Fire alarm operation and control Flashcards
In larger buildings, the fire detection and alarm
system may be divided into zones. These are
n Detection zones
n Alarm zones
Discuss detection zones
- detectors and manual call
points based on zones within the protected
premises. - This enables the indicator boards to
show more precisely the whereabouts of the origin
of the signal. - Detection zone indication will help direct firefighters to the area of the fire
- Be aware ,where only manual call points are provided, M systems, a person might operate a point some distance from the fire.
BS 5839: Part 1 makes certain recommendations
for the size and configuration of detection zones
(i) Where manual call points are provided on
landings, the call point should be incorporated
in the zone served by that landing.
(ii) If the total floor area of the building exceeds
300m2 zones should be restricted to a single
storey. If the total floor area of the building
is 300m: or less, the building may be considered
a single zone even though there may be
more than one storey.
(iii) The floor area of a single zone should not
exceed 2,000m2 except in a single storey
open plan area (e.g. warehouse) where the
zone should not be more than 10,000m2.
(iv) Where a zone is served by non-addressable
detectors, the ‘search distance’, i.e. the distance
that may have to be travelled by a person
responding to a fire alarm signal seeking
to locate the fire visually should not exceed
60m.
(v) Where the zone is served by addressable
detectors, the control equipment should
automatically give a visual indication of the
zone affected together with a text display of
the location of the first detector to respond to
the fire. This display should be supported by
other information, e.g. a building floor plan,
to enable firefighters who may be unfamiliar
with the building to proceed directly to the
location of the fire.
(vi) Automatic detectors in an enclosed stairwell,
liftwell or other flue like structure are considered
as a separate zone.
What is an alarm zone
In buildings with simple evacuation strategies, the
actuation of a call point or detector will cause the
alarm system throughout the building to operate.
In larger buildings the means of escape arrangements
may be designed around more sophisticated
evacuation process. That may be:
n Staged evacuation
n Phased evacuation
n Progressive horizontal evacuation
BS 5839 recommends that alarm zones
(i) The internal boundaries between zones
should be of fire resisting construction.
(ii) Where audible alarms are used, alarm zones
should be acoustically separated from each
other. Where an overlap of signals does
occur, it should not cause confusion to occupants
of the building.
(iii) The evacuation signal should be sufficiently
different from the alert signal to avoid confusion.
(iv) Alarm zones may incorporate more than one
detection zone but should coincide with
detection zone boundaries. No detection
zone should cover more than one alarm zone.
Alarm signals
signals may be
Audible
Visual
Sensual
Discuss audible alarm systems
1.Bell or siren
2.distinctive and unambiguous signal throughout the alarm zone.
3.sufficient to rouse them from their sleep.
4. if appropriate in a place of public entertainment, the audible signal can be confined to staff areas
5. Once activated, audible signals should operate
continuously until silenced manually.
6.Generally, the minimum levels in most areas
should not be less than 65dB(A
7. Voice systems are useful in buildings
having a two-stage evacuation system
8.
discuss visual alarms
1.Visual alarm signals can be used to supplement
audible signals in areas where an audible signal
might be ineffective, such as in areas of high background
noise.
2. They may also be used as the single
form of alarm where an audible alarm might cause
unwanted disturbance or distress, such as in places of public assembly or entertainment or in certain parts of hospitals.
3. designed to be distinctive against other backgrounds
4.Normally red
discuss sensual alarms
- Sensual or vibrating alerters can be used as part of an internal paging system to alert staff in circumstances where public alarms are inappropriate.
- profoundly deaf people
- devices under pillows in place where deafos sleep
- Checks for radio interference
The control unit is the nerve centre of any system
and is usually placed in a prominent position in a
building to ensure that its signals will be easily
seen and heard by the building’s occupants, fire
brigade, etc. Its functions can include:
(i) Automatically monitor and control the
equipment in the system, such as the fire
detection and fire alarm device circuits and
the power supply to that equipment.
(ii) Indicate fire and fault signals and their location.
(iii) Provide manual control facilities for testing
the circuit, triggering fire alarm signals,
silencing audible fire warnings and resetting
the system after a fire signal.
(iv) Operate the alarm either throughout the
building or in any particular sequence
related to an evacuation plan for the building
with manual override facilities as necessary.
(v) Transmit the signal to an Alarm Receiving
Centre (ARC) for onward transmission to the
fire brigade (if a fire signal) or other interested
parties, e.g. the keyholder, maintenance
engineer.
(vi) Indicate from which zone the signal is coming,
not only at the main indicator panel but,
if necessary, at repeater indicators throughout
the premises, e.g. the gatehouse.
(vii) Operate other equipment e.g. de-activation
of door-holding devices, opening smoke
ventilators.
(vii) Operate fixed firefighting equipment
The control and indicating equipment is designed
to give a visual and audible warning of a fault and
its location, e.g.:
(i) A fault in any detector or manual call point
in the circuit.
(ii) A short or break in any circuit forming part
of the system.
(iii) A failure of the mains power supply (within
30 minutes) or of a standby supply (within
15 minutes
(iv) A failure of the transmission links between
the system and an ARC.
(v) Any other fault that may render the system
or part of the system inoperative.
BS 5839 requires that a system should have
a silencing device, which can silence general
alarm sounders or, in a two-stage system, alert
signals.
The operation of the device should:
(i) Require a manual operation.
(ii) Sound an audible alarm at the control unit.
(iii) Not cancel any visual signal of the alarm at
the control unit.
(iv) Not prevent the proper receipt of alarms
from any zone not already providing an
alarm.
(v) Not prevent the correct operation of any control
for starting or restarting the alarm
sounder.
(vi) Not prevent the transmission of an alarm to
an ARC. ( alarm receiving company )
Monitoring the system
Other facilities, which could be included in a system,
are:
(i) The unit can be programmed to note the
ambient conditions in certain areas and
adjust its response to the signals accordingly.
(ii) The unit can note, over a period of time,
deterioration in a sensor, e.g. from an excessive
accumulation of dust, excessive insect
inhalation. Taking this into account it can
adjust the sensors response up to a predetermined
limit and beyond that will register a
fault.
(iii) If a sensor is removed for any reason this can
be noted and it will register a fault if the
wrong type of sensor is reinstalled or the
new sensor is not coded with the correct
‘address’.
Connections from premises (FAS)
to ARCs
There are four distinct means of connection
between protected premises and ARCs. These are:
(i) Private wires - most ARCs offer this facility;
(ii) Omnibus circuits - fairly frequent option
offered;
(iii) Connection to ARC satellites - from here
signals are multiplexed to the ARC. A number
of major companies offer this.
(iv) Digital communications - widespread
facility;
