Answer MET Question 20

Question:  A. (i) Discuss the various hazards and problems which are associated with electric cable Insulation in the event of fire.

(ii) Suggest remedies for these problems.

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

Answer: A (i) Electrical cable insulation fire present a hazard of flame and smoke.

Properties of smoke  such as colour, density, toxicity etc. and intensity of flame depends on the material of insulation.

Cable sheath materials commonly in use are organic plastic compounds that are classed as flame retardant, i.e. will not sustain a fire. Most cable materials now achieve this property by developing chlorine gas and acid fumes to smother the flame.

Hydrogen sulphide and sulphur dioxide are produced when the material is rubber

PVC is notorious for its release of deadly acid fumes, like hydrogen chloride, but PCP and CSP do the same. EPR and XLPE do not. Some new materials do not produce acid  fumes when burning - an important feature for fire-fighting personnel. However, burning Cable -materials still tend to produce dense black smoke.

(ii) The remedies for the above stated problems are:
a. Choose the cable with proper rating as per the requirement of the intended services.
b. Instalation of the cable should be carried out properly and no loose contact present.
c. Glands to be used when required.
d. Cables are to be properly inspected for aging and physical damages.
e. Material used for insulation should have oxygen index number above 21%. (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)

B. Ways to reduce spread of fire by the method used for installing electric cables. -
a. All electric cables and wiring external to equipment shall be at least of flame retardant type and shall be so installed as not to impair their original flame retarding properties.
b. Cables and wiring serving essential or emergency power lighting internal communication or signals shall so far a practical be routed clear of galley laundries M/C spaces of category A and their casing and other high risk areas.
c. Where cables which are installed in hazardous areas introduces the risk of fire or explosion in the event of an electrical fault areas special precaution to be taken.
d. Cables shall be installed and supported in such a manner as to avoid chafing or other damage resistance property. Each separate circuit shall be protected against short circuit and against overload.
e. 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.