What are the various electrical cable types and what dictates their construction?
There are many different types of electrical cable used for applications across power distribution, control or signalling, and data transmission, and used in industrial, commercial and domestic installations. Electrical cables can be categorised in several different ways including by voltage rating, application, environment, industry, and material type, and determining any of these will help narrow down the search for the correct cable for any given purpose.
Typically, voltage rating categories for cable types include the following:
- Extra Low Voltage for supplies below 70V
- Low Voltage cables include voltages up to 1000V
- Medium Voltage Cables from 1000V to 35kV
- High Voltage cables from 35kV to 230kV
- Extra High Voltage above 230kV
The insulation layer is designed to withstand the electrical performance demands of the cable, so the choice of material type and thicknesses may vary. In some cases a higher voltage may require additional cable layers as determined by local specifications and national or international standards.
The materials used in cable construction are chosen for their electrical properties such as conductivity and insulation resistance. These materials and the precise construction may also influence reactance, impedance, capacitance and inductance values of the cables.
The intended application also determines the cable design and the materials used. For example:
- Overhead line wires need to be strong to support their weight between pylons or posts, and be corrosion resistant, but they don’t need a material insulation layer to protect against short circuit and electric shock if they are used in areas where the risk of contact or grounding does not exist.
- Underground cables must be insulated to protect against water ingress and possible mechanical damage. Cables suitable for direct burial will often have a metallic armour to provide extra protection.
- Cables for use in data-sensitive areas such as instrumentation cables often need to be screened using metallic tapes to protect against electrical noise (also commonly referred to as electromagnetic interference).
- Fire performance cables designed to support fire safety systems such as alarms and emergency lighting must be capable of withstanding fire conditions and maintaining functionality.
Different industries have their own particular requirements for electrical cable, for instance the mining industry requires cables that are resistant to the harsh a unique environments they operate in. Rubber insulation and sheathing is often used as this can offer additional flexibility but also need enhanced robustness and resistance to the chemicals they may be subjected to as a matter of course in operation. The mechanical properties such as resistance to abrasion and impact and the tensile strengths and the elasticity required must also be considered. Protection can include wire braiding, wire armouring, and metallic taping.
The chemical properties of the materials must be in compliance with national and international regulations such as RoHS and REACH, and be capable of withstanding exposure to the various chemical and environmental stresses they may come into contact with. Cables used for outdoor use must be weather resistant and capable of withstanding sunlight and ozone. Other considerations for material selection and construction include the range of temperatures the cables are required to operate in (both high and low temperatures).
Lastly, both cost and appearance (such as colour coding for easy identification) can influence cable construction but of course, these factors should play a secondary role to that of safe application.
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