A Guide to Intruder Alarm Detectors
Intruder alarm detectors are designed to provide the electronic detection of persons
within their detection field.
There are two main types of intruder alarm detectors in use in schools:
1.
Passive Infra-Red (PIR); and
2.
Dual-Technology.
As will be discussed below, there are technical and practical considerations to both
these technology types which does not guarantee an advantage of one type over the
other.
1.
Passive Infra-Red (PIR) Detectors
Definition
Passive infrared detectors are “designed to initiate an alarm condition in response to
the change in radiation at wavelengths within the specified band of the infrared
spectrum which results from the presence of an intruder.”
(Source: Australian Standard AS2201.3)
Operation
Infrared energy is HEAT. PIR detectors use a temperature sensitive element which is
focussed using a mirror and/or a plastic lens to produce a pattern of protection zones
in the area to be covered (see Figure 1). Essentially, any rapid change in temperature
between two or more of these zones is detected and an alarm is initiated.
The internal electronics which manage and control these devices usually use a
differential algorithm which does not differentiate between whether there is an
increase or decrease in temperature. This makes sense when you consider that if the
temperature of a particular zone actually decreases, the temperature in another zone
must have increased.
2.
Dual-Technology Detectors
Definition
Dual-Technology detectors use two forms of detection methods:
Passive Infrared (as defined above); AND either
Doppler Microwave; or
Doppler Ultrasonic.
Because Doppler Microwave technology is by far more prevalent, Ultrasonic
technology can be safely excluded from discussion.
Doppler Microwave technology is defined as being “designed to initiate an alarm
condition in response to Doppler frequency effect provided by the reflection of
microwaves from a moving intruder.”
(Source: Australian Standard AS2201.3)
Operation
In addition to the PIR technology as described above, these devices have an active
device which produces and transmits microwave energy into the protection area and a
receiver which monitors the frequency of this energy as it is reflected by the
environment. Any rapid change in frequency of these reflections means that an object
in the protection area is moving.
An alarm is initiated if the device detects a moving object (Doppler microwave) AND
detects a moving heat source (PIR).
Installation
Reliable and optimal detection can only be possible if detectors are focussed or
adjusted to suit their actual environment AND their environment is suitable to their
form(s) of detection technology.
While a contractor will need to take in account many environmental considerations
when determining the type and placement of detectors, the following guide
demonstrates one such consideration and may be used to determine how best to test
the operation of these intruder detectors.
PIR
Because these devices detect rapid changes in infrared energy between the zones, they
work most effectively when the influencing activity crosses the zones at a right angle.
For this reason, PIR detectors are optimally positioned in locations where intruders
are most likely to cross over the zones rather than approach or retreat from the
detector head-on (see Figure 2).
Dual Technology
Because these devices use PIR technology (which is most effective when the zones
are crossed at a right angle) and Doppler microwave (which is most effective if
movement is performed directly towards or away from the detector), these devices are
optimally positioned where intruders will cross the detection area at an angle.
Maintenance
Because these devices are continually powered and operating, their performance
degrades over time.
All detectors need to be regularly maintained to ensure they are operating to their full
ability and their environment needs to be regularly examined to ensure that any
changes are not detrimental to the reliable and optimal operation of any intruder
detection device.
All detectors have a limited life expectancy and must be considered as consumable
items. Since, the cost for the replacement of these devices is borne by schools under
their maintenance budget, schools should prepare for the necessary replacement of
these items when it is required.
The life expectancy of a detector will depend highly on its location and environment.
Detectors which are exposed to direct sunlight have a severely reduced life
expectancy. Other factors such as dust, humidity, insect infiltration and power supply
voltage level and fluctuations can and do impact on the life expectancy of intruder
detectors.
A typical intruder detector, properly installed and maintained, can be expected to last
between 8 and 10 years before degradation of operation reaches a level where
replacement is necessary.
The service maintenance carried out on your alarm system should identify any
detector which is not providing adequate protection and if adjustment or calibration
cannot provide an improvement.
Undetectable Situations
Intruder detectors are designed to detect PEOPLE; not objects. In fact, under the
Australian Standard AS2201.3, a “Standard Target” is defined and each type of
detector technology is required to pass a particular method of testing using this
“Standard Target”. This is a minimum requirement and they can, of course, detect
smaller objects and under different situations, but they must pass this test to be AS
approved.
The “Standard Target” is defined as “A person having a height of 178±8 cm, weight
of 77±9 kg, having arms folded, facing the reference point, clothed in fully fastened,
light coloured, pre-washed cotton overalls of cloth weight 310±40 g/m2 and having
footwear selected for minimal noise generation.”
(Source: Australian Standard AS2201.3)
1.
2.
3.
4.
Detectors cannot “see” through objects. The PIR element will simply monitor
the temperature of any object on which the protection pattern falls and
microwave signals will simply reflect off any non-metallic object on which
this radiation falls.
Detectors cannot detect objects, whether moving or not, which do not emit
infrared radiation (heat).
Even objects that do emit infrared radiation need to interrupt two or more
detection zones to be detected.
Objects which emit infrared radiation but are smaller than the “Standard
Target” may not be detected, even if they do interrupt two or more detection
zones.
Some examples of where detection is not or may not be possible are where:
The detection pattern is blocked by posters, flags, netting, decorations,
screens, dividers, partitions, etc.;
Detectors are painted or otherwise damaged;
Small objects are thrown into the detection pattern, eg. rocks thrown through
windows;
An offender simply leans through a window or doorway to reach and steal
items;
The environment is preventing the detector from operating correctly;
Keep in mind that aluminium foil is metallic and will absorb microwave energy.
Decorations made from aluminium foil in rooms protected by dual-technology type
detectors will have a detrimental effect on the ability of the device to detect activity in
the area.
Similarly, compounds or enclosures made from steel mesh will absorb most of the
microwave energy produced by a dual-technology detector and probably prevent the
detection of an intruder.
False Alarms
Since PIR detectors produce an alarm if they detect any rapid movement of heat, any
environmental influence which results in the movement of heat may cause a false
alarm.
Because dual-technology devices rely on both forms of detection, there is a perception
that they are more stable and less prone to false alarms than PIR devices. This is not
necessarily the case. Experience in schools has demonstrated that dual-technology
detectors are actually more reliant on the environment and may be more susceptible to
false alarms or failures to optimally protect an area (see above advice on aluminium
foil and steel mesh).
Every alarm deserves some investigation and action in an attempt to ensure a similar
alarm is not repeated. This investigation does not have to be exhaustive or take a lot
of time; it may be as simple as being aware of the alarm history of the system,
considering a few aspects and perhaps asking some questions.
Several tools are readily available for schools to conduct a quick investigation into a
suspected false alarm. Such tools are:
The Review Events function of the alarm system
A report from the system event printer
A statement obtained from the monitoring station detailing all events reported
over a particular time period
Historical evidence has shown that there are four main reasons for a false alarm.
User Error
Environmental Disturbance
Insects or Vermin
Detector Fault
In most cases, the reason for a false alarm can be quickly identified as one of the
above and action can be taken in each of these cases in an attempt to prevent a reoccurrence. Any action taken to address a false alarm is better than none, as an
action, even if it is not successful, will help to identify the real cause of the alarm.
False alarms resulting from detector or system faults are very rare. Contacting your
alarm system service maintenance contractor to attend, investigate and make repairs
or rectifications is an expensive action which should only be undertaken as a last
resort after all efforts to determine and address the reason for a false alarm have
failed.
Types of Alarms
Intruder detection systems are capable of monitoring and reporting the THREE states
of each detection device:
1.
SECURE;
2.
ALARM; and
3.
TAMPER.
A SECURE state is where the detector is not currently detecting any activity in its
detection area.
An ALARM state is where the detector is currently detecting activity in its detection
area.
A TAMPER state is where the system detects that the detector is currently being
tampered with or the cabling to the detector is currently broken or short circuited.
ANY TAMPER ALARM SHOULD BE IMMEDIATELY INVESTIGATED.
Figure 1: Detection Patterns of Typically Installed PIR Detectors
Figure 2: Environmental Considerations for Typically Installed PIR Detectors