Question SR 12 2008.

Discuss the various hazards and problem which are associated with electric cable insulation in the event of fire.

Suggest remedies for these problems.

State how the spread of fire may be reduced by the method used for installing electric cables

The insulation of an electric conductor is normally made of rubber or plastic

The type and amount of smoke generated by burning plaistic material depends on the nature of plastic the additives present whether the fire is flaming or smoldering and what ventilation is available

Most plastic decomposes when heated yielding dense to very dense smoke

Ventilation tends to clear the smoke but usually not enough for clear visibility

Those plastic that burn clearly yield less dense smoke under condition of heat and flame when exposed to flaming or non flaming heat

Hydrogen chloride is a product of combustion of chlorine containing plastic such as polyvinyl chloride [PVC] a plastic used for insulating most electrical wiring

Hydrogen chloride is a deadly gas it has a pungent and irritating odor

Burning rubber produces dense black oily smoke that has some toxic qualities

Two of the noxious gases produced in the combustion of rubber are hydrogen sulphide and sulphur di oxide

Both are dangerous and can be lethal under certain condition

Suggest remedies for these problems

Cables with E.P.R. ethylene propylene rubber insulation with flame retarding sheathing of PCP OR CSP [poly chloride prene or chlor sulphurated polyethylene] and G.I. armour may be used to protect the insulation from fire

The G.I. armour has to be earthed

Burning capability of an material depends on its oxygen index number

The number is allotted to a material depending on the minimum percentage of oxygen required to sustain combustion

When the oxygen index number is less than 21% the material will continue to burn

When it is more than 21% or more it is self extinguishing

Material used for insulation should have oxygen index number above 21%

State how the spread of fire may be reduced by the method used for installing electric cables

All electric cables and wiring external to equipment shall be at least of a flame retardant type and shall be so installed as not to impair their original flame retarding properties

cables and wiring serving essential or emergency power lighting internal communication or signals shall so far as practical be routed clear of galley laundries M\C spaces of category A and their casing and other high risk areas

Where cables which are installed in hazardous areas introduces the risk of fire or explosion in the event of an electrical fault in such areas special precaution to be taken

Cables shall be installed and supported in such a manner as to avoid chafing or other damage

Terminations and joint in all conductors shall be so made as to retain the original fire and resisting property

Each separate circuit shall be protected against short circuit and against overload

Fire proof compression gland is to be fitted when cables passes through bulkhead or comes out of a gland box

This is required to prevent spread of fire.