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Fire Safety for Facilities Management Personnel – Fire Prevention – Part 97

May 10, 2021 10:56 am

Lawrence Webster Forrest (LWF) is a specialist fire engineering and fire risk management consultancy whose aim is to give information on best practice in fire safety for facilities management personnel through this blog series. In part 96, LWF discussed electrical faults and their role in causing fire. In part 97, we will consider the reasons electrical fires can start and continue to discuss the Wiring Regulations, as per CIBSE Guide E.

The primary causes of electrical fires are as follows:

  • Cables being overloaded by currents they were not designed to carry. The cables overheat causing the life of the cable insulation to be shortened. This situation can be caused by an appliance demanding too much current during normal operation, or by an abnormal condition, such as mechanical load on a motor or faults in equipment.
  • The conductors may short-circuit due to damage to insulation. In this situation, vast currents may flow through and cause an extreme form of overload.
  • Current may be leaked to earth if the cable insulation fails.
  • A loose connection can cause components, cables or combustible materials to overheat.
  • Electrical faults can cause arcs and sparks.
  • The bunching of several cables or the presence of thermal insulation can cause overheating.

To avoid fires of this nature, the Wiring Regulations require the use of fuses or miniature circuit breakers, both of which work to prevent the overcurrents arising from overload or short-circuits. The currents at which these devices isolate the circuit must be matched to the current-carrying capacity of the cables. The maximum current a cable of a particular size can carry is determined, in part, by the maximum temperature at which the insulation is safe, although there are also constraints on voltage drop along the cable.

De-rating is the reduction of the maximum permissible current that the cable may carry. It is necessary where semi-enclosed (rewirable) fuses are used as protection against overcurrent. Such devices are slower to operate than cartridge fuses or miniature circuit breakers. It is also required where the cables are surrounded by thermal insulation, because the heat generated by the cable cannot be readily dispersed. PVC insulated cables should not be laid in contact with polystyrene, which is sometimes used for thermal insulation, as the plasticizer will migrate from the PVC, causing the insulation to become brittle and hazardous.

In part 98 of this series, LWF will continue to discuss electrical fires and residual current devices. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact LWF on freephone 0800 410 1130.


Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 25 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.

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