False alarm of the intrusion alarm system brings losses to the user. There has been a lot of effort by the scientific and technical personnel to develop low-mispronounce products, and the results have been impressive. But false alarms persist, and where are the problems? How to solve it?

There is no authoritative definition of false alarm at present, and it is generally believed in China that “there is no dangerous situation in, and the alarm signal issued by the alarm system is a false alarm.” According to this definition, the false alarm rate of the intrusion alarm system (in a certain period, the ratio of the number of false alarms and the total number of alarms) is generally more than 95%. In other countries, the definition of false alarm is: “false alarms refer to alarm signals that do not require police action in actual situations, excluding those caused by harsh natural weather and other special environments that cannot be manipulated by the alarm company or users.” According to this definition, the American UL standard stipulates that each alarm system can only have a maximum of four false alarms per year.

The above two definitions are from different angles and each makes sense. This article only analyzes the causes of false alarms in the alarm system for the first definition, and specifically develops practical measures to reduce false alarms.

1 Caused by alarm equipment failure

A product that fails to perform the prescribed function under the prescribed conditions and within the prescribed period is called a failure. The types of failures are damaging failures and drifting failures.

• Damaging failures include full performance failures and sudden failures. Such failures are usually caused by damage to components or poor production processes (such as virtual welding).
• Drifting failures are fault caused by the drift of the component parameters and the supply voltage. For example, if the temperature is too high, the resistance value of the resistance will change, and the equipment is sometimes good and sometimes bad. In fact, factors such as ambient temperature, component manufacturing process, equipment manufacturing process, use time, storage time and power supply load may cause changes in component parameters, resulting in drifting failures.

Solution: Whether the damaging failures or drifting failures will make the intrusion alarm system false alarm, to reduce the false alarms should be done as the following aspects：

(1) The manufacturer of alarm equipment: the design level and process level of the products must be improved. At the same time as the design, it is also necessary to make reliability design, such as redundant design, electromagnetic compatibility design, three-proof design (moisture, salt spray, anti-mildew), drifting reliability design, etc. On this basis, improve the reliability of the product manufacturing process, such as strict screening of the quality of components, strict quality supervision and management of the production process, etc., to ensure that product quality meets the requirements of the standard.

(2) The unit or individual of selling alarm equipment: strict purchase inspection should be carried out, the inspection content is: product quality inspection certificate, production enterprise industrial production license certificate or safety certification certificate or production registration approval.

(3) Management departments: regular or irregular security market inspection, spot check should be organized. Any illegal act in the production or sale of alarm equipment shall be dealt with in strict accordance with the provisions.

(4) Intrusion alarm system construction unit (user): in the corresponding engineering documents, the construction unit should be clearly required to use qualified products testing by authorized institutions. Foreign equipment should be imported through formal channels, and qualified products should be tested according to international advanced standards. In order to ensure the good working condition of the intrusion alarm system, a regular inspection and regular maintenance system must be established. Problems occur, it is best to be repaired by the engineering contractor or professional maintenance company, which is not only conducive to the improvement of the utilization and maintenance level of maintenance resources (maintenance personnel, maintenance equipment, maintenance spare parts, etc.), but also more importantly, can improve the reliability of the intrusion alarm system.

2 Caused by improper design of intrusion alarm system

The choice of alarm equipment is the key to the intrusion alarm system design, and there are many types of alarm equipment, each with its own characteristics, scope and limitations, and improper selection will cause false alarms. For example, the use of vibration detectors near the source(airport, railway side) can easily cause false alarms in the intrusion alarm system; the use of ultrasonic detectors in places where bats are often present also makes the intrusion alarm system false alarms, because bats emit ultrasound, high-frequency sounds such as metal impact sound can cause false alarm of glass detector.

Solution: Intrusion alarm system designers should be very familiar with the principles, characteristics, scope and limitations of various alarm equipment. At the same time, it is also necessary to master the on-site environmental conditions, climatic conditions, electromagnetic field strength and illumination changes, so as to select alarm equipment according to local conditions.

In addition to the selection of alarm equipment, improper design of intrusion alarm system is also reflected in the installation location of alarm equipment, installation angle, protection measures and wiring. For example, when installing the PIR motion sensor towards air conditioning and ventilation fans, it will cause false alarms. If the outdoor active infrared sensor is not properly protected from sun shading (preferably with a sun shield), it will inevitably cause false alarms. When the distance between the alarm line and strong current lines such as power lines and lighting lines is less than 1.5m and no anti electromagnetic interference measures are taken, the intrusion alarm system will also generate false alarms.

3 Caused by improper construction of intrusion alarm system

This part of the problem is mainly manifested in the following aspects：

• It does not strictly follow the design requirements for construction.
• The equipment is not installed firmly or the inclination is not suitable.
• The solder joint has brazing, burr phenomenon, or shielding measures are improper.
• The sensitivity adjustment of the equipment is not good.
• The testing equipment for construction does not meet the measurement requirements.

Solution: Strengthen the supervision and management of the construction process and implement the supervision system of security engineering as soon as possible, which is very conducive to improving the quality of the project and reducing the false alarms caused by the construction process.

4 Caused by improper use by users

Due to improper use by users, it often causes false alarms to the intrusion alarm system. For example, a window with a door contact is not inserted and is blown open by the wind at night. The staff mistakenly entered the alert area. Accidentally triggered emergency alarm device. The error of operation of the personnel and the failure to pay attention to the change of work procedures are all the reasons for the false alarm of the intrusion alarm system.

Solution: The improper use of user analysis, to find out where the error is, improve the users’ abilities; can greatly reduce the number of false alarms.

5 Caused by environmental noise

The false alarm caused by environmental noise means that the intrusion alarm system is generated in normal working conditions, which is inevitable in principle, and the fact is unnecessary. It is a false alarm. For example, the hot airflow causes the false alarm of the PIR motion sensor; the high-frequency sound causes the false alarm of the glass detector; the ultrasonic source causes the false alarm of the ultrasonic detector.

Solution: the use of a variety of detection technology combined with the detector, such as the use of dual detection detector (two different principles of the detector at the same time to detect “ target”, alarm signal), the current products are: microwave-passive infrared dual detector, acoustic control-vibration glass broken double detector, Ultrasonic-passive infrared double discriminator, etc.

With the development of sensing technology and computer technology, the promotion and application of large-scale integrated circuits and the intelligent degree of intrusion alarm systems will continue to increase, and the false alarm phenomenon caused by environmental noise will be reduced accordingly.