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.
