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Encyclopedia of Electrical Engineering
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HOME TABLE OF CONTENTS
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Encyclopedia of Electrical Engineering
HOME TABLE OF CONTENTS PROJECTS TECHNOLOGY
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Electrical Cables

What is Electrical cable?

An electrical cable is an assembly of one or more wires running side by side or bundled, which is used to carry electric current. The term cable originally referred to a nautical line of specific length where multiple ropes are combined to produce a strong thick line that was used to anchor large ships.
Fig.no.1: Electrical Cable
As electric technology developed, people changed from using bare copper wire to using groupings of wires and various sheathing and shackling methods that resembled the mechanical cabling so the term was adopted for electrical wiring. In the 19th century and early 20th century, electrical cable was often insulated using cloth, rubber or paper. Plastic materials are generally used today, except for high-reliability power cables. The term has also come to be associated with communications because of its use in electrical communications.

Uses of Electrical cables

Electrical cables are used to connect two or more devices, enabling the transfer of electrical signals or power from one device to the other. Cables are used for a wide range of purposes, and each must be tailored for that purpose. Cables are used extensively in electronic devices for power and signal circuits. Long-distance communication takes place over undersea cables. Power cables are used for bulk transmission of alternating and direct current power, especially using high-voltage cable. Electrical cables are extensively used in building wiring for lighting, power and control circuits permanently installed in buildings.

How do Electrical cables are made?

Physically, an electrical cable is an assembly consisting of one or more conductors with their own insulation and optional screens, individual covering(s), assembly protection and protective covering(s). Electrical cables may be made more flexible by stranding the wires. In this process, smaller individual wires are twisted or braided together to produce larger wires that are more flexible than solid wires of similar size. Bunching small wires before concentric stranding adds the most flexibility. Copper wires in a cable may be bare, or they may be plated with a thin layer of another metal, most often tin but sometimes gold, silver or some other material. Tin, gold, and silver are much less prone to oxidation than copper, which may lengthen wire life, and makes soldering easier. Tinning is also used to provide lubrication between strands. Tinning was used to help removal of rubber insulation.
Fig.no.2: Insulation of electrical cable

Skin effect in electrical cables

Skin effect is a tendency for alternating current (AC) to flow mostly near the outer surface of an electrical conductor, such as metal wire. The electric current flows mainly at the "skin" of the conductor, between the outer surface and a level called the skin depth as shown in Fig.no.2. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor.
Fig.no.3: Current distribution Skin depth in steel at different frequencies.(current shown in blue)
Skin effect causesConductors, typically in the form of wires, may be used to transmit electrical energy or signals using an alternating current flowing through that conductor. The charge carriers constituting that current, usually electrons, are driven by an electric field due to the source of electrical energy. An alternating current in a conductor produces an alternating magnetic field in and around the conductor. When the intensity of current in a conductor changes, the magnetic field also changes. The change in the magnetic field, in turn, creates an electric field which opposes the change in current intensity. This opposing electric field is called counter-electromotive force (back EMF). The back EMF is strongest at the center of the conductor, and forces the conducting electrons to the outside of the conductor
Fig.no.4: skin effect is produced due to the eddy current effect
Fig.no.5: Litz wire
Increased AC resistance due to the skin effect can be mitigated by using specially woven litz wire shown in Fig.no.4. Litz wire is a type of specialized multi-strand wire or cable used in electronics to carry alternating current (AC) at radio frequencies. The wire is designed to reduce the skin effect and proximity effect losses in conductors used at frequencies up to about 1 MHz. It consists of many thin wire strands, individually insulated and twisted or woven together often involving several levels (groups of twisted wires are twisted together, etc.). The result of these winding patterns is to equalize the proportion of the overall length over which each strand is at the outside of the conductor. This has the effect of distributing the current equally among the wire strands, reducing the resistance. Litz wire is used in high Q inductors for radio transmitters and receivers operating at low frequencies, induction heating equipment and switching power supplies.

Solid vs. Stranded Conductors

Solid conductors are constructed of one, single piece of metal. It is tougher than a stranded conductor, but rigid and less flexible than a stranded conductor. Solid conductors are more likely to break if subjected to frequent flexing than stranded conductors. Stranded conductors are made of multiple small strands, which group together to make up a single conductor. It is more flexible than a solid conductor, but less durable.
In general, the electric power that is to be supplied to the household and most of the electric equipment of industries is AC type, i.e. Alternating Current. One of the properties of alternating current is that it has tendency to flow in the surface of any conductor. It means the flow of alternating current is highly concentrated near the circumference instead of being evenly distributed in the whole cross-section of the conductor. This tendency is increased with the increment of frequency of alternating current. This phenomenon is called skin effect. The skin effect concludes that for flowing of the higher amount of AC requires more circumferences instead of more cross-sectional area of conductor.
Hence, instead of single copper strand, a copper wire has numerous multi-strands that increases its circumference dimension so that for the same amount of the copper, a wire of higher rated current can be manufactured. A multi-strand wire also offers more mechanical strength then a single-strand wire.