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Cable Fires in Hidden Voids [Hardfacts]


The continuing rapid growth in the use of information technology and the necessity to regularly upgrade existing systems has resulted in a corresponding increase in the amount of cabling found in buildings. Consequently the fire loading of a building can be substantially increased by the sheer volume of cabling used, particularly as new cables are more often than not laid alongside old ones.

It is common practice to make use of concealed spaces to carry cables resulting in the potential for rapid, undetected spread of fire and smoke throughout a building.

Fire brigades have reported that, where cables have been involved in fires, greatly increased amounts of smoke and toxic gases have been released. These impair vision making escape more difficult and can cause asphyxiation through smoke inhalation.

The effect on property of a cable fire is likely to be damage to computer systems and communication and power equipment, i.e. facilities which are vital for the day to day operation of most businesses. Even a small localised fire has the potential for serious business interruption, as repairs to cables may not be practical or effective, resulting in the replacement of an entire cable system.

Specifiers and building occupiers need to be aware of the hazards and the appropriate measures which are presently available to minimise the risk.

The Problems Introduced by Cables and Hidden Voids

  • Communication and power cables are commonly laid in the hidden voids in a building. They can be found in under floor spaces, ceiling spaces and vertical risers. The voids often form part of the normal heating, ventilation and air conditioning system within a building, introducing the potential to spread heat, fire and dense toxic and corrosive fumes throughout the building.
  • It is common practice to install new cables alongside existing ones, as it is not cost effective to remove the older cables. This can result in the build up of a particularly large volume of cables of different ages and materials.
  • The voids carrying cables are frequently large and without compartmentation. Even where cavity barriers are installed these have often been breached to allow the passage of new cables or other services and are then imperfectly resealed.
  • Deep cavities or voids have more oxygen readily available to support combustion and are able to carry a greater volume of cabling, resulting in a corresponding increase in the fire load.
  • Many of the halogenated polymers used for cable insulation are inherently flame retardant. However the addition of plasticisers to give cables flexibility also considerably increases their flammability.
  • Commonly used Low Smoke Zero Halogen cables have been found to burn intensely along their whole length, spreading fire rapidly and producing significant amounts of smoke.

Fire Protection Measures

Section 17 of the FPA Design Guide core document ‘Protection of Openings and Service Penetrations from Fire' recognises the potential fire risk in hidden voids. The basic principle in the design of any building is that any material used in its construction should not make a significant contribution to a fire. Consequently the type of cables used should be specially formulated to resist fire or should be protected from the effects of fire. A flexible approach to the protection of cables against fire allows for one or more of the following alternatives to be used:

  • The use of fire resistant cables tested and approved by the Loss Prevention Certification Board (LPCB) to UL 910 will ensure that they have passed the most stringent of large scale fire tests currently available
  • The provision of adequately designed, installed and maintained cavity barriers at not more than 20 metre centres in any direction and with a minimum fire resistance of 30 minutes in respect of integrity and insulation
  • Ensuring that cables in voids are adequately protected by providing the void with a fire resistance of not less than 30 minutes in respect of integrity and insulation. However, for ceilings a fire resistance of 60 minutes integrity and insulation can be recommended. Some non-combustible materials may not necessarily provide the required fire resistance and otherwise fire-resisting ceilings may fail because of light fittings or other features.
  • The protection of cavities with an automatic extinguishing system
  • The installation of an automatic fire detection system with an ability to detect the exact location of a fire

Key Action Steps

  • Be aware of the increased fire load that cables can introduce into a building and assess the extent of the problem in your premises
  • Consider the removal of obsolete cables
  • Provide separate routes for power and communication cables
  • Use LPCB approved cables
  • Provide (minimum 30 minute) fire resistant voids for cables
  • Ensure hatches / doors in to the voids have a similar fire resistance as the void
  • Provide cavity barriers and ensure that their integrity is maintained
  • Consider providing automatic fire detection and/or extinguishment in cable voids, particularly for high risk areas such as computer suites and control rooms

Reference Documents

Loss Prevention Certification Board List of Approved Fire and Security Products and Services at

FPA Design Guide Core Document ‘Protection of Openings and Service Penetrations from Fire' available at

The Association of British Insurers guidance note ‘Fire Hazard of Communication Cables in Ceiling, Floor and Vertical Voids'.

Next Steps:

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Please Note
This document contains general information and guidance and is not and should not be relied on as specific advice. The document may not cover every risk, exposure or hazard that may arise and Aviva recommend that you obtain specific advice relevant to the circumstances. AVIVA accepts no responsibility or liability towards any person who may rely upon this document.

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