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Sunday, 18 September 2016

Manufacturing of Power Cables

Power cables are insulated conductors used to carry electrical power from one point in the electrical circuit to another point or to any electrical equipment. They may be installed as permanent wiring within buildings, buried in ground or provided overhead depending on the site specific requirements. 
Modern power cables come in a variety of sizes, materials and types, each particularly adapted to its use. These cables comprise of conductors, insulation, inner sheath, armour and outer sheath.

The construction and materials of cables are determined by three main factors:

1.       Working voltage, which determines the thickness of the insulation,
2.       Current carrying capacity, determining the cross-sectional area of conductors, and
3.       Environmental conditions such as temperature, water, chemical, mechanical impact etc.

Power Cables have aluminium or copper conductors and polymer insulation. Conductors may be solid, stranded circular or shaped. Stranding makes the cable flexible and easy to handle whereas shaping is done to make them compact. All multi-core cables of 16 sq.mm size or above are sector shaped. The conductor is manufactured in equal segments and compacted, and then laid together. This also reduces losses due to skin and proximity effect.

Conductors:
Cables with copper conductors have higher tensile strength, better conductivity and better flexibility. They are usually used in providing power connections to electrical equipments in industries, instrumentation, submarine and ship wiring and in mining applications also. These cable conductors are manufactured in accordance to IS: 8130 (Indian specifications) and IEC: 60228/ BS: 6360 (International standards). Manufacturing process of both copper and aluminium conductor cables are the same.

Insulation:
Cables are usually made with polymer dielectrics as insulation, so as to have better thermal and thermo-mechanical properties under both normal and abnormal conditions. Usually PVC (thermo-plastic dielectric) and XLPE (thermo-setting dielectric) insulation are used. Cables with PVC insulation use PVC compound whereas XLPE insulated cables use XLPE compound with certain addition to improve the electrical and mechanical characteristics. Insulation process in cable manufacturing are governed by IS: 5831/ IS: 7098 and IEC: 60502/ BS: 6746/ BS: 5467.

Screening:
XLPE cables with voltage rating exceeding 3.3 kV are provided with both conductor and insulation screening. Conductors are screened with extruded screen of semi-conducting compound. Insulation screening has a non-metallic screen part combined with a metallic part. Non-metallic part consists of semi-conducting compound tape applied directly over the insulation. Over this copper tape is applied helically which forms the metallic part of the insulation screen.
Perfect bonding of insulation and screening is essential to avoid cavities and void formation in dielectric. Conductor screen, insulation and the non-metallic part of the insulation screen, all the three, are usually applied in one operation to ensure perfect bonding.

Inner-sheath:
Inner-sheath of PVC acts as bedding for steel armouring. Sometimes filler cords are also provided to maintain the circular shape. Inner-sheaths are compatible with temperature rating of cables.
The inner sheath is applied either with extrusion or by wrapping. The dimensions of the inner sheath are maintained as per IS: 1554/7098.

Armouring:
Armouring provides the mechanical protection to the cable and are made of low resistivity material. Armouring of single core cables are either wires or strips of Aluminium or aluminium alloy to avoid hysteresis losses. Multi-core cables are provided with galvanized steel wire or strips. Galvanized wire armouring is used where cables are subjected to stresses. Armouring is done as recommended in IS:3975.

Outer Sheath:

Power cables   are usually provided with outer sheath made up of PVC/ polymer. These outer sheaths are harder than inner sheath and manufactured with various characteristics of sheathing compounds for example general purpose, heat resistant, fire retardant, flame retardant, UV radiation resistant, anti rodent/termite compound etc. Compounds for outer sheath are supposed to meet IS: 5831 specifications.