can be used. Multiple reeds switches are encouraged as they are significantly harder to defeat.  

Vibration detectors. Vibration detectors are sensors that detect movement through vibrations and can be used on windows and doors or placed on freestanding objects, such as ATMs, filing cabinets and remote service.  

Traditionally, a small ball is seated on top of several metal contacts. Vibration moves the ball and breaks the contact, alerting the control panel. Modern variants of vibration detectors use a small crystal that moves under vibrations, generating a small amount of electricity, which alerts the control panel. Vibration detectors are best deployed on perimeter doors to offer an early warning as they can register an intruders approach. Unlike protective switches, which require the door to be opened.  

Vibration detectors may false alarm due to environmental conditions, such as rain or hail.  

Break glass sensors. A break glass sensor detects when a window has been broken, they can either register the sound caused by the glass breaking or by detecting a physical change to the window. Audio break glass sensors use microphones to register the sound of breaking glass and send out an alert. False alarms can occur due to registering similar sound waves, such as the sound of keys being dropped. For physical detection, either a physical circuit or ultrasonic waves are used for detection.  

A thin metal strip is placed on or embedded into the window. When the window is broken so it’s the circuit and the alarm is triggered or with an ultrasonic break glass detector, a wave is sent from one corner of the window to the other. When the glass breaks, this wave is disturbed, and the alarm is triggered. Brake glass sensors must be deployed where a security officer can oversee and inspect them as the thin metal strips or microphones are susceptible to tampering.  

Beam detectors. A beam-detector sends a beam from one tower to another. If that beam is broken, the alarm is raised. Beam detectors are normally deployed in large flat open areas, such as warehouses and are ideal for large doorways as they can be switched on and off, leaving their physical infrastructure, obstructing the opening. It's important to deploy, leaving no gaps in the detection pattern.  

Beam detectors can be compromised by the towers physically being moved or by having foreign beams being introduced to them, to fool the system. Volumetric security. Volumetric motion sensors are designed to detect intruder emotion within the interior of a protected space. There are different types of volumetric motion sensors, which include PIRs and microwave detectors.  

Passive Infrared Receivers, PIRs work by detecting the IR radiation from other objects. An intruder's body emits heat and is sensed by the detector. Therefore, it does not need to be powered to be operational. The detection pattern of a PIR looks like fingers, radiating out from the central sensor. Sensitivity is better across the fingers rather than along them. PIRs can false alarm due to a change in the general room temperature and it can also be triggered by excessive IR light reaching the sensor. Because of this, they are best deployed within a building, facing away from the windows and where they can be overseen and inspected by a security officer.  

Microwave detectors. These are similar to PIRs but are a more robust technology. The detector emits a wave and measures the ping of the reflected returning wave, any disturbance in the ping results in the alarm being raised. Microwave detectors are best deployed where a PIR would struggle due to excessive IR lighting. For example, in a bright glass atrium, choosing the correct IDS for your environment is of utmost importance as is optimizing their deployment.  

You want to match their placement and configuration to suit your security needs and the physical properties of your site. They must be regularly maintained and inspected, and any walk lights must be turned off as this can allow attackers to probe the system and work out any blind spots. When securing a large area, multiple intruder detection systems may be needed to provide full coverage of that area. It's important to deploy a variation of ideas. This is to prevent false alarms as two PIRs could both be triggered by the same stimulus.  

Deploying a PIR and a microwave detector in one space helps to avoid these as they respond to different triggers. Dual technology systems, a combination of microwave and PIR can help to reduce false alarms as both systems need to be activated for the alarm to be triggered. However, this reduction in false alarms can increase your susceptibility to attack.  

IDS are an important component of any security system, but they only form one protective layer of security on your site.  

To keep your site safe, they must be deployed in conjunction with other security measures, such as access control systems, on-site security offices, and CCTV to create a multi-layered security system.

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