火灾报警和安全检测外文翻译.docx

PAGE PAGE 7 Fire Alarms and Security Detection A 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 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 the 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 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 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 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 closed. 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