About Cables

The overview of cable construction covers how typical low voltage, high voltage and instrumentation cables are put together and outlines the different constituent parts that make up a cable.
common types of materials used in cables,

• Conductor materials
• Insulation materials
• Sheath materials

Cable Conductor Materials

In order to transmit electrical current with as few losses as possible, a cable conductor needs to be of low resistivity (or high conductivity). There are two main cable conductor materials used in practice, copper and aluminium, because of their low resistivity characteristics, coupled with their relatively low cost. Silver has better resistivity characteristics than either copper and aluminium, but being a precious metal, is far too costly.

Copper
The resistivity of copper is in the order of 1.7 - 1.8 Ωmm2 / m. Copper is a denser material than aluminium and has a higher melting point, hence has better performance under short circuit conditions and is mechanically stronger. However the high density of copper makes it less flexible than aluminium. Copper conductors also need to be very pure, and small traces of impurities (e.g. phosphorous) can significantly affect conductivity.

Copper is typically used more commonly in industrial plants, generating stations and portable equipment because of its mechanical properties. Furthermore, it is used in applications where space restrictions abound, e.g. offshore platforms and aircraft

Aluminium
The resistivity of aluminium is around 2.8 Ωmm2 / m, which makes it roughly 60% less conductive than copper. Therefore, aluminium conductors need to be oversized by a factor of 1.6 in order to have the equivalent resistance of copper conductors. However aluminium is also 50% lighter in mass than copper so it has a weight advantage. Additionally, it is more malleable and flexible than copper.
Aluminium is inherently corrosion resistant due to the thin oxide layer that is formed when aluminium is exposed to the air. Aluminium also performs better than copper in sulfur laden environments (in terms of corrosion resistance).
Aluminimum is typically used for overhead aerial lines because of its light weight and high conductivity. It is also used in applications where space restrictions are not a large factor, e.g. underground cables

Cable Insulation Materials
1 Thermoplastic
2 Thermosetting
3 Paper Based
4 Comparison of Materials

Thermoplastic
Thermoplastic compounds are materials that go soft when heated and harden when cooled:

PVC (Polyvinyl Chloride) – is the most commonly used thermoplastic insulator for cables. It is cheap, durable and widely available. However, the chlorine in PVC (a halogen) causes the production of thick, toxic, black smoke when burnt and can be a health hazard in areas where low smoke and toxicity are required (e.g. confined areas such as tunnels). Normal operating temperatures are typically between 75C and 105C (depending on PVC type). Temperature limit is 160C (<300mm2 140c="" and="">300mm2).
PE (Polyethylene) – is part of a class of polymers called polyolefins. Polyethylene has lower dielectric losses than PVC and is sensitive to moisture under voltage stress (i.e. for high voltages only).

Thermosetting
Thermosetting compounds are polymer resins that are irreversibly cured (e.g. by heat in the vulcanization process) to form a plastic or rubber:
XLPE (Cross-Linked Polyethylene) – has different polyethylene chains linked together (“cross-linking”) which helps prevent the polymer from melting or separating at elevated temperatures. Therefore XLPE is useful for higher temperature applications. XLPE has higher dielectric losses than PE, but has better ageing characteristics and resistance to water treeing. Normal operating temperatures are typically between 90C and 110C. Temperature limit is 250C.
EPR (Ethylene Propylene Rubber) – is a copolymer of ethylene and propylene, and commonly called an “elastomer”. EPR is more flexible than PE and XLPE, but has higher dielectric losses than both. Normal operating temperatures are typically between 90C and 110C. Temperature limit is 250C.

Paper Based 

Paper Based insulation is the oldest type of power cable insulation and is still used mainly for high voltage cables. The paper insulation must be impregnated with a dielectric fluid (e.g. oil resin or a synthetic fluid). A lead sheath is commonly applied over the insulation to prevent water or moisture ingress into the paper insulation, which is sensitive to moisture.

Cable Sheath Materials

Thermoplastic compounds are materials that go soft when heated and harden when cooled:
PVC (Polyvinyl Chloride) – as a sheath material, PVC is used extensively because of its low cost and good overall properties – high physical strength, good moisture resistance, adequate oil resistance, good flame resistance and excellent resistance to weathering and to soil environments. PVC contains halogens which produces thick, black toxic smoke when burnt. Most commonly used sheath material for LV cables.
PE (Polyethylene) – is usually categorized under three different densities – 1) Low density (0.91 – 0.925 g/cm3), 2) Medium density (0.926 – 0.94 g/cm3), and 3) High density (0.941 – 0.965 g/cm3). PE sheaths have good physical strength, excellent moisture resistance, good ageing properties, but poor flame resistance. Like PVC, PE will melt at high temperatures. Does not contain halogens.
CPE (Chlorinated Polyethylene) – similar to PVC, but with better high temperature properties. Contains halogens.
TPE (Thermoplastic Elastomer) – provides flame resistance, good low temperature performance, good abrasion resistance and good physical strength. Does not contain halogens.
Nylon – provides good physical strength, reasonable abrasion resistance, very low friction when in contact with conduit materials which aids in pulling cables. Excellent resistance to oils and organic solvents, but very sensitive to strong acids and oxidizing agents.

Thermosetting compounds are polymer resins that are irreversibly cured (e.g. by heat in the vulcanization process) to form a plastic or rubber:
XLPE (Cross-Linked Polyethylene) – provides a tough, moisture, chemical and weather resistant sheath material. Used mainly as an outer sheath material for “rugged” cables.
PCP (Polychloroprene) or trade name "Neoprene" – provides good heat resistance, flame resistance, resistance to oil, sunlight and weathering, low temperature resistance and abrasion resistance. Due to its ruggedness, neoprene is used widely in the mining industry. Does not deform with high temperatures and does not contain halogens.
CSP (Chloro-sulphanated Polyethylene) – similar properties to neoprene, though superior in resistance to heat, oxidizing chemicals, ozone and moisture, and has better dielectric properties. However CSP contains halogens.
EPR (Ethylene Propylene Rubber) – not commonly used as a sheath material, but can be useful if increased cable flexibility is required (especially in low temperature applications).