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Smoke Detector Power Supply

When smoke detectors are used in an alarm system, their internal electronic circuits are usually powered from the main fire alarm power supply.

Figure 7-2.\Typical dc power supply and battery charger.

Some types of smoke detectors have a more strict power supply requirement than other parts of the fire alarm system, especially with regard to purity of the dc voltage level. The power supply of those smoke detectors must have output voltage regulation and filtering not otherwise required by the fire alarm system. In those cases, the basic power supply may be upgraded to power the smoke detectors as well as the control unit, or a separate smoke detector power supply may be used in addition to the basic supply. In either case, if the system has battery standby, it is usually common to both power supplies.


The fire alarm control unit provides termination points for all initiating circuits, indicating circuits, remote annunciators, and other auxiliary devices. The control unit accepts low current signals from the alarm-initiating circuits and, through relays or other circuitry, provides the larger current required to operate the alarmindicating devices and/or auxiliary devices. The control unit also continuously monitors the condition of the alarm initiating and indicating circuit wiring and provides a trouble indication in the event of an abnormal condition in the system, such as an ac power failure or a wiring failure.

The control unit is usually housed in a sheet metal cabinet (fig. 7-3). The control unit usually provides annunciation of signals (telling where a signal originates).

Because all circuits end at the control unit, it is a convenient test location. Test switches (if provided) are usually inside the locked door of the control unit. If the switches are key operated, they may be on the control unit cover rather than inside the cabinet.

Local Alarm Signaling

Because of the critical nature of fire alarm systems, a feature known as "electrical supervision" has been designed into these systems. Alarm systems must be in service at all times; electrical supervision causes a warning (trouble) signal if some potential or actual electrical problem exists in the alarm system. This trouble signal is clearly distinguishable from a fire

Figure 7-3.\Control unit with annunciation.

alarm signal. Figure 7-4 shows a typical local alarm signaling circuit using electrical supervision.

A continuous small electrical current, supplied by the fire alarm control panel, flows through the series loop formed by one side of the initiating circuit, the end-of-line resistor, and the other side of the initiating circuit as indicated by the arrow. The fire alarm control panel reacts to this constant low current as a no-alarm or normal condition.

Under normal conditions, the alarm and trouble relay coils have the same low value of supervisory current flow. This value is inadequate to close the normally open contacts of the alarm relay. The trouble relay, being more sensitive, is energized by the supervisory current, and the normally closed contacts (TR1) are held open. If the supervisory current drops to zero because of a broken wire anywhere in the initiating circuit, the trouble relay is de-energized, and the TR1 contacts close, causing an audible and visual trouble signal. Also, the portion of the circuit beyond the broken wire will not operate in the event of an alarm.

If no wires are broken, closing the contacts of an initiating device provides a low-resistance current path, short-circuiting the end-of-line resistor and increasing the alarm relay coil current. The alarm relay is energized, causing its contacts (AR1) to close and the alarm bells to ring. Continued fire alarm operation with a broken wire depends upon the location of the break and which initiating device is actuated.


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