ELECTRIC CABLESThe ship wiring cables have to with stand a wide variety of environmental conditions, like temperature, humidity and salinity of normal and extreme conditions. Cable consist of conductors and insulators of specially improved materials to stand to the above environmental working conditions.Conductors : Conductors areof annealed copper. In many cables copper conductors are TIN COATED. This coating act as a barrier between the insulation and the copper , reducing adhesion and preventing possible chemical action of copper with certain constituents of rubber insulation.Aluminium requires a large crossection for the same conductance as compared to copper. Its tensile strength is less than copper and is not used on board the ship.Insulation : The thickness of the conductor insulation depends upon CABLE VOLTAGE RATINGS. The normal voltage on ships is 440V and cables for use at this voltage are designed at 600/1000V. That means 600V to earth or 1000V between conductors.INSULATION MATERIALS:Polyvinyl Chloride (PVC)- Max.temp.-120. Class-E .PVC is flame retardant . when burnt,formation of hydrochloric acid gas tends blanket the area, but these fumes are toxic. 1. Butyl rubber Max.temp.80-85 resistant to oxidation,heat and moisture but effected by solvents,oil &grease.2. Ethylene propylene rubber (EPR).- Max.temp.90 .It is not flame retardant and is effected by solvent and oil ,hence needs to be protected by sheathing.3. Silicone rubber-Max.temp.150 .4. Mineral insulation-Max.temp.150 This insulation is used for high temperature and high fire risk areas.Sheathing : Purpose of sheathing is to protect the insulation from external damage. It is not classified as insulation.Sheath materials are required to be heat, oil,and chemical resistance and flame retardant (HOFR). The sheath must also be tough and flexible.Armouring : Extra mechanical protection is provided by armouring with basket woven wire braid of :-1. Galvanised steel 2. Tinned phosphor bronze.Wire braiding acts as a screen to reduce interference caused by magnetic fields in communication and instrumentation circuits.Owing to magnetic properties ,steel armouring is unsuitable for single core cables carrying heavy alternating currents. Protective outer sheath of PVC or Neoprene covers the wire braid.Ratings of the cable : The current passing through the cable conductor results in rise of temperature due to the resistance of the conductor.(IR).Therefore maximum current rating is based upon maximum operating temperature of the insulation and sheathing.Short circuit rating : Under short circuit condition there is rapid rise in coductor temperature,effecting the cable insulation.Also there is mechanical forces due to magnetic effect and the longitudinal expansion of conductors due to heat also needs to be considered. It is therefore necessary to match the cable size with the short cicuit protection equipment (fuse or circuit breakers) which maytake certain time (fraction of secs.to few seconds) to trip.Special consideration for A.C. wiring to prevent heating of steel bulkhead due to magnetic induction.In three phase system, with single core cables laid parallel, if each cable passes througha separate hole in a steel bulkhead, heating of steel due to magnetic effect will occur. Therefore:-1. Where cables pass through steel bulkheads, all conductors should pass through non-ferrous plate or gland. The clearance between the magnetic material (steel)and cable should be 75mm so that steel does not heat due to magnetic induction.2. Single core cables carrying more than 250 amps. Should be atleast 50mm away from adjacent steel deck or bulkhead.3. For single core metal sheathed cables carrying heavy currents e.g.generator cables:- a) The cable should run in TREFOIL formation. b) The run from generator to the switch-board should be as straight and short as possible. c) Cables should be secured by non-magnetic cleats at intervals of 0.75m .4. The armour of the cable should be non-magnetic material. 5. If installed in steel conduits , all phases should be in the same conduit.