 The philosophy of a conventional system revolves around dividing the building into a number of areas called zones - the detectors and callpoints within each zone are then wired on dedicated circuits.
In the event of a detector or callpoint being activated, the panel is able to determine which circuit contains the triggered device and thereby identify which zone the fire alarm has come from - see diagram 1.
It is then necessary to manually search the indicated zone to pinpoint the exact cause of the fire alarm. In addition to the detection circuits, there is also a need for separate circuits for alarm annunciation devices such as sounders and beacons, to signal the existence of a fire alarm condition to the building users.
For sounder circuit continuity monitoring to function effectively, sounder circuits must be wired in a single radial circuit - spurs and tees are not permitted.
Almost every conventional fire panel will have facilities for more than one sounder circuit. In general, the number of sounder circuits increases with the number of detection zones provided and/or the specification of the panel. Normally however, there will be less sounder circuits than detection zone circuits, so it will be necessary for a sounder circuit to provide cover for more than one zone - see diagram 2.
This increases installation complexity by forcing the sounder wiring to follow a different route(s) to that of the detector wiring. It is therefore important when designing a conventional system to ensure that the panel has adequate zone capacity for the size and complexity of the building and that the panel can support the intended sounder circuit wiring and loading.
2 wire systems are based on standard conventional system technology, but incorporate additional functionality to enable both the detectors and the sounders for each zone to be wired on a single common circuit.
2 wire technology thereby enables the control panel to use a single circuit per zone for detection and to power the sounders - see diagram 3.
 Even though the panel continually powers the sounders, additional control functionality within each sounder enables them to be activated only in the event of a fire alarm condition.
By combining both the detection and alarm annunciation wiring into a single circuit, considerable savings in installation time and cabling can be achieved.
Although adding 2 wire technology to the detectors and sounders slightly increases equipment costs, this is normally more than offset by savings in installation - resulting in a reduced total cost for the system.
Typical Wiring Arrangements
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