In the majority of tunnels, there is a lighting system that provides a lighting level in accordance with national and international road tunnel lighting standards for ‘normal’ tunnel operation. The lighting system will also provide additional ‘emergency’ lighting in the tunnel during periods where the mains power supply has failed for a limited period. The emergency lighting (approximately 10% of the ‘normal’ lighting level) will be powered by an Uninterruptible Power Supply (UPS). The UPS is a battery backed system.
There is an additional set of lighting installed in tunnels that is used for evacuation purposes. This lighting will not normally be on unless there is a declared emergency in the tunnel, and usually when vehicle users are being instructed to leave their vehicles and evacuate the tunnel. The evacuation lighting should be installed at a low level (around 1.2m to 1.5m above the walkway). Light fittings should point down onto the walkway and be installed to illuminate the evacuation route for pedestrians to follow in smoky conditions (Fig. 1). Light fittings are usually installed every 10 to 20m. This evacuation lighting will be powered by UPS.