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Fire Alarm
Systems
A fire
alarm system is an active fire protection system that automatically
(or manually) detects a fire, or the effects of fire, and performs
one or more of the following functions:
-
notifies the building occupants via alerting devices
-
automatically summons the fire service (if remotely connected)
-
indicates the location/zone of the activated detector or manual
call point
-
self-supervises for fault conditions
-
resists false activation due to typical non-fire phenomena
-
initiates ancillary fire-related control functions in the
building (e.g lift recall, air conditioning shutdown, smoke-stop
door release).
A fire
alarm system will typically include manual call points, detectors,
control and indicating equipment, fire alerting devices,
interconnections, safety control outputs, power supplies, and
wiring. If connected to a remote receiving centre it will also
include remote signalling equipment.
The
protected premises will generally be subdivided into detection
“zones”, and alarm indication of these will be provided to attending
fire fighters in an “index”. Zone area is usually limited to what
can readily be searched in a short period of time.
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Fire Alarm System Design
The
overall design objective of a fire detection and alarm system is to
detect fire as early as possible consistent with good tolerance to
adverse environmental influences and other potential sources of
false activation. Once a fire is detected, the actions taken by the
system need to be consistent with the building’s overall design, its
evacuation plan, and the integrated fire protection strategy for the
premises.
This
system design should be performed by competent engineers with
experience in fire protection, and who are trade certified to act in
this capacity. This design is usually done in conjunction with the
Architect’s design team during the design phase of the building
project. The detailed component selection and final equipment layout
and programming is provided by a certified contractor during the
building’s construction and fit-out phases.
The
design of building fire alarm systems in New Zealand is almost
invariably required to be in compliance with New Zealand Standard
NZS 4512:2003 Fire Detection and Alarm Systems in Buildings.
This national standard covers design, installation, extension,
modification, commissioning, testing and maintenance. It also sets
down minimum levels of trade qualification for those working on fire
alarm systems, and requires independent third-party inspection/audit
of all new systems, and all significant system alterations and
extensions.
NZS
4512:2003 is the only “Acceptable Solution” for fire detection and
alarm systems under the NZ Building Code Approved Documents, and
full compliance with this standard ensures acceptance under the
Building Code by the Territorial Local Authority.
Alternative “Engineered Solutions” are permitted, however these
require a specific Professional Fire Engineer’s design for that
particular building, and in general such designs are usually invoked
to allow trade-offs between one compliance parameter and another, or
in the case of unusual building features.
Extensions of existing systems should be performed with the same
attention to design, but must also consider the originally installed
fire alarm system, and more than likely will need to be proprietary
to match the existing equipment. NZS 4512:2003 has requirements for
independent third party inspection of system extensions.
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Detection Devices
Fire
alarm systems have devices connected to them to detect the
fire/smoke. Below is a list of common detection devices found on a
fire alarm system:
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Manual Call Points
– devices to allow people to manually activate the fire alarm.
These are usually located near exits and on exit routes within
the premises so that building occupants will be able to locate
one within a reasonable travel distance after discovering (or
causing) a fire. Manual call points must be at least 115mm x
100mm, coloured red, have operating instructions. New Zealand
also requires two-stage operation (“the breaking of a frangible
element followed by the manual operation of a switch”).
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Heat Detectors
– devices which are designed to operate when the temperature or
rate-of-rise of temperature exceeds a predetermined value.
These are most commonly based around either a simple bi-metal
thermostatic switch, or a thermistor-based electronic circuit.
Eutectic alloy (low melting point metal) devices were used
historically, but are now prohibited. Other (less common) heat
detection technologies include thermoplastic cable (shorts when
plastic melts) and fibre-optic time domain reflectometry
systems.
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Smoke Detectors
– devices which detect visible or invisible products of
combustion, usually emitted prior to the flaming stage. These
are most typically point type devices operating on photoelectric
(light-scattering) or ionization chamber principles. Other
(less common) device types include: projected beam smoke
detectors and air-sampling (aspirated) smoke detectors. Some of
these can be incredibly sensitive indeed, so precautions need to
be taken to avoid false activations.
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Carbon Monoxide Fire Detectors
– devices which detect the poisonous carbon monoxide (CO) gas
characteristic of smouldering fires. These are primarily used
in life safety applications (e.g. sleeping occupancies). For
the highest level of life safety CO detectors are usually
employed in conjunction with heat or other smoke detection (e.g.
multi-sensor detectors), or sprinklers.
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Flame Detectors
– devices which detect the infrared or ultraviolet radiation
from a flaming fire. These are most typically used in
commercial and industrial applications where a fast flaming fire
can be expected (e.g. aircraft hangar, fuel storage depot), and
can also be used outdoors.
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Water Flow Switches
– devices which detect when water is flowing through the fire
sprinkler system. These will typically indicate on zone index
of the fire alarm system to assist fire fighters in locating the
source of a fire, but are not generally permitted to initiate
alerting (building evacuation) or a brigade call.
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Sprinkler System Alarm Valve
– the main alarm output from a fire sprinkler system will
generally indicate on the zone index of a fire alarm system and
initiate alerting (general building evacuation). The sprinkler
system will be required to have its own remote connection to the
fire brigade.
Advances in technology in recent years have allowed for multiple
detection principles (heat, smoke, CO, flame) to be incorporated
into one detector. These so-called multi-sensor detectors are
becoming more common. Their particular attractions are improved
detection and improved immunity to false activations (nuisance
alarms).
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Alerting
Devices
The
days of fire alarm bells are well and truly gone for new
installations, although bells and sirens may still be present in
historical systems. For new installations, a specific standard
“rising whoop” tone is now mandatory, with an interspersed verbal
message (except for very small buildings). The voice message will
generally be something like “Evacuate the building using the nearest
fire exit”, and provides positive direction to building occupants.
Although point-type sounders are available to produce this standard
alerting signal, it is now most common to have a central tone and
voice generator/amplifier located at the fire alarm control panel,
with a network of loudspeakers to reproduce the signal around the
building.
The
alerting signals through a building must be identical. For
additions to existing systems, it is permissible to retain the
existing sound (siren, bell). Also on large sites where there is a
uniform alerting signal, it is permissible for additional (new)
systems to retain the same signal (even if it is, say, a bell
sound), however a voice message must be provided on the new system.
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EWIS
Systems
A EWIS (Emergency Warning and Intercommunication System) is an
enhanced system for fire alarm alerting and evacuation control in
larger or more complex buildings. Instead of the whole building
receiving the (evacuation) alerting signal simultaneously, the
premises are instead subdivided into multiple evacuation zones. In
response to signals from the fire alarm system, the EWIS system
controls a zone-by-zone staged/phased evacuation according to a
pre-programmed scheme.
As with “all out” evacuation, the EWIS system generates the standard
“rising whoop” evacuation signal and plays a voice message with
evacuation instructions. With an EWIS these messages can be
customized for various types of installations, and multi-lingual
capabilities are usually available. In addition an earlier “Alert”
tone, with separate verbal message, can be played to zones that have
not (yet) reached the evacuation stage.
A EWIS system is also designed to enable either the fire service or
a designated building warden to take manual control of the
evacuation, including directing Public Address (PA) messages to all
or selected evacuation zones.
The (optional) Emergency Intercommunication (warden telephone)
system allows building wardens or fire-fighting personnel to
communicate with the master evacuation control panel to coordinate
evacuation efforts.
The rationale behind audio evacuation systems is, though
conventional fire alarm notification devices alert occupants of a
building of the presence of an emergency, they do not provide
detailed information to the occupants, such evacuation routes or
instructions. Nor do they allow occupants in the greatest danger to
have unimpeded access to escape routes.
EWIS
systems usually permit multiple messages. For instance, "non fire"
messages can be programmed for situations such as a hazardous
material spill, gas leaks, security breaches, etc.
They
can also be used to provide non-emergency building Public Address
facilities.
Under
the New Zealand Building Code Approved Documents, EWIS systems are
designated as “Type 8” Voice Evacuation Systems and are mandated for
new buildings primarily in healthcare, high-rise, and large crowd
occupancies (cinemas, stadiums, shopping malls). They are required to comply with AS 2220 part 1: 1989
and to be installed to AS 2220 part 2: 1989
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Reliability
The
reliability of fire alarm systems is required to be much higher than
some other building systems, due to the reliance placed on active
fire protection systems for life safety. This is a fundamental
consideration underpinning the NZ building Code and the associated
Approved Documents. Designs therefore tend to be conservative,
based on risk and experience.
Mains
power supplies are expected to fail under fire conditions, so
battery back-up is expected. Batteries are known to fail, so are
tested almost continuously both for continued presence and
capacity. Circuits are automatically supervices for fault
conditions, producing fault signals in case of failure. Sometimes
wiring circuits are required to be installed with redundant paths.
Because reliability is so important, monthly testing and annual
surveying of fire detection and alarm systems by a qualified trade
practitioner is mandatory. Fire detection systems are invariably
listed on a building’s compliance schedule, and evidence of
satisfactory test and survey must be submitted to the Territorial
Local Authority before the building’s annual warrant of fitness (for
continued occupation) can be issued.
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Analogue Addressable
Systems
“Conventional“fire detection and alarm systems are hard-wired to
each group of detection devices. The control and indicating
equipment is unable to distinguish alarm and fault conditions from
individual devices within the group and the actual alarm decision is
made at each device.
“Analogue Addressable” systems provide knowledge of the exact
location and status of every device at the control and indicating
equipment. Furthermore, the alarm decision is generally made by the
control equipment, rather than the detection device itself.
In
general, Analogue Addressable systems offer the following distinct
advantages:
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Pre-alarm Indication
– Incidents can be investigated, and possibly resolved, before
the Fire Service is called.
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Maintenance Alert
–
dirty detectors can increase false alarms, but unnecessary
cleaning wastes money. Maintenance alert facilities show which
detectors need cleaning.
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Individual Detector Identification
–
each detector is uniquely identified. A full description of a
detector’s precise location is displayed at the fire control
panel, or on remote annunciators. This speeds the location of a
fire or a fault.
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Wiring Faults
–
cut wires or short circuits on wiring do not generate false
alarms, and are easier to locate.
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Adjustable Sensitivity
–
each detector can be tailored to its environment to give optimum
sensitivity to fire phenomena and resistance to environmental
influences or permitted activities (e.g. food preparation).
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Multi-Sensor
Detectors – multi-sensor/multi-criteria detectors use
sophisticated algorithms to make them less likely to generate a
false alarm, but more likely to respond promptly to a real fire.
Overall, these technical advantages increase system performance and
reduce unwanted building evacuations and calls to the Fire Service.
Although analogue addressable systems can cost a little more
initially, this can be recouped in reduced maintenance and false
alarm costs, while offering the benefits of superior performance.
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False Alarms
Technology is improving all the time; however the false alarm rate
from fire detection and alarm systems remains fairly constant – NZ
Fire Service figures indicate that around 96% of such calls are
false alarms. With more and more systems being installed, the
call-out rate is increasing.
False
alarms disrupt – business, staff, and customers. Repeated false
alarms lead to complacency, and alarms being ignored. The Fire
Service also levies callout charges for systems which repeatedly
false alarm.
Correct detector selection and placement is of utmost importance in
reducing false alarms. Other important measures are the control of
building work, good building maintenance, regular detection system
maintenance, and premises security.
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Domestic Smoke Alarms
Smoke Alarms are primarily used in domestic residential situations.
They are different from the Smoke Detectors used in Fire Detection
and Alarm Systems because they have a built-in alerting device, and
are designed, tested, and manufactured to different standards. The
most common examples are the battery-operated units available in
many hardware stores.
Early warning of the presence of smoke in a building is critical to
life safety. Some fires can grow rapidly making escape very
difficult, especially if the occupants are asleep, and/or the escape
routes are smoke-logged. The time difference between escaping from a
building or dying in a fire can often be measured in seconds.
Sometimes fires can smoulder for hours, filling the premises with
toxic fumes. Without the early warning provided by a smoke alarm,
occupants can perish as they sleep without ever waking.
“Type 1” Smoke Alarms are mandatory under the New Zealand Building
Code Approved Documents for new installations in domestic
residential situations, including detached dwellings. Often
stand-alone battery operated units are sufficient, however for some
larger multi-level dwellings interconnected mains-powered units may
be required to meet sounder audibility requirements.
Because they have none of the inherent self-supervision or
maintenance regimes of commercial Fire Detection and Alarm Systems,
and they are not listed on a building’s compliance schedule for
occupancy warrant of fitness, it is imperative that dwelling owners
conduct their own regular maintenance and testing of their smoke
alarms.
These procedures include: annual cleaning with a vacuum cleaner (no
disassembly), monthly testing with the alarm’s “test” button, and
regular battery replacement (interval depends on type – annual for
ordinary dry-cell batteries).
A New Zealand Standard NZS 4514:2002 Interconnected Smoke Alarms
for Single Household Units exists, however it is not mandatory.
Under the current “Type 1” Approved Document regime, the
requirements are contained in section F7/AS1, and installation is
required to be to AS 1670.6 and the manufacturer’s instructions.
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Ionisation
or Photoelectric?
Commercially available domestic smoke alarms are usually either
ionisation chamber or photoelectric (light scattering) types.
Considerable debate has arisen recently in the popular press as to
which technology gives better performance, especially considering
the price difference between the two nowadays is minimal
(photoelectric used to be considerably more expensive and drained
batteries much faster).
Ionisation smoke alarms respond well to fast flaming fires, but can
give much slower response to smouldering fires. Escape routes can
therefore become more smoke-logged before a warning is given.
Ionisation detectors are also prone to nuisance alarms from cooking
activities, so must be sited well away from kitchens. They also
contain a (very small) radioactive source, which in some countries
presents environmental issues (not in New Zealand, where, under
current legislation, they are permitted to be disposed of in the
landfill with normal household rubbish).
Photoelectric smoke alarms respond to visible smoke, so can give a
slower response to fast flaming fires with invisible products of
combustion. They are also prone to nuisance alarms from steam, so
must be sited well away from bathrooms and saunas.
On balance, photoelectric is the preferred technology for new
installations as it offers more consistent detection performance
across all the likely fire scenarios in a residential environment
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