Emergency Lighting Introduction This handout describes the following fire alarm and emergency lighting systems: Emergency lighting Maintained emergency lighting Non-maintained emergency lighting Segregation Fire alarm systems Emergency lighting Emergency lighting should be planned, installed and maintained to the highest standards of reliability and integrity, so that it will operate satisfactorily when called into action. Emergency lighting is not required in private homes because the occupants are familiar with their surroundings, but in public buildings people are in unfamiliar surroundings and in an emergency people will require a well illuminated and easily identified exit route. This would help to prevent the panic that is caused in emergency situations. Emergency lighting is provided for two reasons: - To illuminate escape routes called ‘escape’ lighting Lighting to enable a process or activity to continue after a normal light failure called ‘standby’ lighting. Local and national statutory authorities under legislative powers usually require escape lighting. Escape lighting schemes should be planned so that identifiable features and obstructions are visible in the lower levels of illumination, which may prevail during an emergency. This lighting provides lighting for escape routes from a point within a building to a final exit. The level of illumination required is 0.2 lux, which is similar to the brightness of the full moon. Standby lighting is required in, for example, hospital operating theatres and in industry, where an operation or process once started must continue, even if the mains lighting fails. The cash points in commercial buildings may need to be illuminated at all times to discourage acts of theft occurring during a mains failure. Maintained emergency lighting In this type of system the luminaire is supplied with a single light source which may be switched on and off as required. The emergency lamps are continuously lit using the normal supply when it is available, and change over to an alternative supply when the mains supply fails. The advantage of this system is that the lamps are continuously proven healthy and any failure is immediately obvious. It is a wise precaution to fit a supervisory buzzer and indicator lamp in the emergency supply to prevent accidental discharge of the batteries, since it is not otherwise obvious which supply is being used. Maintained emergency lighting is normally installed in theatres, cinemas, discotheques and places of entertainment where the normal lighting may be dimmed or extinguished whilst the building is occupied. The emergency lamps are wired in parallel from a low voltage supply. Non-maintained emergency lighting In this type of circuit the emergency lamps are only illuminated if the normal mains supply fails. Failure of the main supply de-energises a solenoid and a relay connects the emergency lamps to a battery supply, which is maintained in a state of readiness by a trickle charger from the normal main supply. When the normal supply is restored, the relay solenoid is energised, breaking the relay contacts, which disconnects the emergency lamps, and the charger re-charges the battery. The disadvantage with this type of installation is that broken lamps are not detected until they are called into operation in an emergency, unless they are regularly maintained. A battery contained within the luminaire, together with a charger usually provides the emergency supply and relay making the unit self contained. Segregation The wiring to self-contained emergency lighting luminaires is not considered part of the emergency lighting installation and no special segregation is necessary. Otherwise emergency lighting circuits must be separated from other cables and from each other in compliance with BS 5266. See table 7.4 below. Table 7.4 Segregation requirements of fire alarm and emergency lighting standards. Fire alarms BS 5839 Emergency lighting BS 5266 Physical segregation of at least Physical segregation or use of mineral 300mm, or physical segregation, or cables, or cables to BS 6387 Cat B Use of mineral cables, or Cables to BS Cat AWX or SWX and Incorporating an earthed metallic screen and an overall insulating sheath. Fire alarm systems A correctly installed fire alarm system installation is of paramount importance and can be compared to any other electrical undertaking. Life could be lost and property damaged resulting from carelessly or incorrectly connected fire detection and alarm equipment. It is essential that the installation be carried out complying with the requirements of BS5839 Part 1, BS 7671 and manufacturer instructions. Fire alarm circuits must be segregated in accordance with table 7.4. In order to comply with BS 7671 a dedicated circuit must be installed to supply main power to the fire alarm control panel. In practice, wiring would be terminated in an unswitched fuse connection unit to BS 1362; the size of fuse would depend on the manufacturer’s recommendation. Fire alarm systems can be designed and installed for one of two reasons. For life protection Property protection. Property protection A satisfactory fire alarm system for the protection of property will automatically detect a fire at an early stage, indicate its location and raise an effective alarm in time to summon the fire fighting forces (both the resident staff and the fire brigade). The general attendance time of the brigade should be less than 10 minutes. Therefore an automatic direct link to the fire brigade is essential. Protection for property is classed as: - P1 All areas of the building must be covered with detectors P2 Defined areas only require cover by a detector. A fire resisting construction should separate unprotected areas. Life Protection Here a satisfactory fire alarm system for the protection of life can be relied upon to sound a fire alarm while sufficient time remains for the occupants to escape. Life protection is classed as: - L1 Same as P1 L2 Specified areas where a fire could lead to a high risk to life e.g. sleeping areas, store rooms, kitchens, plant rooms, and places where the occupants are especially vulnerable owing to age, illness or are unfamiliar with the building. L3 Protection of escape routes. o Corridors, passageways and circulation areas. o In rooms opening onto escape routes o Top of stairs o On landing ceilings o Top of vertical risers such as lift shafts o At each level within 1.5m of access to lift shafts or other vertical risers. Zones To ensure a fast and unambiguous identification of the source of fire, the protected area should be divided into zones. The following guidelines indicate the size of zone that should be used. If the floor area of each building is not greater than 300m² then the building only needs one zone, no matter how many floors it has. This covers most domestic installations. The total floor area for one zone should not exceed 2000m². The search distance should not exceed 30m. This means the distance that has to be traveled by a searcher inside a zone to determine visually the position of a fire should not exceed 30m. The use of remote indicator lamps outside doors may reduce the number of zones required. Where stairwells or similar structures extend beyond one floor but are in one fire compartment, the stair well should be a separate zone. If the zone covers more than one fire compartment then the zone boundaries should follow compartment boundaries If the building is split into several occupancies, no zone should be split between two occupancies. Note: a fire compartment is an area bordered by a fire resisting structure usually at least 30 minutes resistance. Break glass and manual call points The break glass call point is a device to enable personnel to raise the alarm in the event of a fire, by simply breaking a fragile glass served with a thin plastic membrane to protect the operative from injury sustained by broken or splintered glass. A sturdy thumb pressure is all that is required to rupture the glass and trigger the alarm sequence. The following gives correct siting and positioning of break glass call points: They should be located on exit routes and in particular on the floor landings of staircases and at all exits to the open air. They should be located so that no person need travel more than 30m from any position within the premises in order to raise the alarm Generally, call points should be fixed at a height of 1.4m above the floor, at easily accessible well-illuminated and conspicuous positions free from obstruction. The method of operation of all call points in an installation should be identical unless there is a special reason for differentiation. Manual and automatic devices may be installed on the same system although it may be advisable to install the manual call points on separate zones for speed of identification. Automatic detectors When choosing the type of detector to be used in a particular area it is important to remember that the detector has to discriminate between fire and the normal environment existing within the building, i.e. smoking in hotel bedrooms, fumes from forklift trucks in warehouses, steam from kitchens and bathrooms. There are several automatic detectors available detailed as follows overleaf; Heat detectors (fixed temperature type) The fixed temperature heat detector is a simple device designed to activate the alarm circuit once a predetermined temperature is reached. Usually a choice of two operational temperatures is available 60° C and 90° C. this type of detector is very suitable to monitor boiler rooms or kitchens where fluctuations in ambient temperature are commonplace. Heat detector (rate of rise type) This type of detector responds to rapid rises of temperature by sampling the temperature difference between two heat-sensitive thermocouples or thermisters mounted in a single housing. (A thermistor is a device whose resistance quickly decreases with an increase in temperature). It is wise not to install this type of unit in a boiler room or kitchen where fluctuations in ambient temperature occur regularly. This will help to avoid nuisance alarms. Smoke detector The optical smoke detector, sometimes known as the photoelectric smoke detector, operates by means of the light-scattering principle. A pulsed infrared light is targeted at a photo-receiver but separated by an angled non-reflective baffle positioned across the inner chamber. When smoke and combustion particles enter the chamber, light is scattered and reflected onto the sensitive photo receiver and triggers the alarm. Detector heads for fire alarm systems should only be fitted after all trades have completed work that could create dust. Alarm sounders Normally a bell or electronic sounder, which must be audible throughout the building in order to alert (and/or evacuate) the occupants of the building. The following gives guidance for the correct use of alarm sounders. A minimum level of either 65dBA or 5dBA above any background noise likely to persist for a period longer that 30s, whichever is greater should be produced by the sounders at any occupiable point in the building. If the alarm system is to be used in premises such as hotels, boarding houses etc, where it is required to wake sleeping persons then the sound level should be 75dBA minimum at the bedhead. All audible warning devices used in the same system should have a similar sound A large number of quieter sounders rather than a few very loud sounders may be preferable. At least one sounder will be required per fire compartment The level of sound should not be so high as to cause permanent damage to hearing. Wiring systems for fire alarms Fire detectors and sounders must be wired in a continuous parallel formation arrangement as shown in diagram 3. No spurs are permitted, as this would prove difficult to monitor for breaks and short circuits, which might occur. Circuits from the control panel should be as short as possible as many manufacturers require a maximum of 100 ohms per fire detection loop, as shown in diagram 4. In practice this will not cause undue problems as the nominal impedance of a single loop of 1.5mm² two core cable lies between 19 and 22 ohms per kilometre. In order to continually monitor both cables and terminations for open and short circuit conditions an end of line resistor, typically valued between 20 and 20000 ohms should be connected across the terminals of the last sounder Cables must be segregated in accordance with table 7.4. However segregation is not necessary when the fire alarm system is wired in MI cables or FP200 cable manufactured to BS 7629. It is wise to leave the wiring of the fire alarm system until most of the constructional work has been completed. This will help avoid accidental damage occurring to the cables. Keep the control panel in the packing carton and only remove when building work has been completed in the area where it is to be mounted, thus avoiding possible contamination to the unit.
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