Fire Suppression design

Designing the fire detection, for a fire suppression system is similar to what would be required for a standard building fire alarm system.
When designing a fire suppression system, the type of detector to use used is as follows:
The selection of detectors of the fire alarm panel should be in accordance with the recommendations given in BS 5839-1 and, where applicable, BS 6266.
In some circumstances, fire detection considerations might dictate the
need for use of two different principles of detection (e.g. optical smoke
detectors and ionization chamber smoke detectors) to ensure the
earliest warning of fire and give the fire suppression system the maximum amount of time to extinguish the fire. In such circumstances, an even distribution of each type of detector should be provided throughout the protected
space. Where coincidence is used, normally it should be possible to
achieve coincidence from two detectors of the same operating
principle. In these cases, if, for example, two independent circuits are
used to achieve coincidence, there should normally be an approximately equal number of detectors of each principle connected to each of the independent circuits. For example, where four detectors are required to protect the space and these comprise two optical smoke detectors and two ionization chamber smoke detectors, there should be one optical smoke detector and one ionization chamber smoke detector on each circuit.
However, it is not always necessary to use two different principles of
fire detection. For example, given the type of fire anticipated and the
speed of detection required, it might be acceptable to use detectors of a single type.
See a video of a Fire suppression system in action.

Fire alarm design - Zones

Fire alarm design should take into account the recommendations applicable to fire detection zones that contain non-addressable automatic fire detectors. The following recommendations are applicable.
a) The floor area of a single fire alarm zone should not exceed 2 000 m2.
b) The search distance (the distance that has to be travelled by anyone responding to a fire alarm signal after entry to the zone in order for the location of the fire to be determined visually,should not
exceed 60 m.
Having entered the zone, the person responding to the alarm signal need not reach the seat of the fire within the recommended search distance; it is only necessary to become aware of the location of the fire.
In measuring search distance, the worst case, in which the fire is located at the furthest concealed area from any likely
point of entry to the zone, should be considered.
Automatic fire detectors within any enclosed stairwell, liftwell or other enclosed flue-like structure should be considered as a separate detection zone. Manual call points located at final exits to open air
from stairwells may be incorporated within these detection zones ,but may alternatively, be incorporated within the detection zone.

Fire Alarm Zones

In many buildings, the evacuation strategy will be very simple; on operation of any fire alarm manual call point, or detection of fire by  automatic fire detection, fire alarm sounders will operate throughout the building to indicate the need for evacuation of the entire building.
In larger, more complex buildings, the fire “Evacuate” signal may, in the first instance, be restricted in extent (e.g. to a single floor, a limited number of floors or a limited area of the building). In other areas, an fire “Alert” signal may be given to warn occupants of the fire alarm signal, without any requirement for those occupants to
evacuate.

In order to support such arrangements, it is necessary for the building to be divided into a number of individual alarm zones and for the installation of an addressable fire alarm system.  The operating state of fire alarm sounders in any alarm zone is independent of the operating state of fire alarm sounders in all other alarm zones. The fire alarm sounders control by the addressable fire alarm are thus grouped, so that, at any point in time, all alarm sounders within any given alarm
zone are in the same state (i.e. silent, giving an “Alert” signal or giving an “Evacuate” signal).
The need for sub-division of a building into discrete alarm zones may arise for a number of reasons,
including:
1.the use of phased evacuation.
2. to avoid unnecessary disruption when false alarms occur.
Any other circumstances in which a two stage alarm arrangement applies.
Since it will be the case that, when occupants of one alarm zone are required to evacuate, occupants of one or more alarm zones above, below or adjacent to that zone will be expected to remain in the building, every alarm zone needs to be separated from all other alarm zones by fire resisting construction. (In some complex
public buildings, such as shopping centres, alarm zones are designed to coincide with smoke control zones,
which are separated from each other by smoke curtains, rather than fire resisting construction. However, in this case, the audible signal is normally given by a voice message, rather than audible fire alarm devices.)
As the sub-division of a building into alarm zones implies that, in the early stages of a fire, occupants in
certain areas will not be expected to evacuate, the configuration of alarm zones might require approval by
the authority responsible for the enforcement of fire safety legislation in the building.

Fire alarm control panel – Fire Systems Ltd

Below is information for convention fire alarm panels used by Fire Systems Ltd.
Detection zone wiring
The detection zones from our convention fire alarm panel, provide a nominal 24V DC to power the conventional fire detection. The fire alarm wiring is monitored for open and short circuit fault conditions by removing the 6K8 end of line monitoring resistors that are supplied fitted to the fire alarm control panels’ terminals and placing them across the last device that is wired to the zone circuit.
Fire detection zone circuits must be wired as a single, radial circuit with no spurs or T junctions to enable the monitoring circuit to work correctly.
On T series fire alarm panels, polarised sounders must also be wired across the detection zone but in reverse polarity to that shown by the zone terminals. At least one sounder should be fitted to each fire detection zone. Each zone can be configured individually as a 2-wire type zone or a non 2-wire type zone using configuration options C1 to C8.
For non 2-wire fire alarm system that are required to comply with BS5839 Part 1: 2002 detector removal requirements, detector bases fitted with a Schottky diode should be used and the end of line resistor replaced with an LCMU active end of line monitoring device.

Radio linked fire alarm systems

Some of the recommendations, applicable to wired fire alarm systems, are unsuitable for, or cannot be applied to, radio fire alarm systems. These include, in particular, those relating to power supplies and fault monitoring.
Additional recommendations apply to radio-linked fire systems in order to address the integrity and performance of the radio communications link between components and the fire alarm control and indicating equipment.
In practice, no fire systems can have total reliability, but one of the objectives of good fire system design is to reduce the probability of the fire alarm system being inoperative in the area of a fire when the fire starts. It is useful that the advantages and disadvantages between the two technologies, hardwired and wireless fire alarms and radio linked systems, are different because this allows for a system to be installed that suits the site. Great care needs to be taken when assessing a site and choosing the technology to use, as neither the wireless fire alarm nor hardwired system is singularly suitable
for every site.
Components of a fire system interconnected by radio-links may include fire alarm devices, such as fire alarm
sounders, as well as detectors and manual call points. However, exercising the fire system with testing and evacuation drills in excess of those recommended by the manufacturer can reduce the life of the radio-linked fire alarm sounder batteries. Automatic silencing of radio-linked sounders on wireless fire alarm systems, is sometimes employed to overcome the potential for undue discharge of sounder batteries. However, sounders with this facility need to reactivate with any new fire or phased evacuation command. Such a facility ought not to impair the system’s ability to provide an adequate audible warning to the occupants of the building in the event of fire.
Other components of a radio fire alarm system may include radio relay units, which are installed remote from
the control and indicating equipment so as to extend the range of the system. Where such relay units are separate control and indicating panels, this is a form of networked system .
Radio communications may also be used to link a small number of detectors and other components to what is essentially a wired system. The recommendations of this standard apply equally to integral radio-linked systems of this nature.

Wireless fire alarm systems

Radio fire alarms, or also refer to as wireless fire alarms are becoming more popular as the technology improves.
A Radio fire alarm/Wireless fire alarm should comply with all recommendations BS EN 54-25.
In accordance with BS5839-1, clause 27 All fire alarm radio-linked components should be supplied from at least two independent power supplies. These can be either:
i) the normal mains supply plus a reserve battery (primary or continuously charged secondary); or
ii) a primary battery plus a second primary battery; or
iii) a primary battery plus a secondary battery.
Components, other than fire alarm control and indicating equipment, may utilize batteries to provide the normal power supply;
Wireless fire alarm/Radio fire alarm, that use power supplies incorporating one or more primary batteries should give 30 days warning of impending battery failure". This should be indicated as a low battery warning condition at the control and indicating equipment;
At the point at which the power supply(ies) to any radio-linked component can maintain the component in normal operation for no more than seven days, and, in addition, in the case of fire alarm
devices, 30 min in the alarm condition, a fault warning should be given at the control and indicating equipment;

Fire alarm spacing and siting of fire detectors

When designing a fire alarm, the heat and smoke detectors installed, depend on convection to transport hot gases and smoke from the fire to the detector.
When designing fire alarms, spacing and siting of these detectors needs to be based on the need to restrict the time taken for this
movement and to ensure that the products of combustion reach the fire detection in adequate concentration. In a building, the hottest gas and the greatest concentration of smoke will generally form at the highest parts of the enclosed areas, and it is here, therefore, that the fire detection need to be sited.
As the smoke and hot gases from a fire rise, they become diluted with clean, cool air, which is drawn into the plume. Hence, the size of fire required in order to operate heat or smoke detectors increases rapidly as the height of the ceiling above the fire increases. To some extent, this effect can be countered by the use of more sensitive detectors.
fire alarm system, incorporating optical beam detectors are less sensitive to the effects of ceiling height than are point-type detectors, since the increased size of plume will involve a greater proportion of the path length of the optical beam and help to alleviate the effects of reduced smoke density. In addition, the entrainment of air cools the gases. If the ceiling is high and the ambient temperature in the
uppermost areas within the protected space is high, the plume of smoke and hot gases may reach ambient temperature before reaching the ceiling. If the temperature of the surrounding air increases with height, (e.g. as a result of solar gain), it is possible for the air at the uppermost levels to be at a higher temperature than that of the plume. The plume will then spread out to form a smoke layer before it reaches the ceiling, as though there were an “invisible ceiling” at a specific height within the protected space. This is known as
stratification, and, at this stage of the fire growth, the smoke and hot gases will not operate ceiling mounted detectors, regardless of their sensitivity.

Fire alarm manual call points

Fire alarm manual call points need to be prominently sited, readily distinguishable from non-fire alarm call points and need to be distributed such that, from any point in the building, it is impossible to leave the storey or the building without passing a fire alarm manual call point.
When designing a fire alarm system, sufficient call points need to be provided to minimize, to a reasonable extent, the delay between discovery of a fire and the sounding of the fire alarm system. Where the fire hazard level is high and rapid fire development is
anticipated, this delay needs to be commensurately shorter.
Manual call points can, if present in unsupervised areas, be subject to malicious operation. For this reason,they are not normally provided in, for example, public common areas of shopping complexes and certain
public houses. In public car parks within a building, use of an emergency voice communication system could be considered instead of manual call points.

Fire alarm installation practices and workmanship

The nature and quality of the fire alarm installation work needs to be such as to maintain the integrity of the fire alarm system and minimize the duration and extent of disablement of the system during maintenance or modifications. Installation practices and workmanship need to conform to the requirements of BS 7671.
Penetration of construction (e.g. for the passage of cables, conduit, trunking or tray) need to be “fire stopped” to avoid the free passage of fire or smoke, regardless of whether the construction has a recognized degree of fire resistance.
Recommendations:
The following recommendations are applicable.
1) The entire electrical fire alarm system installation should conform to the requirements of BS 7671. In general, the recommendations of this standard supplement, but do not conflict with, these requirements. Where any such conflict is considered to exist, the recommendations of this standard should take precedence.
2) Cables which are directly fixed to surfaces should be neatly run and securely fixed at suitable intervals, in accordance with the recommendations of the cable manufacturer. Cables should not rely on suspended ceilings for their support.
3) The installer should ensure that all wiring complies with, at least, 26.2f) to 26.2o).
d) Joints in fire alarm cables, other than those contained within the enclosures of equipment, should be avoided
wherever practicable [see 26.2g)].
4) So far as is reasonably practicable, cable routes should be selected to follow the recommendations .
Care should be taken to ensure the electrical continuity of electromagnetic screens, including metallic
sheaths of cables.
5) Where new conduit, trunking or tray is installed, its capacity should be in accordance with the
recommendations given in BS 7671.
6) Where a cable passes through an external wall, it should be contained in a smooth-bore sleeve of metal
or other non-hygroscopic material sealed into the wall. This sleeve should slope downwards towards the outside and should be plugged with a suitable non-hardening waterproof compound to prevent the entry of rain, dust or vermin.
7) Where a fire alarm cable passes through an internal wall, a small clearance hole should be provided. If additional mechanical protection is necessary, a smooth-bore sleeve should be sealed into the wall.
8) Care should be taken to ensure that the ends of any sleeves are free from sharp edges which might
damage cables during installation.
9) When a cable passes through a floor, the considerations a
sleeve should extend as far above floor level as is required for protection of the cable it is to carry, but never less than 300 mm.
10) Where cables, conduits, trunking or tray pass through floors, walls, partitions or ceilings, the surrounding hole should be as small as reasonably practicable and made good with fire stopping
materials that ensure that the fire resistance of the construction is not materially reduced. Spaces through which fire or smoke could spread should not be left around the cable, conduit, trunking or tray.
11) If cables or conduits are installed in channels, ducts, trunking or shafts that pass through floors, walls, partitions or ceilings, barriers with the appropriate level of fire resistance should be provided
within the channels, etc. to prevent the spread of fire unless, in the case of ducts and shafts, the construction of the duct or shaft affords equivalent fire resistance to the structure penetrated; in the
latter case fire stopping need only be provided where cables pass into, or out of, the duct or shaft.

Grade of fire alarm system

Fire alarm System Grades relates to the engineering aspects of the fire detection and fire alarm system. Higher Grades of fire system design, tend to provide a greater level of control and monitoring of the system, or greater reliability and availability to perform correctly in the event of fire. The Grade of fire alarm system that needs to be installed depends on the nature of the property, the level of fire risk and the characteristics of the occupants.
Grade F -  fire alarm systems, comprising one or more battery-operated smoke alarms, are the least reliable in the long
term because of the need for battery replacement. For new dwellings, a Grade D system (in which the normal power supply for each smoke alarm is derived from the mains electricity supply and a standby supply is provided) or higher is justified.
However, because of their low cost and ease of installation, Grade F systems (comprising battery-powered
smoke alarms) may be considered for installation in existing dwellings. Their reliability can be improved by use of batteries that have a long anticipated life and that cannot be removed without the use of a tool.
Nevertheless, Grade F fire detection systems are not appropriate in dwellings in which the fire risk to occupants is high
nor where there cannot be a reasonable certainty that, when the dwelling is occupied, batteries will be replaced within a short time (typically no more than five days) of a battery fault warning indication.
Otherwise, a system in which the normal supply is derived from the mains needs to be used (e.g. a Grade D
system).
If, in an existing dwelling, a Grade F fire alarm system would be acceptable, it is acceptable to install a Grade E system
(comprising mains powered smoke alarms). However, if there are likely to be periodic interruptions to the
mains supply, whether due to the inability of the occupier to pay for supplies or due to unreliability of the
mains electricity supply, a Grade D system or higher (in which there is a standby supply) is then necessary.

Where there is a need for readily accessible control of the fire detection and fire alarm system, a Grade C
system or higher might be provided. Where the fire risk calls for a high standard of system monitoring and
availability, it might be appropriate to install a Grade B system. If the dwelling is very large, or is subdivided into a significant number of dwelling units, a Grade A system is likely to be appropriate.
If the purpose of the system is property protection, fire insurers might require a Grade A system. However, for smaller properties, a Grade B or Grade C system might be sufficient.