3.5.2  - Fire protection

Where an electrical installation or a piece of equipment which is part of it is, under normal circumstances, likely to become hot enough to set fire to material close to it, it must be enclosed in heat and fire resistant material which will prevent danger. Because of the complexity of the subject, the Regulations give no specific guidance concerning materials or clearance dimensions. It is left to the designer to take account of the circumstances arising in a particular situation. When fixed equipment is chosen by the installation user or by some other party than the designer or the installer, the latter are still responsible for ensuring that the installation requirements of the manufacturers are met.

The same general principle applies in cases where an equipment may emit arcs or hot particles under fault conditions, including arc welding sets. Whilst it may be impossible in every case to prevent the outbreak of fire, attention must be paid to the means of preventing its spread.

For example, any equipment which contains more than 25 litres of flammable

Fig 3.11 Precautions for equipment which contains flammable liquid

liquid, must be so positioned and installed that burning liquid cannot escape the vicinity of the equipment and thus spread fire. {Figure 3.11} indicates the enclosure needed for such a piece of equipment, for example an oil-filled transformer. In situations where fire or smoke could cause particular hazards, consideration should be given to the use of low smoke and fire (LSF) cables. Such cables include those with thermosetting insulation and mineral-insulated types.

Perhaps a word is needed here concerning the use of the word 'flammable'. It means something which can catch fire and burn. We still see the word inflammable in everyday use, with the same meaning as flammable. This is very confusing, because the prefix 'in' may be taken as meaning 'not', giving exactly the opposite meaning. 'Inflammable' should never be used, 'non-flammable' being the correct term for something which cannot catch fire.

Under some conditions, especially where a heavy current is broken, the current may continue to flow through the air in the form of an arc. This is more likely if the air concerned is polluted with dust, smoke, etc. The arc will be extremely hot and is likely to cause burns to both equipment and to people; metal melted by the arc may be emitted from it in the form of extremely hot particles which will themselves cause fires and burns unless precautions are taken. Special materials which are capable of withstanding such arc damage are available and must be used to screen and protect surroundings from the arc.

Some types of electrical equipment, notably spotlights and halogen heaters, project considerable radiant heat. The installer must consider the materials which are subject to this heat to ensure that fire will not occur. Enclosures of electrical equipment must be suitably heat-resisting. Recessed or semi-recessed luminaires mounted in ceiling voids must be given special attention to ensure the heat they produce cannot result in fire. Equipment that focuses heat, such as radiant heaters and some luminaires, must be mounted so that excessive temperatures are not reached in adjacent surfaces, The installation of a protecting RCD with a rating not exceeding 300 mA will sometimes prevent a fire in the event of an earth fault.

Additions to an installation or changes in the use of the area it serves may give rise to fire risks. Examples are the addition of thermal insulation, the installation of additional cables in conduit or trunking, dust or dirt which restricts ventilation openings or forms an explosive mixture with air, changing lamps for others of higher rating, missing covers on joint boxes and other enclosures so that vermin may attack cables, and so on.