In the event of a fire it is vital to the safety of the tunnel occupants that certain electrical systems remain functioning until people have escaped. Such systems will therefore require to be protected from fire for a specified period of time and include:

• Lighting for means of egress (emergency escape route lighting) and areas of refuge
• Exit signs
• Communications
• Electrically operated extinguishing systems
• Electrically operated fire alarms
• Ventilation systems (smoke extraction)
• Tunnel drainage and fire pumps

In addition to protection from fire outside the duct, it is normally vital that any fire within the duct is contained e.g. if cable sheathing ignites due to an electrical overload. A suitably designed duct will:

• Prevent the propagation of fire from one compartment to another
• Assist in maintaining escape routes
• Ensure the continuing operation of services
• Reduce damage to localised areas
• Contain smoke and toxic fumes from burning cables if the fire was caused within the cable enclosure