A typical security system is a device or installation that helps protect people, property, and buildings from theft, fire, aggression, or intrusion. Some security systems help in law enforcement by providing information on the location of individuals who have had their movements restricted, such as early-release convicts on a tagging program.
Some security systems are acquired and operated privately, such as household intruder alarms. Others are maintained and operated by security firms that charge for their services, as is the usual case for the defense of a large commercial building. Public security systems are operated by the police and other security forces as a deterrent against crime, and as a means of collecting video evidence to help secure convictions.
Although many aspects of security systems are automated and secure against tampering, there are frequently human elements, such as security guards, that could be vulnerable to blackmail, corruption, or temptation. Hence, careful selection of personnel of high integrity is fundamental to an effective security system.
One form of selection technique is vetting, whereby a candidate’s integrity is investigated by interviewing family, associates, and former employers, as well as by consulting police records if available. Other techniques include polygraph (lie-detector) tests and psychometric analysis, which can provide some indication of character through a person’s responses to a series of apparently neutral questions.
Depending on the sensitivities of their positions, other employees can be subjected to vetting as part of their appointment procedure. Also, the hiring contracts of many manufacturing and retail companies include clauses that allow the security personnel of the company to inspect the employees’ baggage, vehicles, and even homes for evidence of pilfering or theft of information.
Protection against intruders
A major part of the security of a building is its resistance to unauthorized access. The precise details of security systems vary to suit different applications, but most systems include some means to defend the perimeter of a secure location; they may be combined with motion sensors that detect intruders and other systems to detect when doors open or windows break.
Perimeter defense. Perimeter fences or walls are physical barriers that keep casual or accidental intruders out, and they may be supplemented by motion-sensitive floodlights or audible alarms that warn potential intruders that further ingress is likely to meet resistance from security guards, often accompanied by trained guard dogs.
Alternatively, the perimeter defense system might give the intruder no indication that he or she has been detected, alerting instead a security guard stationed in a central control room. The guard can then proceed to inspect the area where the intruder has been detected or view the images from closed-circuit television (CCTV) cameras in that area. At night, these cameras may use light from constantly lit lamps, or they can use image intensifiers to make the meagre ambient light sufficient to illuminate the intruder. Alternatively, the intruder can be viewed using both invisible infrared light from appropriate lamps and infrared-sensitive CCTV cameras.
Motion sensors. A variety of techniques can be used to detect moving intruders, either in the grounds of a building or within the building itself. Beams of infrared light can span the gaps between sources and photocell detectors, so the passage of an intruder through the beam temporarily interrupts the current from the detector, so triggering the alarm. Microwave and ultrasound transceivers can detect moving objects by the frequency shift of reflected radiation, just as Doppler radar does. Other detectors, called passive infrared detectors, sense the presence of humans by the infrared radiation given off by a warm body.
Many motion sensors can operate over a range of sensitivities and must be set according to the environment where they will be used. This feature helps minimize the number of false alarms caused by movement of leaves and wildlife and even drapes or sheets of paper in drafty rooms.
Other detectors. Various electromechanical devices can detect when windows are broken and doors forced or when a window or door is opened outside normal working hours and the alarm system has been armed. Thin strips of conducting foil attached to a window can split if a window is broken, triggering an alarm when the current that they carry is interrupted. Opening doors and windows can be detected by a simple switch in the frame, similar to the switches that operate refrigerator lights. Also, the impact that breaks a window or door lock can be detected by vibration sensors similar to those of car alarms.
Patrols. Another approach to the protection of buildings is a security patrol, whereby one guard or more makes regular inspections of vulnerable premises. The effectiveness of such patrols can be improved by using a network of checkpoints, where the guard registers his or her presence by entering a secure code on a keypad or turning a key. The alarm is raised if a guard fails to report on time at any point along the route, indicating a possible attack. The patrol system has several weaknesses, however. Determined intruders can often establish the patrol pattern by observation and then attack when no patrol is due; also, guards can be coerced into giving up the key or code for the checkpoint, rendering the checkpoints useless.
