PERIMETER PROTECTION
Electronic Alarms - The False Alarm Scene

Both in perimeter protection systems and in building or house alarms systems false alarms can nullify the usefulness of an alarm system. The causes of false alarms, the reasons why false alarms nullifies the effectiveness of systems as well as the methods to overcome false alarms and its effects must be investigated and fully understood to ensure effective Security Systems being installed protecting property, life and limb. One must start at the basic principles in the investigation:
CRITERIA:
Any effective security system must comply to a number of criteria:
A) Border Demarcation: Both the potential intruder / perpetrator and the protector must fully understand what constitutes a breach of the protectors rights and therefore at what stage the intruder / perpetrator can be punished, detained or arrested. In more simple terms, 'what are the rules?' or 'where is the border/fence?'.
B) Detection: A method must be devised by which any breach of the above can be determined or detected i.e. 'what type of alarm should be used?'.
C) Communication: The fact that a breach has occurred must be communicated to a reaction force i.e. 'how do I find out about the alarm?'.
D) Analysis: Before action is taken it is wise to make sure that:
1) a breach has actually occurred i.e. 'is this a false alarm?' or 'should we react?' and
2) the breach was not caused legally i.e. 'don't shoot until you are sure'.
E) Reaction: It must be very clear what measures will be taken should a breach occur i.e. 'there must be an effective reaction force'.
F) Delay: The time it takes the intruder/perpetrator to cause damage must be long enough that effective measures can be taken i.e. 'reaction time must be shorter than delay time'. 

The first five criteria constitutes criteria to which the protector must comply and the last criteria is something one must force the perpetrator/intruder to comply to.

All of the above according to the detailed theory and expert system (computer program) developed in the early '80's and the principles of which are ever more widely applied.

A recurring problem with electronic security are that the equipment work but are not useful.
Any security installation (note - not yet a system) which does not effectively cover all the above aspects, and in a balanced way, is not a system and will not be fully effective and therefore not fully, if at all, useful. As a simple check envisage a security installation where any of the above aspects are missing and imagine the effectiveness of the installation. A security system can therefore be defined as a system consisting of all the above aspects clearly defined and designed as a balanced whole. Where the security manager needs an effective system which he can use, he might well find the engineers in his organisation proving that his installation works. With a lot of theory and measurements it can be proven that the alarm will go off whenever a specific input is given or that a signal will be passed through the radios or cables or even that he signals will be displayed in the security control room. Even so the security manager finds that he does not obtain useful information and would like to switch off the system.
Compared to the cost of human guards the cost of an electronic system could be fractional but still guards are preferred because of the electronic installation not having been designed to be useful while the human guard (or even dog) meets all the above criteria even without it having been designed in as a system:
The guard knows where the border is (and normally who may pass),
he can detect an intrusion,
he can communicate the alarm,
he can verify the alarm,
he can react to the alarm and
he can possibly delay or arrest the intruder -
Still some of these functions would have been performed much better by a well designed electronic system, integrated with the human response. Such an integrated system also gives more value to the task of the security man.
Typical of the lack of system design, electronic installations suffer from the lack of proper consideration of false alarms and of alarm verification or analysis, the specific area within the system where electronics should play the most powerful role.
ALARMS:
Alarms can be categorised as four types: 
True alarms for which a response is essential (and which seldom occurs in normal installations).
False alarms caused by some electronic instability or electronic noise inherent in the electronics and the equipment design. This rarely occurs in modern, high quality equipment.
False alarms cause by external electrical interference (e.g. motors starting or lightning) or bad installation design (e. g. noise on the cables).
Nuisance alarms caused by the electronics working correctly but picking up an effect which should not be reacted upon (e.g. paper blowing in the wind or monkeys or baboons).
The last three types are all sometimes called false alarms since they can cause an alarm to be raised even when no reaction is required.

Alarm choice / Environment:
The only way to limit the number of false alarms generated by a single type of detector is to choose the alarm technology in such a way that the environment has the least effect on the detector. I.e. the characteristic in the intruder (or bullet or bomb or whatever must be detected) must be a clear characteristic of the intruder while this same characteristic must not be typical in the environment or area covered by the detector. For instance if a passive infra red detector (PIR) is going to be used to pick up the body heat from an intruder, there must not be another heat source within the detection range of the detector. This limit is overcome in modern PIR's in various ways specifically to ensure that the body heat source has characteristics not present in the environment: e.g. 
measure for the very specific human body temperature (cats, dogs, birds and mice body temperatures differ),
measure that different 'beams' or areas detect the body temperature in logical sequence (a fire or other hot object is supposed to stay in the same place) etc.
Since only one type of intruder characteristic (with detection probability which is not wonderful) is not generally present in the environment, a properly designed system should consist of two or more detectors (or have adequate analysis ability - see later) which each detect different characteristics of the intruder.
As an example let us consider a system consisting of three detectors, detecting different characteristics of an intruder:
If all three detectors detects the intruder within a short time span and especially in a logical sequence, one can be fairly certain that an actual intrusion has occurred. On the other hand one or more of the detectors might have been overcome by the intruder. I.e. although the probability of false alarms has decreased the probability of detection has also decreased.
In a case similar to this example the correct design requires a very careful balancing of the probabilities of false and nuisance alarm of each equipment as well as the probability of detection of each system while taking into consideration that these probabilities are not mutually exclusive. I.e. the mathematics is not straightforward but with the correct design a system can be put together with a very low false alarm and nuisance alarm rate and also with a very high probability of detection. The above type of design required a number of expensive equipment to be utilised together and was the most cost effective possible method of obtaining reliable systems until the late eighties.
The latest technology created new possibilities but this requires new considerations.

Allowable false alarm rate:
Most people regard false alarms in a security system as unacceptable or even caused by bad equipment or bad design. While it is often true that many false or nuisance alarms are caused by bad installation practices or incorrect installation methodology caused by a lack of knowledge of the installation environment, false alarms can not be totally eliminated. The most cost effective design must take into account that false and nuisance alarms will occur and must design in such a way that the number of false and nuisance alarms are limited to an 'acceptable' level. With an 'acceptable' number of false alarms is understood such a number that the effectiveness of the reaction force is not adversely affected. Clearly as more and more false or nuisance alarms occur, a stage will be reached where the reaction force will stop reacting on alarms (crying wolf). Conversely when false or nuisance alarms seldom occurs the reaction force will react fully to every alarm. A simplification of a complex mathematical relationship reduces to a rule of thumb: 'The false alarm rate must be less frequent than 100 X the time taken to verify the cause of the alarm'. Here the term false alarm include nuisance and false alarms and must be taken over all the alarms generators supervised by the reaction force.
Example: Consider a site with a circumference of 6 km with detection zones of 60 meters each. Let us assume each detector on average generates only one false or nuisance alarm per day (but can be desensitised to give fewer alarms but at the same time making it less likely to detect an intruder) and the reaction force on average takes 5 minutes to get going and another 8 minutes to reach an alarm point. (These assumptions are better than experience at many haphazardly designed sites show). A quick calculation show that on average an alarm can be expected every 14,4 minutes while the reaction force will take 21 minutes to return to the guard room. Obviously the reaction force will start ignoring alarms or otherwise desensitise or switch off the system. Using the above rule of thumb it is clear that the verification of alarms must either be brought down to 8 second or less or the combined false alarm rate from all 100 detectors must be reduced to one in 24 hr. I.e. each detector must on average generate only one false or nuisance alarm in 2 400 hr. or one per 15 weeks. The ideal way there fore is to reduce verification time or increase the analysis capabilities of the electronics which is the subject for a separate article but CCTV cameras go far to solve this problem.