of Fire Alarm Systems
|All Fire Alarm Systems essentially operate on the same
principle. If a detector detects smoke or heat, or someone
operates a break glass unit, then alarm sounders operate
to warn others in the building that there may be a fire
and to evacuate. For the system protecting property,
it is additionally likely that the Fire Alarm will incorporate
remote signalling equipment which would alert the fire
brigade via a central station.
|Wired Fire Alarm Systems can be broken down into three
categories, Conventional, Addressable and Analogue Addressable.
Fire Alarm System
|In a Conventional Fire Alarm System, a number of call
points or a number of call points and detectors are wired
to the Fire Alarm Control Panel in Zones. A Zone is a
circuit and typically one would wire a circuit per floor
or fire compartment. The Fire Alarm Control Panel would
have a number of Zone Lamps. The reason for having Zones
is to give a rough idea as to where a fire has occurred.
The accuracy of knowing where a fire has started is controlled
by the number of Zones a Control Panel has, and consequently,
the number of circuits that have been wired within the
building. The Control Panel would then be wired to a
minimum of two sounder circuits which could contain bells,
electronic sounders or other audible devices. Sounder
Circuits and Detection Zones are wired in a star configuration.
Each circuit would have an end of line device which is
used for monitoring purposes.
|Photain Controls currently use 20K resistors for use
on the end of all sounder circuits and HRMODULES for use
on the end of the Detection Circuits for all of their
Conventional Control Panels except the PCS800HR Panel
and PCS1200HR range which uses a 4K7 resistor.
of Detectors while maintaining the Break Glass Operation
|BS5939 part 1 1988 (6.6.2) - amendment 6317
January 1991, requires that :
|A where Detectors are designed to be removed
from the circuit, removal of any detector from the circuit
should not affect the operation of any manual call point.
|On Conventional Fire Alarm Systems one of
the following three methods of wiring could be used to
meet the Head Removal requirements:
||All Manual Call Points could be wired to
one Zone or Zones and all Smoke or Heat Detectors could
be wired to a separate Zone or Zones
||All Manual Call Points and Smoke Detectors
could be wired to the same Zone providing all manual Call
Points are wired in front of all the automatic detectors.
||An active end of line module (HRMODULE)
could be wired to the end of the Zone/s in place of the
normal end of line unit. This would allow for the wiring
of Call Points and Automatic Detectors in any combination
on a circuit zone. If any Detector head is removed from
its base then all call points will continue to be operative
|It should be noted that with Addressable
and Analogue Addressable Fire Alarm Systems the method
of wiring to a detector base is different to the method
of wiring to a Conventional Base. In addition the method
of operation of Addressable and Analogue Addressable Systems
means that the requirements of the amendment 6317 (as
detailed on the previous page) are always complied with,
irrespective of the sequence in which the devices are
|The detection principle of an Addressable System is
similar to a Conventional System except that the Control
Panel can determine exactly which detector or call point
has initiated the alarm.
|The detection circuit is wired as a loop and up to 99
devices may be connected to each loop. The detectors
are essentially Conventional Detectors, with an address
built in. The address in each detector, is set by dil
switches and the Control Panel is programmed to display
the information required when that particular detector
is operated. Additional Field Devices are available which
may be wired to the loop for detection only ie: it is
possible to detect a normally open contact closing such
as sprinkler flow switch, or a normally closed contact
|Sounders are wired in a minimum of two sounder circuits
exactly as a Conventional System. Loop Isolation Modules
are available for fitting on to the detection loop/loops
such that the loop is A sectioned @ in order to ensure
that a short circuit, or one fault will only cause the
loss of a minimal part of the system.
Fire Alarm Systems
Analogue Fire Alarm Systems are often known as Intelligent
Fire Alarm Systems.
There are several different
types of Analogue Systems available which are determined
by the type of protocol which they use. The bulk of
Analogue Detectors available are A fairly stupid @ as the Detectors can only give output signals representing
the value of sensed phenomena. It is left up to the
Control Unit to decide whether there is a fire, fault,
pre alarm or whatever. With the Photain True Intelligent
Analogue System each detector effectively incorporates
it= s own computer which evaluates the environment around it,
and communicates to the Control Panel whether there
is a fire, fault or the detector head needs cleaning.
Essentially however, Analogue
Systems are far more complex and incorporate far more
facilities than Conventional or Addressable Systems.
Their primary purpose is to help prevent the occurrence
of false alarms.
With the Photain Analogue
Addressable System up to 127 input devices ie: Smoke
Detectors, Call Points, Heat Detectors, Contact Monitors
and other interface devices may be wired to each detection
loop. In addition to the 127 Input Devices, up to 32
Output Devices such as Loop Sounders, Relay Modules
and Sounder Modules may also be connected.
Photain Analogue Systems
are available in 2,4 and 8 loop versions which means
large premises can be monitored from one single panel.
Isolator units should be connected between Asections@ of detectors as described for Addressable Systems.