Handbook of Electrical Installation Practice - BeKnowledge

500 Handbook of Electrical Installation Practice
But what do we mean by ‘emergency lighting’? There are two types of emergency
lighting: escape lighting and standby lighting. Escape lighting is provided to ensure
the safe and effective evacuation of the building and has special requirements.
Standby lighting is provided to enable some activities to continue for a period
after the mains lighting fails, for example in a hospital operating theatre. Standby
lighting can be regarded as a special form of conventional lighting and dealt with
accordingly.
There are two main types of system available for powering emergency lighting:
self-contained or central. For the former, in the event of an emergency the luminaires
operate from their own batteries. Each luminaire is fully equipped with
battery, charger, charge indicator and changeover device. This can be within the
luminaire of in a separate unit less than one metre from the luminaire. The normal
unswitched lighting circuit, which charges the batteries and provides mains failure
detection, is the only extra connection required for a self-contained emergency luminaire.
With central systems, the power is provided by batteries or generators and
the output is distributed through sub-circuits to feed a number of slave luminaires.
These systems require comparatively large battery/generator rooms. Sub-circuit
monitoring is used to protect against local failure, and high integrity wiring must be
used between power source and luminaire. Due allowance must be made for voltage
drops.
Types of emergency lighting luminaire are classified according to modes of operation
and duration. The battery is always being charged when the mains supply is
present.
Non-maintained In this mode the lamp(s) is only lit when the mains fails and is
operated by an emergency power source.
Maintained In this mode the lamp(s) is lit at all times. It is powered by the mains
supply under normal conditions and under emergency conditions operates
from an emergency power source.
Combined/sustained This is a variant of the maintained luminaire in which one
lamp is powered by the mains supply during normal conditions. A second lamp
only operates under emergency conditions or by an emergency power source.
This type of luminaire provides light at all times.
Duration is the period that the emergency luminaire remains lit to the minimum
design output, after a mains failure. Various premises require different durations, as
set out in BS 5266, from 1–3h.
The European committee CENELEC has published a common European standard,
entitled ‘Lighting applications – Emergency lighting’ and dual numbered as
BS 5266 Pt 7: 1999/EN 1838: 1999. Major changes are as follows.
Emergency escape route illumination
On the whole there are two types of routes to be considered for the purposes of
escape lighting requirements:
(1) Clearly defined routes – corridors and gangways. Here the lighting should
ensure that the horizontal illuminace at the floor on the centre line of the escape