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Back Electrical Electrical Engineering Dictionary Electrical Equipment Fire Alarm Circuits - Definition of Fire Alarm and Alarm Circuits

Fire Alarm Circuits - Definition of Fire Alarm and Alarm Circuits


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 warning.

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. The external circuitry is similar whether the control panel consists of electronic components or electromechanical relays.


Circuits of Fire Alarm | Fire Alarm Circuits

Several alaram 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. An end-of-line resistor placed at the remote end of the break-glass points or detectors allows a monitoring current to flow so that any breaks in the wiring can be detected by the cessation of a current flow.

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. The 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 cabled in parallel and are normally open, while in a closed circuit they are cabled in series and are normally closed.

When an alarm has been given it is often desirable to silence the audible alarm before the operating point which actuated the alarm is replaced or reset. An alarm stop/reset unit is made commercially which diverts the current from the general alarm to a supervisory buzzer or indicator but restores the current to its normal condition when the alarm initiating point has been reset. In a large building it may be desirable to have an indicator at some central position to show which warning point in the building has caused the alarm to sound. A closed scheme in which each pair of points is connected to a separate signal on an indicator board. The board can have either flags or luminous signs. The circuit can easily be adapted so that each individual point has its own signal or so that a larger number of points is grouped together to one signal. All the points so grouped are cabled in series and are connected to their own operating relay in the relay box. The alarm contacts are closed when any one of the relays is energized. The bells can be silenced when required, but neither the supervisory buzzer nor the indicator can be reset until the alarm initiating point has been restored to its normal position.

Other refinements can be made for more complicated schemes in large buildings. The exact circuit arrangement must depend largely on the features of the equipment used, and in practice a satisfactory scheme can only be designed round a chosen manufacturer’s equipment and with the aid of data from the relevant catalog.

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