The most acceptable metals for conductor are copper and Aluminium due to their higher conductivity and ductility. As copper has got higher affinity for sulphur, in the atmosphere where sulphur fumes are present. In these conditions tinned copper should be used.
Aluminium oxide film, which is always present on Aluminium conductor surface, acts as a barrier and it protects the Aluminium conductor from corrosion in fumes laden atmosphere.
The most economical construction is solid conductor is made of one single wire. As the area of conductor increases, solid conductor becomes stiffer and hence difficult to handle. In this case stranded construction is adopted. Here the conductor is made of number of strands. The strands are arranged in spiral layers in 1+6+12+18+... formations. This construction provides more flexibility. Where crimping of lugs are required, the conductor has to be stranded construction only.
To economies in insulating material, weight and overall diameter, shaped conductors are employed in bigger sized cables. Here the stranded conductor is shaped in to a segment of a circle so that when all the cores are laid, they form a complete circle.
I.S. 1554 permits solid conductor construction up to 10 sq.mm. in Aluminium and up to 6 sq.mm in copper. It permits the use of shaped conductors for sizes from 16 sq.mm onwards.
The cores are laid up with suitable lay. The final layer always has a right hand lay i.e. if you look along the cable, the cores move to your right hand.
Inner sheath is provided over the laid up cores. It is provided to give circular shape to the cable and it provides bedding for the armouring.
I.S. 1554 permits following two methods applying the innersheath of any thermoplastic material i.e. PVC, polyethylene etc.
Here the innersheath is provided by extrusion of thermoplastic over the laid up cores. This type of innersheath is generally provided in cables having round cores i.e. in control cables and power cables up to 10 sq.mm. Size.
This type of innersheath is also acts as a water barrier & between cores and Outersheath. In case of puncture in the outersheath the water cannot reach to the core and hence we recommend that cables for outdoor underground uses should have extruded inner sheat
Here wrapping a thermoplastic tape over the laid up cores provides the innersheath. It is generally employed in cables having sector shaped cored i.e. multicore cables of 16 sq.mm. And above. This method saves a process and hence manufacturers always provide this type of innersheath ueless the purchase specifications ask for extruded innersheath.
In case of armoured cables, generally galvanized steel wire /strip armouring is provided over the innersheath in multi-core cables and over the insulation in single core cables. It provide the mechanical protection to inside cores and it carries earth return current in case of short circuit of core with armour.
As per I.S 1554 (Part 1) 1988 round wire armouring is provided in cable where calculated diameter under armour is up to 13mm.Above this the armouring is either with round wire or strip of size 4mm x 0.80mm. As per strip construction is economical, the manufacturers always provide steel strip armouring unless wire armouring is specially specified.
In long run of cables and in case of mine, round wire armoring is must, as strip construction provides higher resistance to earth fault current and sometimes this current may not be sufficient to operate the circuit breaker in case of earth fault.
In mines, the resistance of armour in no case should exceed the resistance of the main core by more than 33% for safety reasons. To achieve this sometimes-tinned hard drawn copper wires are required to be used along with galvanized steel wires. Sometimes two layers of steel wires are provided to give extra protection.
In case of single core armoured cables for use in AC circuits, the material for armouring has to be non magnetic, as in this case the return current is not passing through the same cable and hence it will not cancel the magnetic lines produced by the current. These magnetic lines which are oscillating in case of AC current will give rise to eddy current in magnetic armouring and hence armouring will become hot, and this may lead to the failure of the cable. Generally hard drawn aluminium wire /strip are used for armouring in this case.
The PVC covering on armouring in case of armoured cables and over the innersheath in unarmoured cables is called outersheath I.S. 1554 specifies nominal and minimum thickness of outersheath for unarmoured cables and only minimum thickness of outersheath for armoured cables.
It permits the following types of outersheath PVC compounds.
Outersheath with type ST1 PVC compound as per IS-5831, which is suitable for 70° C continuous operation.
Outersheath with type ST1 PVC compound as per IS-5831, which is suitable For 85° C continuous operation.
PVC has got fire retardant properties due to its halogen content. The fire in the gets extinguished immediately on removal of the fire source. In the modern power, Chemical, fertilizer and cement plant many PVC cables are bunched in the cable shaft or on cable trays. In case of fire in these cables the fire becomes self-sustaining. Moreover due to the burning of PVC a dense corrosive smoke is emitted which makes fire fighting very difficult, due to poor visibility and toxic nature of the smoke. HCL content of the smoke, not only damages other costly equipment lying near by, but also penetrates the RCC and corrodes the steel reinforcement. Due to this there is an extensive damage to the property.
To overcome these deficiencies FRLS i.e. Fire Retardant Low Smoke PVC was developed. If required, we can provide Fire Retardant Low Smoke (FRLS) PVC inner sheath and/or outer sheath. This PVC compound, apart from meeting the requirements of Type ST2 as per IS-5831, has got better fire retardant properties and it emits lower smoke and acid fumes When it catches fire (For more information please refer our catalogue on FRLS cables)
YWY means copper conductor, PVC insulated, round wire armoured and PVC sheathed cable. AYFY means Aluminium conductor, PVC insulated, steel strip armoured and PVC sheathed cable.