Guide to Fire Alarm Systems
Introduction
The following Guide is
based on BS5839 Part 1 on the design and installation of Fire Alarm
Systems for general
applications. It is intended as a reference only, and not a replacement for
BS5839 part 1.
When is
a Fire Alarm System Required ?
There are various
statutory documents covering the need for fire protection in various types of
premises. The principle
documents are:
_ Health and Safety at Work Act
1974
_ Fire Certificates (Special
Premises) regulations 1976
_ Factories Act 1971
_ Offices, Shops and Railway
Premises Act 1963
_ Private Places of
Entertainment Act 1967
_ Fire Precautions (Workplace)
Regulations 1997
The Fire
Precautions Act 1971
Under this Act, all
shops irrespective of staff numbers and contents must have:
_ Adequate means of escape while
an employee is in the premises (unlocked doors and
unobstructed access to
them)
_ Appropriate means for fighting
fire provided and maintained
A Fire Certificate must
be obtained from the Fire Authority for premises in which:
_ More than 20 persons are
employed at any one time
_ More than 10 persons are
employed at any one time other than on the ground floor
_ The shop employees are working
in the same building as others and the total in all the
premises exceeds 20 or
10 elsewhere other than on the ground floor
_ Highly flammable or explosive
materials are stored or used
_ Sleeping Accommodation is
provided
_ A Hotel or Boarding House
contains sleeping accommodation for 6 or more people,
which includes staff
and guests, or a Hotel or Boarding House that sleeps a member of
staff or any guests
above the first floor or below the ground floor.
The majority of
Industrial and Commercial premises therefore require a Fire Alarm System with
legislation both
nationally and locally covering a large proportion of the various types of
buildings and their
requirements.
Automatic Fire
Detection will normally be required in premises with:
_ Flammable or Explosive
materials are used or stored
_ Where people are sleeping as
part of the premises business activity eg: Hotels, Nursing
Homes, Hospitals etc
_ When the premises has special
evacuation problems eg: disabled and elderly persons,
cellars and high
buildings
All of the above will
probably need some degree of Automatic Fire Detection to obtain a Fire
Certificate, however
with the new Fire Precautions (Workplace) Regulations 1997 it is not
always necessary to
apply for a certificate. CAUTION - You almost certainly must have a Fire
certificate OR must
comply to the regulations.
The Fire
Precautions (Workplace) Regulations 1997
These new regulations
came into force on 1st December 1997 and have an important message for
those responsible for
ownership/management of a property where persons are employed.
Unfortunately there are
numerous interpretations placed on the regulations and some places are
exempted because they
are already covered by other legislation or already hold a Fire
Certificate.1
To understand the
detailed application to your own requirements please contact the following:
1) For advice on Fire
Detection and Fire Alarm System Requirements, contact Photain
Controls plc
2) For copies of the
regulations contact: The Stationary
The
important points to note are as follows:
1 It is
now the LEGAL RESPONSIBILITY of EMPLOYERS, PERSONALLY, TO
COMPLY WITH THE
LEGISLATION, and it will no longer be the responsibility of
others, eg: the Fire
Service to proscribe to employers the measures to be taken to
minimise risk. The Fire
Service will, however, continue to provide advice and guidance
relating to Fire
Precautions.
2 The
risk assessment requirements of the 1992 Health and Safety Regulations are
extended to include
fire risk. Fire precautions are to be based upon the risk assessment.
2a Employers
must inform their employees of the results of the risk assessment
3 Employers
must provide appropriate means for detecting a fire, raising a fire alarm and
for fire fighting. What
is considered appropriate will depend upon the size and the nature
of the premises, the
number of people present and activities undertaken. Current British
Standards such as
BS5839 Part 1 1988 provide guidance to system design requirements.
4 Employers
must provide emergency routes and exits for use in case of fire. These must
be kept clear, be
available in an emergency and be provided with emergency lighting if
requiring illumination.
5 Equipment
provided to warn of fire, fight fire and to aid escape from fire must be
suitably maintained in
good working order.
6 Where
fire fighting measures are necessary, employees must be adequately trained and
equipped.
7 If
employers fail to meet their obligations, the Fire Service has the
responsibility for
enforcement by means of
a series of procedures dependant upon the seriousness of the
situation.
Consultation
It is always advisable
to consult the Local Fire Prevention Officer at an early stage, regarding the
legislation covering a
particular premises. The Fire Officer will interpret the Fire Precautions Act
or any other act
covering a particular premises and advise on the particular type of Fire Alarm
System that may be
required.
It must be remembered
that the Fire Prevention Officer is concerned with LIFE, his concern for
property is secondary.
We would advise that
you only consult with a Fire Prevention Officer with the client=s consent.
When designing a Fire
Alarm System, it is important to consult with all other interested parties,
for example:
The Local Fire
Authority
The System Installer
The Health and Safety
Officer
Any Consultant or
Architect
The Insurance Company
During early
discussions it is important to establish the purpose of the Fire Alarm System,
ie:
A) To enhance the
safety of the occupants
B) To minimise damage
to the property
Whilst Insurance
Companies give good discounts to clients who fit sprinkler systems the fitting
of complex Fire
Detection Systems seldom lead to a reduction in premiums sufficient to
encourage a client to
fit a Fire Alarm System for property protection. The vast bulk of Fire
Alarm Systems fitted
are normally for the protection of Life.
Types 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.
Conventional
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.
Removal
of Detectors while maintaining the Break Glass Operation
BS5939 part 1 1988
(6.6.2) - amendment 6317 January 1991, requires that :
AWhere 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:
TYPE 1 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
TYPE 2 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.2
TYPE 3 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 connected.
Addressable
Systems
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 opening.
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 Asectioned@ in order to ensure that a
short circuit, or one fault will only cause the loss of a
minimal part of the
system.3
Analogue
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 Afairly 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.
Life or
Property
BS5839 Part 1,
classifies systems and divides them into six different types:
L1 Intended for life
safety and covering the whole building
L2 Intended for life
safety and covering escape routes and other areas of high
risk
L3 Protecting only the
escape routes
P1 Intended for
property protection and covering the entire building
P2 Intended for
property protection covering any potentially high risk areas
M Intended for giving
the alarm in response to operation of a manual call
point and having no
means of automatic detection
System
Design
Before staring the
design you will need to ensure that certain information is available. This may
be given in the
specification or it may have to be obtained by consultation. As well as the
purchaser, there may be
a requirement to consult with other interested parties. The most
important of which will
probably be the Fire Prevention Officer of the local Fire Brigade.
The information which
should be available includes:
A The type of system
required ie: L1, L2, L3 etc and where appropriate, parts
of the premises to be
covered.
B The action to be
taken in the event of fire
C Whether other
occupants of a multi occupancy building will be affected
D Whether other work is
to be done at the same time. If so then consultation
with other contractors
may be required.
E A Method of calling
the Fire Brigade
F Whether the type of
occupants or activity in the building will require a
greater provision of
Manual Call Points than normal
G A likely attendance
time of the Fire Brigade
Coverage
L1 Systems
In an L1 System, all
areas of the building should be covered. It is normal not to cover the
following areas:
_ Lavatories and Water Closets
unless they contain electric hand dryers
_ Voids less than 800mm in
height
Coverage
L2 Systems
Coverage of L2 systems
depends on the vulnerability of the likely occupants and the probability
of ignition of fires,
ie:
_ Sleeping Areas without
supervision
_ Areas having a high
probability of ignition ie: day accommodation,
store rooms, kitchens
and plant rooms
_ Where occupants are especially
vulnerable due to illness, age or are unfamiliar
with the building. It
should be noted that L2 systems always include L3 coverage.
Coverage
L3 Systems
In L3 systems, coverage
should be provided for escape routes and any areas in which the
occurrence of a fire
would directly threaten escape routes. The following areas should therefore
be covered:
_ Corridors, passages and
circulation areas
_ Stairwells
_ The top of vertical risers
_ At each level within 1.5m of
an access point to a lift shaft or other vertical riser
_ In all rooms opening onto an
escape route
Coverage
P1 Systems
In a P1 system all
areas of the building should be covered with the exception of:
_ Lavatories and Water Closets
_ Voids less than 800mm in
height and such that extensive spread of fire or fire products
cannot take place in
them prior to detection by detectors outside the void
Coverage
P2 Systems
P2 systems give coverage
of only part of the building. The areas covered would normally have a
high fire risk and
unprotected areas should be separated by fire resisting construction.
Both P1 and P2 systems
would invariably be connected to the Fire Brigade via a central station.
It would be on an
extremely irregular basis that you would ever be requested to fit a P type of
system.
Manual
only Systems
An AM@ system provides for Manual
only alarm and systems intended for multi occupancy
buildings are given the
suffix AX@.
The
Survey
If you are surveying a
building we would suggest you start at the top of the building and work
down. Two basic reasons
for this are as follows:
A If you are as
athletic as the writer of this guide then you can ride the elevator to the top
floor and the rest of
the survey is downhill!
B If you start at the
top of the building then you can be sure to check the top of each
stairwell. This will
need a detector (except a Manual only System) even on an L3 System.
As you work your way
down the building you can then check how often detectors are
required vertically
down the stairwell. On Type L Systems, BS5839 states that this shall
be at vertical
intervals not exceeding 10.5 metres which normally works out to be every
third floor. On Type P
Systems, there should be detectors on every main landing.
The
Design
If you are designing
onto a set of drawings then we would recommend that you adopt the
following procedure.
1 Identify and plan out
where all break glass points are required.
2 Note where all the
Sounders are required. Indicate where the Control Panel will be. You
now have an >M= System.
3 Consider fitting door
holders/closers onto doors which might otherwise get propped open.
Doors leading onto
stairwells should not be fitted with Door Holders. There was an old
GLC regulation which
requires a Smoke Detector to be fitted within 2m either side of the
door or pair of doors
fitted with a Door Holder/s - Closer/s and many authorities ie: West
Sussex still require
this.
4 Mark down where all
detectors are required in escape routes, top of stairs, landings,
ceilings at vertical
intervals not exceeding 10.5m, top of vertical risers, within 1.5m of
access to lift shafts
and within rooms opening onto escape routes.
5 You should now
effectively have the design of an L3 System and can now go on to add
detectors to bring the
system up to L2 or L1 as required.
6 Indicate the number
of Zones that will be required.
Siting
of Manual Call Points
A Break Glass Call
Point is a device which enables personnel to raise the alarm by breaking the
frangible element on
the facia. They should be mounted 1.4m from the floor and sited where they
can be easily seen.
Manual Call Points
should be sited on the floor landings4 of
stairways and at exits to open air. It
should be noted that
many Fire Officers prefer Call Points to be fitted on the floor side of an
access door to a
staircase so the floor of origin is indicated at the Control Panel. Where
necessary, extra points
should be sited5 so that the greatest travel distance from any point in the
building to the nearest
call point does not exceed 30m. A greater number of Call Points may be
needed in high risk
areas or if the occupants are likely to be slow in movement.
Siting
of Sounders
An Alarm Sounder may be
a bell or electronic sounder and it must be audible throughout the
building.
A minimum sound level
of either 65db(A) or 5db(A) above any background noise likely to persist
for longer than 30
seconds, which ever is the greater, should be produced by the sounders at any
point in the building.
It is unlikely that more than 65DB will be available if the sound has to
carry through more than
one door.
If the alarm system is
used in premises such as hotels, boarding houses etc where the alarm is
intended to wake
sleeping persons then the sound level at the bedhead should be at least 75db(A)
with all doors closed.
We would strongly recommend that you allow one sounder per bedroom.
A few bells sprinkled
down the corridor in hotel will not produce 75db(A) at all the bedheads.
It is important to note
that the above audibility levels must be produced with all doors shut, after
the works on site have
been completed. If a Fire Officer even expects that there is a lack of
audible sounders, then
he is sure to check each area with a db metre and prove it. It can be costly
and very inconvenient
to have to return to site and fit additional sounders.
A minimum of two
sounder circuits should be wired and a larger number of quieter sounders are
preferable to a small
number of very loud sounders.
At least one sounder
should be installed in each fire compartment and all sounders throughout an
installation must
produce a similar sound, ie: you cannot mix an electronic sounder and bells.
Choice
of Detectors
Smoke Detectors will
generally detect a fire far sooner than heat detectors. It is therefore
preferable to fit Smoke
Detectors unless there is any possibility of false or unwanted alarms. It is
not advisable for
example to fit a Smoke Detector in a kitchen as anybody burning toast would
cause an unwanted
alarm. Heat Detectors should be fitted in boiler rooms, generator rooms,
garages and dusty
areas. The products of combustion produced by a boiler, a leaky exhaust on a
generator or exhaust
fumes from a vehicle could all cause a smoke detector to operate and
produce an unwanted
alarm.
Fixed Temperature Heat
Detectors should be installed in areas where one would normally expect
a sudden rise in
temperature for instance kitchens and boiler rooms.
Rate of Rise Heat
Detectors should be installed where Smoke Detectors would be unsuitable but
one would not expect a
sudden rise in temperature for instance, garages, car parks, dusty
workshops etc.
There are two basic types
of Point Smoke Detectors:
1 Ionisation chamber
Smoke Detectors which are very sensitive to smoke with small
particles ie: fresh
cellulosic smoke and the source of almost invisible smoke one gets with
burning paper and
spirit. They are relatively insensitive to smoke with large particles for
example, smoke produced
by burning plastics or stale smoke.
2 Optical Smoke
Detectors are sensitive to optically dense smoke ie: smoke with large
particles and they are
relatively insensitive to optically thin smoke.
Some countries ie:
Italy, Japan, Qatar only use Optical Smoke Detectors and within parts of the
Middle and Far East,
only Ionisation Detectors are used. Within the UK systems can comprise of
a mixture of the two.
The demise of most people is caused by thick dense choking smoke which
is normally a greater
problem than getting burnt. For this reason Optical Detectors are normally
used on escape routes
such as corridors and stairwells. Ionisation Smoke Detectors are normally
fitted within office
and other general areas.
Siting
of Detectors
In a building the
greatest concentration of Smoke and Heat will generally collect at the highest
parts of the enclosed
areas and it is here therefore, that the detectors should normally be sited.
Smoke
Detectors
Smoke Detectors should
be sited so that the sensing element is not less than 25mm, nor more than
600mm below the ceiling
or roof. If a protected space has a pitched or northern light roof, then
Smoke Detectors should
be installed in each apex.
The maximum horizontal
distance between any point in the area being protected and the nearest
detector should be as
follows:
Under flat horizontal
ceilings and corridors more than 5m wide, then the maximum distance for
Point Type Smoke
Detectors should not exceed 7.5m. The maximum area of coverage of a Point
Smoke Detector is 100
square metres. On the rear of all Photain Smoke Detector Data Sheets, a
diagram showing the
relevant coverage they provide is shown.
In corridors the number
of detectors required depends on the corridor width. When installing
Smoke Detectors the
following data can be used:
The maximum height that
smoke detectors should be installed at is as follows:
Point Smoke Detectors
10.5m
Optical Beam Smoke
Detectors 25m
If detectors are to be
fitted in the apex of a pitched or north light roof then a row of detectors
should be sited within
the apex. One row of detectors should be sited at the highest point a
minimum distance of
0.5m from the vertical wall. Add to the maximum horizontal distance 1%
for each degree of the
slope up to a maximum of 25%. For instance a point type detector at the
apex of a 20 degree
slope would work out as follows: 20% of 7.5m = 1.5m. Therefore the
maximum distance
between detectors = 7.5 + 1.5 = 9m. The maximum area of coverage may
also be increased
proportionally.
Where the passage of
Smoke or Hot Gases from a position to a detector is likely to be disturbed
by a ceiling
obstruction such as a beam having a depth greater than 150mm but less than 10%
of
the height of the
ceiling, then the horizontal distance should be decreased by twice the depth of
the obstruction. For
instance for a Point Type Smoke Detector obstructed by a 200mm depth
beam then the maximum
distance between detectors = 0.2m x 2 = 0.4m.
7.5m - 0.4m = 7.1m.
Where a ceiling
obstruction, such as a beam is greater than 10% of the height of the ceiling
then
the area either side of
the obstruction should be considered as separate rooms.
Ceiling beams less then
150mm in depth can be ignored.
Optical beam smoke
detectors are useful for covering large unobstructed roof areas such as those
found in most
warehouses. They can be quite cost effective as one smoke beam can provide the
coverage of many
individual point detectors. A smoke beam typically comprises of a Projector, a
Receiver, a Remote
Manual Reset Unit and a Local Power Supply with battery standby. Optical
Beam Smoke Detectors
should be mounted as follows: (m = Metres)
• The
minimum height above floor level = 2.7m
• Maximum
height above floor level = 25m
• Minimum
Optical Beam length = 10m
• Maximum
Optical Beam length = 100m
• Minimum
distance of Optical Beam = 0.3m
From a flat ceiling or
apex
• Maximum
horizontal distance between Optical = 14m
Beams measured at right
angles to a Beam
• The
Maximum horizontal distance between = 7m
Optical Beam and an
adjacent wall or partition
Heat
Detectors
Heat Detectors should
be sited so that the heat sensitive element is not less than 25mm, nor more
than 150mm below the
ceiling or the roof.
The maximum horizontal
distance between any point in the area being protected and the nearest
detector should be as
follows:
Under flat horizontal
ceilings and corridors more than 5m wide then the maximum distance
between any heat
detector and any wall or partition should be 5.3m.
The maximum area of
coverage per heat detector is 50 square metres.
On the rear of all
Photain Heat Detector Data Sheets, a diagram showing the relevant coverage
they provide is shown.
There is also
information regarding detector coverage in corridors using Heat Detectors. As
Heat
Detectors are very
seldom used in corridors then please consult British Standard BS5839 Part 1,
should you require this
information.
The maximum height that
Heat Detectors should be installed at are as follows:
Grade 1 Heat Detector
9m
Grade 2 Heat Detector
7.5m
Grade 3 Heat Detector
6m
High Temperature Heat
Detectors 6m
Control
Equipment
The Fire Alarm Control
Equipment should normally be sited in an area as follows:
Preferably in an area
of low fire risk and on the ground floor by the entrance used by the Fire
Brigade and preferably
viewable from outside of the building. It should be located in an area
common to all building
users and where automatic detection is in use, the Control Panel should
be in a protected area.
An alarm sounder should be sited next to the Control Unit, but not too
near the telephone
position.
A suitable zone chart
of the building should normally be installed adjacent to the Control Panel.
Power
Supplies
Two power supplies are
required ie: mains and battery and these are normally built into the Fire
Alarm Control Panel.
Standby batteries must allow the system to operate without mains for 24
hours longer than the
building is likely to be unoccupied and then support the sounders for an
additional half hour.
If the mains supply is supported by an emergency generator then six hours
standby plus half an
hour alarm load is sufficient. All modern Fire Alarm Systems are 24 volts.
On the medium and
larger sized Fire Alarm Systems, the standby batteries will often not fit
within the Control
Panel. Where standby batteries are contained within a separate housing, then
this housing must be as
close as possible to the main Fire Alarm Control Panel. If the power
supply or battery
housing is located more than 10 metres from the main Fire Alarm Control Panel
then serious volt drop
problems can arise. Standby batteries are invariably of the sealed lead acid
variety. Use of nickel
Cadmium Batteries is not cost effective and automotive batteries must not
be fitted.
Fire
Compartments
Buildings are normally
split into fire compartments with each compartment so constructed as to
prevent the spread of
fire from one compartment to another.
Each floor and each
stairwell within a building is normally a separate fire compartment. Within a
small factory, the
factory unit will normally be separated from the offices by >firewalls= to
prevent the spread of
smoke and fire from one to the other. The factory and offices will therefore
be in separate fire
compartments. A zone should normally only cover a single fire compartment.
Zoning
If the total floor area
(ie: the total of the floor areas of each floor of the building) is not greater
than 300 square metres
then the building need only be one zone, no matter how many floors it
has.
In general, if the
total floor area is greater than 300 square metres, then each floor should be a
separate zone (or set
of zones, if the floor is big enough).
There are two
exceptions:
A If the building is
sub divided into fire compartments, then any compartment
communicating with
other compartments only at the lowest level of the building can be
treated as if it were a
separate building ie: if a floor area is not greater than 300 square
metres then it can all
be one zone, irrespective of the number of storeys.
B Where stairwells or
similar structures extend beyond one floor, but are in one fire
compartment, the
stairwell should be a separate zone.
There are two restrictions
on the maximum size of a zone, irrespective of the size of the building
A Its total Floor area
should not exceed 2000 square metres
B The search distance
should not exceed 30 metres. This means that a searcher entering the
zone by the normal route
should not have to travel more than 30 metres after entering the
zone in order to see a
fire big enough to operate a detector, even if the fire is only visible
from the extreme end of
his search path. Remote indicators show an alarm in a closed
area and their fitting
can enable larger areas to comply to the search distance
requirements.
There are two
restrictions on the configuration of a zone, irrespective of its size.
A If the zone covers
more than one fire compartment, then the zone boundaries should
follow compartment
boundaries
B If the building is
spilt into several occupancies, then each occupancy should lie within a
separate zone (or set
of zones), no zone should be split between two occupancies
For the zoning of
special risks or complex areas please consult BS5839 Part 1
Recommended
Cable Types
All cables used in Fire
Alarms must have a minimum conductor size of 1.0mm squared.
BS5839 Part 1,
recommends 11 types of cable which may be used on a Fire Alarm System where
prolonged operation in
a fire is not required. Therefore 1.0mm twin and earth cable for instance,
may be used on
detection circuits of Conventional Fire Alarm Systems and the detection loops
of
Addressable and
Analogue Systems providing sounders are not connected to them.
Only two types of cable
may be used on Fire Alarm Circuits where prolonged operation in a fire is
required.
1 Mineral - insulated
copper - sheath cables (MICC) complying with BS6207
AND
2 Cables complying with
BS6387, and meeting at least the requirements of categories AWX
or SWX
In other words, on
sounder circuits and for wiring between a power supply and or battery housing
and the main fire alarm
control panel you must use one of the following types of cable.
MICC, Flamsil, Firetuff
or similar
On Addressable and
Analogue Addressable Fire Alarm Systems we would recommend the use of
a screened cable such
as BICC Flamsil or Firetuff or MICC for all wiring so as to minimise the
possibility of
interference being picked up by or being transmitted by the data loops.
In the larger buildings
within the London area (old section 20 buildings) only bare MICC cable is
often specified.
In summary therefore
MICC cable used for all your fire alarm wiring would be acceptable
anywhere. However,
ordinary twin and earth 1.0mm cable may be used on detection circuits of
Conventional Systems in
certain circumstances.
As far as possible,
joints should be avoided except where a joint is inside one of the systems
components ie: Control
Panel, detector, Call Point, Sounder etc. Where joints are required
elsewhere they should
be enclosed in a suitable junction box marked fire alarm to ensure that the
fire alarm systems is
not accidentally interfered with.
Fire Alarm Cables,
should always be segregated from cables for other systems. The segregation
of MICC cables with a
plastic sheath is of course not so critical as the segregation of ordinary
twin and earth cable.
Installation of cables
should be in accordance with good practices recommended in the latest
edition of the IEE
wiring regulations
Connection to the mains
supply should be via an isolating switch fuse reserved solely for the
purpose. Its cover must
be painted red and labelled AFire
Alarm - do not switch off@.
Conductor size should
take voltage drop into account. In any case conductors should have a cross
sectional area of not
less than 1 square millimetre.
Where possible cables
should be routed through areas of low fire risk. Cables installed in damp,
corrosive or
underground locations should be PVC sheathed and where there is a risk of
mechanical damage
should be protected accordingly. If Cables are installed less than 2.25m
above the floor should
they normally be protected.
Volt
Drop in Cables
Unless a detection
circuit or detector loop exceeds 1 kilometre in length, it is unlikely to give
rise
to a concern about volt
drop.
If there are fairly
long sounder circuits or a sounder circuit has a large number of Sounders,
Buzzers, Voice Alarms
or Flashing Beacons etc on it, then voltage drops can cause problems.
Providing the overall
volt drop does not exceed 4 volts on sounder circuits then the system should
operate satisfactorily.
The calculation of the
precise voltage drop at each point in the system is a long and tedious
calculation and way
beyond the scope of this guide. However, to get a rough idea as to whether a
system will operate
satisfactorily one can use the following calculations.
To start with we need
to know approximate volt drop characteristic of different sizes of cable
1.0mm cable = 42mV per
amp per metre
1.5mm cable = 28mV per
amp per metre
2.5mm cable = 17mV per
amp per metre
4.0mm cable = 10mV per
amp per metre
6.0mm cable = 7mV per
amp per metre
If one is using 1.0mm
cable:
_ Multiply 42 by the length of
the cable in metres
_ Multiply this by the current
of all the devices on the length of the cable
_ Divide the entire figure by
1000
This gives a rough idea
of the voltage drop.
Lets take an example
where you have 30 Sounders, each with a current consumption of 20mA on
200 metres of 1.0mm
cable.
If you were to wire in
1.0mm cable then the calculations would look something like this:
42 x 200 metres x 30
sounders x 0.02 amps
1000
The answer is 5.04
volts. This is more than the 4 volts previously discussed and therefore we
would suggest that
1.0mm cable would be unsuitable in this instance.
Lets now try the
calculation using 2.5mm cable. In this instance we have the following:
17 x 200 metres x 30
sounders x 0.02amps
1000
The answer is 2.04
volts. A two volt drop is of course acceptable.
REF: &MARKET018
Issue 2
Date:17/3/98
-23-
Should you be on a
budget and be considering using 1.5mm cable, the answer after making the
calculation would be
3.36 volts and this is indeed acceptable. However do not disclaim the
possibility that at a
later date you may wish to add extra sounders, and therefore you would be
pushing the system to
its full limitations by utilising the 1.5mm cable.
You may encounter
examples where even 2.5mm cable is not sufficient. Rather than use a larger
cable which would be
extremely difficult to terminate in the rear of most sounders, it is usually
better to run
additional sounder circuits and spread the load.
Should you be using a
remote power supply or battery housing to power the control panel, then
the voltage drop
becomes very significant. As well as the consumption of the Control Panel, one
must consider the
operating load of the sounders. It is particularly important to keep voltage
drop
as low as possible and
preferably below 1 volt or power levels will decrease even before you have
commenced consideration
regarding the calculation of the volt drop to the sounders from the
control panel.
An example of this now
follows.
We have a control panel
which consumes 260ma and has a number of sounders connected, which
in total use 3amps in
the alarm condition. If you wired between the remote power supply and the
control panel which was
only 20 metres away in 1.0mm cable then the calculation would be as
follows:
42 x 20 metres x 3.26
amps = 2.7 volt drop
1000
This would clearly be
unacceptable.
Should we be able to
locate the remote power supply within 10 metres of the control panel and
wire it in 2.5mm cable
the calculations should look as so:
17 x 10 metres x 3.26
amps = just over half a volt
1000
The above example
should be acceptable. However when calculating the volt drop on your
sounder circuits it
would be advisable not to allow any volt drop to exceed 3.5 volts.
A word of warning
however, the writer of this guide has seen several examples where electricians
have installed cable
that is too thin on sounder circuits and consequently the system has
encountered substantial
volt drops ie: in excess of 12. A way around this has then been sought
and the 24 volt bells
have been substituted with 12 volt bells. This does not work, as if you lower
the voltage the current
increases and so the problem gets worse.
Routine
Testing of the System
The system should be
regularly tested and serviced and BS5839 Part 1 makes the following
recommendations:
DAILY Check that the
panel indicates normal operation. If not record any fault indicated
in the event log and
report the fault to a responsible person. Check that any fault
recorded from the
previous day has received attention.
WEEKLY Operate a manual
call point or smoke detector to ensure the system operates
properly. Each week a
different detector or call point should be checked. Check
that the sounders have
operated and then reset the system. Check the battery
connection. Any defect
should be recorded in the log book and reported. Action
should be taken to
correct the defect.
QUARTERLY Check entries
in the log book and take any necessary action. Examine the
batteries and their
connections. Operate a manual call point and smoke
detector in each zone
to ensure that the system operates properly. Check
that all sounders are
operating. Check that all functions of the alarm
control panel operate
by simulating fault conditions. Visually check that
structural alterations
have not been made that could have an effect on the
siting of detectors and
other trigger devices. Complete the event log with
details of the date,
time, trigger device tested and >Quarterly
Test= in the
event section. Any
defects or alterations to the equipment should also be
entered
ANNUALLY Carry out an
inspection as detailed for this quarterly inspection. Every detector
should be tested in
site. All cable fitting and equipment should be checked to
ensure that they are
secure and undamaged.
A qualified engineer
should carry out the quarterly and annual inspections and issue a certificate
after each annual
inspection. It is normal practice for 1/4 of all detection systems to be
cleaned
and checked on each
quarterly visit so that the entire system has been properly maintained after
the fourth visit.
Whilst the end user of
the fire alarm system may be expected to carry out the daily and weekly
functions very few
would be adequately equipped or trained to carry out the quarterly and annual
tests.
Photain Controls plc
would be please to submit a price for the maintenance of any Fire Alarm
System which has been
installed using Photain Fire Alarm Equipment.
The
intention of this guide is to keep the information given as simple as possible.
This
necessitates
the omission of much information contained within the various British
Standards
and the requirement of the various fire acts. Photain Controls can therefore
not
take
any responsibility for the way in which any information contained in this guide
is used.
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