3.5.11
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Intrusion detection design is, without a doubt, one of the most variable practices in the security
field. If you ask ten installers or designers, you’d probably hear ten opinions on how the system
should be designed. Some believe door contacts are all that is needed. Others believe motion and glass break detectors are absolutely necessary throughout a facility. The truth typically lies
somewhere between these two philosophies. There is a fine balance between risk and cost, as in all security design. Most intrusion detections are built in layers, usually as follows:
1. Perimeter:
The first layer to be secured, the perimeter consists of doors and operable windows. These are equipped with contacts to detect intrusion. Inoperable windows may be protected with glass break sensors, but care should be taken, and the drawbacks of these sensors considered (see below), before they are installed. Door contacts are the minimum protection any intrusion detection system should be designed with. Their low cost contributes to their use as well. Contacts on entrance doors are programmed so that they do not alarm instantly, but begin a countdown, called the entry delay. This delay is used so that staff may enter and disarm the system without triggering an alarm.
The first layer to be secured, the perimeter consists of doors and operable windows. These are equipped with contacts to detect intrusion. Inoperable windows may be protected with glass break sensors, but care should be taken, and the drawbacks of these sensors considered (see below), before they are installed. Door contacts are the minimum protection any intrusion detection system should be designed with. Their low cost contributes to their use as well. Contacts on entrance doors are programmed so that they do not alarm instantly, but begin a countdown, called the entry delay. This delay is used so that staff may enter and disarm the system without triggering an alarm.
2. Outer interior:
The next area to consider securing is the interior. Specifically, the outer reaches of the facility, such as hallways near entrances and exits. Motion detectors are added in these areas to provide a second layer of security, so that if any perimeter sensors fail or are defeated, the motion detectors provide a second opportunity for detection. These zones are typically programmed for instant alarming, so any disturbance immediately causes an alarm, with no delay. Motion detectors near entrances are sometimes programmed to respect the entry delay, to avoid accidental activation when staff enters.
3. Inner interior:
The final areas to be secured are the inner areas of the facility. These areas
normally receive the least attention, since criminals must make it past the perimeter, and then the first interior layer of sensors in order to reach them. The only other way to move freely about the interior is to hide while the building is not secured, and avoid detection until all occupants have left. Then the subject may operate freely, only passing through the alarmed zones at the perimeter when leaving, at which point it’s generally too late for responders to intercept.