Selection and sizing of conductors supplying electrical equipment that must remain functional during a fire

Cables that are exposed to fire while being expected to retain their functionality and provide power to essential equipment at another location must be appropriately selected and sized to take account of the increased electrical resistance at elevated temperature. Manufacturers offer cables and accessories that will survive a standard cellulose fire for 30, 60 or 90 minutes when correctly specified and installed.

Cables, including fire safety cables, are specified in terms that reflect their normal duty conditions; design parameters under fire conditions are rarely, if ever, specified. The objective of this paper is to provide a clear methodology for designing fire safety circuits based on the derivation and application of correction factors and standard cable parameters.

Cables that are exposed to fire while being expected to retain their functionality and provide power to essential equipment at another location must be appropriately selected and sized. This is not only a question of an appropriate insulation. Designers must take account of the increased electrical resistance at elevated temperature.

Manufacturers offer cables and accessories that will survive a standard cellulose fire for 30, 60 or 90 minutes when correctly specified and installed.

A first step to specifying a suitable fire safety cable is a good knowledge of the temperature rise characteristic in areas affected by the fire.

A second step is the correct selection and erection of the cable. This includes the correct sizing of the conductor. Cables, including fire safety cables, are specified in terms that reflect their normal duty conditions; design parameters under fire conditions are rarely, if ever, specified. The designer must take into account the consequent effects of the increased resistance on current carrying capacity, voltage drop, and short circuit capacity of the conductors. Special care should go to the current carrying capacity of the conductor if it is to supply electrically driven fire pumps drawing high starting currents. The circuit protection should also be adapted to fire conditions, as it must be designed to function with significant higher loop impedance than normal.

This paper provides a clear methodology for designing fire safety circuits based on the derivation and application of correction factors and standard cable parameters.

Having selected the appropriate cable, it must be installed properly, using suitable accessories and following the manufacturer’s restrictions.

Comments

Will Gubb's picture

THE CONTINUATION OF ELECTRICAL CIRCUITS IS ESSENTIAL

Fire alarms need to opperate, sprinkler systems may have electric pumps to maintain pressure, and fire detection systems are generally connected to a power scource.

Minimising the risk of fires breaking out needlessly should be the number one priority in any fire safety strategy. Good housekeeping, including such simple actions as avoiding smoking indoors, regularly testing electrical appliances, removing
electrical heaters, and training staff can help to reduce the risk of such fires breaking out. Safety precautions like these should also be taken in the home. As a home owner, you should always have an appropriate fire safety and evacuation plan in place.

This excert is taken from the Safety first with SafeQuip booklet which is free at www.safequip.co.za

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Highlights

  • Provides a clear methodology for designing fire safety circuits based on the derivation and application of correction factors and standard cable parameters.
  • Suggests that cable design parameters are rarely specified under fire conditions.
  • Knowledge of the temperature rise characteristic in areas affected by a fire is essential.
  • Correct cable selection includes correct conductor sizing.
  • Appropriate cable needs to be installed properly following the manufacturer’s restrictions.

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