Emergency Lighting by 8IFfEZ

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									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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