消防报警和安全系统检测外文文献翻译.docx

1、消防报警和安全系统检测外文文献翻译Fire Alarms and Security DetectionA fire alarm circuit, as its name implies, sounds an alarm in the event of a fire. There can be one or several alarms throughout a building, and there can be several alarm points, which activate the warming. The alarm points can be operated manually

2、 or automatically; in the latter case they may be sensitive to heat, smoke or ionization. There are clearly many combinations possible, and this chapter gives some systematic account of the way they are built up. Several alarm points are connected in parallel, and whenever one of them is actuated th

3、e circuit is completed and the alarm sounds. This is described as an open circuit, and it will be seen that it is not fail safe, because if there is a failure of supply, the fire alarm cannot work. Another characteristic of this circuit is that every alarm point must be capable of carrying the full

4、current taken by all the bells or hooters working together. The alarm points are connected in series with each other and with a relay coil. The relay is normally closed when de-energized, and opens when the coil is energized. Thus when an alarm point is activated the relay coil is de- energized, the

5、 relay closes and the alarm sounds. This system fails safe to the extent that if the coil circuit fails the main circuit operates the alarm. It is not of course safe against total failure of the supply because in that event there is no supply available to work the bells. The alarm points do not have

6、 to carry the operating current of the bells or hooters. This arrangement is called a closed circuit in contrast to the open circuit. We can notice that in an open circuit the alarm points are wired in parallel and are normally open, whilst in a closed circuit they wired in series and are normally c

7、losed.A typical manually operated fire alarm point is contained in a robust red plastic case with a glass cover. The material is chosen for its fire resisting properties. The case has knock out for conduit entries at top and bottom but the material can be sufficiently easily cut for the site electri

8、cian to make himself an entry in the back if he needs it. Alternative terminals are provided for circuits in which the contacts have to close when the glass is smashed and for circuits in which the contacts have to open when the glass is smashed. In the former case, there is a test switch which can

9、be reached when the whole front is opened with an Allen Key. In the latter case, the test push is omitted because the circuit is in any case of the fail safe type. The alarm point illustrated is suitable for surface mounting. Similar ones are available for flush fixing and in weatherproof versions.

10、The current carrying capacity of the contacts should always be checked with the makers catalogue. A thermally operated alarm point consists of a bi-metal strip that deflects when the temperature rises, and thereby tilts a tube half full of mercury. When the tube tilts the mercury flows into the othe

11、r half of the tube where it completes the circuit between two contacts previously separated by air. Alternatively, the arrangement within the tube can be such that the mercury breaks the circuit when the tube is tilted. The casing of the alarm is of stainless steel. Heat detectors of this type are u

12、sually set to operate at 65. They are frequently used in boiler houses. A smoke operated alarm point would be used only in special circumstances which make it necessary to detect smoke rather than heat. This type can cause nuisance operation of the alarm by reacting to small quantities of smoke whic

13、h have not been caused by a fire; they have for example been known to sound the alarm as a result of cigarette smoke in an office. Modern ones have adjustable sensitivity so that they can be set to avoid nuisance operation. An ionization detector contains a chamber which houses some low strength rad

14、ioactive material and a pair of electrodes. The radioactive material makes the air in the chamber conductive so that a small current flows between the electrodes. The size of the current varies with the nature of the gas in the chamber and as soon as any combustion. Products are added to the air the

15、re is a sudden change in the current flowing. The detector also has a second chamber which is permanently sealed so that the current through it never changes. As long as the currents through the two chambers are equal there is no output, as soon as they become unbalanced there is a net output which

16、is used to operate a transistor switch in the main circuit through the detector. The human being is the most efficient fire detector, provided that he is actually there at the time. The combination of his eyes, ears and nose allows him to differentiate between various phenomena. He can smell the dif

17、ference between cigarette smoke, burning toast and something more sinister such as burning paper. He can see smoke and flames and will often hear a fire before he has noticed either the flames or the smoke. Sometimes, the first thing he notices is heat, for example, a hot partition wall. It is these

18、 phenomena that fire detector designers measure to detect the onset of fire. It is the onset of fire that is crucial; very small fires can be put out quickly and safely with portable fire extinguishers, or even a cup of coffee if detected soon enough. Obviously, the smaller the fire, the less it may

19、 be observed. Fire detectors must, therefore, be designed without the risk of false alarms. So what technology is used today to detect these phenomena? Two techniques are used for the detection of smoke: photoelectric smoke detectors and ionization smoke detectors. Photoelectric detectors are arrang

20、ed so that, although visible light cannot enter, smoke can enter freely. This type of detector is based on a matt black chamber about the size of a coffee cup. A dividing wall is placed in the middle of the chamber on either side of which is located a light transmitter and a light receiver. Under no

21、rmal circumstances, light from the transmitter is not received by the receiver, as the chamber is not reflective. When smoke particles enter the chamber, light bounces off these particles which results in a signal being received. A similar effect can be observed when sunshine enters a smoky room. Th

22、ese photo-electric detectors are highly sensitive and reliable, so long as the smoke particles are visible. Certain classes of fire, however, emit invisible smoke particles long before the fire is actually alight and in these cases an ionization smoke detector must be present. Although the design of

23、 these detectors is slightly more complicated than in the case of photo-electric detectors, the basic principle is very simple. Two metal plates are separated by several millimeters and a voltage is applied across them. Since air does not conduct electricity, no current flows. If an ionizing radioac

24、tive source is brought close to the plates then the air itself is ionized, that is, electrically charges particles are present between the plates and these allow a tiny current to flow. Any particles entering the chamber, either visible or invisible, tend to neutralize the ions, thus decreasing the

25、current flow. It is this reduction in current which is monitored in measuring the amount of smoke that is present.Fire detection and its corresponding safety systems are crucial parts of an intelligent building. This paper reviews the current state of development of fire detection and alarm systems

26、in the intelligent building. New technologies and concepts developed in intelligent buildings, such as advanced multi-function sensors, computer vision systems and wireless sensors, real-time control via the Internet, and integrated building service systems, have also been reviewed and discussed. Th

27、ese new technologies and concepts will improve the capability of fire detection systems to discriminate between fire and non-fire threats and will increase the time available for property and life protection. However, much effort is still needed to remove barriers to the further development of these

28、 new technologies.New sensor technologies will be key components in the next generation of intelligent buildings. Current intelligent buildings often have embedded processors and dedicated information networks. The new generation is expected to add the capability to learn about the buildings circums

29、tances and its occupants needs and change the behavior of its control systems accordingly. The employment of a large number of sensors within the building will allow it to operate in a responsive manner, rather than using pre-programmed control models as are employed in the first two generations of

30、intelligent building. The information provided by sensors includes changes in both internal and external environments of a building, such as smoke, temperature and humidity, air quality, air movement, and the number of building occupants as well as a host of other properties. The system will use sen

31、sors to identify how a particular person tends to react to particular circumstances and to learn different behaviors for different people. The number of sensors required to obtain this type of functionality is quite high, especially since one of the major goals of intelligent buildings is to allow i

32、ndividualized control of an environment. This need will increase the cost of intelligent buildings and make it difficult to manage the resulting large amount of data. Development of cost-effective sensors has consequently been identified as a key need for intelligent buildings. Fortunately, many of

33、the properties that need to be monitored can be used for multiple purposes. Security systems that can track the entry and exit of occupants from an office building can also be used to ensure complete evacuation of a building during a fire or even, in more advanced forms, determine where occupants may be trapped and unable to escape. Similarly, parameters such as tempera