Building Services: I

Electrification – Wires and cables

Different types of Wires and Cables

Contents:

• Why wires and cables needed?

• What is a wire?

• What is a Cable?

• How wires are made?

• How Cables are made?

• Different types of wires

• Use and application of different types of wires

• Different types of cables

• Use and application of different types of cables

Why wires and cables needed?

• For the Transmission of Electricity from its generation point to the user.

Conductors of Electricity…….

• In physics and electrical engineering, a conductor is amaterial which contains movable electric charges. Inmetallic conductors, such as copper or aluminum, themovable charged particles are electrons.

• All conductors contain electric charges which will movewhen an electric potential difference (measured in volts)is applied across separate points on the material.

• This flow of charge (measured in amperes) is what ismeant by electric current.

• Most familiar conductors are metallic, in otherword; all metals are good conductors of electricity.

Conductors of Electricity…….

• Copper is the most common material used for electricalwiring.

• Silver is the best conductor, but is expensive.

• Gold is used for high-quality surface-to-surface contacts.However, there are also many non-metallic conductors,including graphite, solutions of salts, and all plasmas.

Thus, A conductor is a wire suitable for carrying an electric current.

What is a wire?

• A wire is a single, usually cylindrical, string of metal.

• Wires are used to bear mechanical loads and to carryelectricity and telecommunications signals.

• Wire is commonly formed by drawing the metal througha hole in a dye or draw plate.

• Standard sizes are determined by various wire gauges

Thus, A wire is a single slender rod orfilament of drawn metal. This definitionrestricts the term to what would ordinarily beunderstood as "solid wire." The word"slender" is used because the length of awire is usually large when compared to itsdiameter.If a wire is covered with insulation, it is aninsulated wire. Although the term "wire"properly refers to the metal, it also includes theinsulation.

Finishing, jacketing, and insulating

• Electrical wires are usually covered with insulatingmaterials, such as plastic, rubber-like polymers, orvarnish. Insulating and jacketing of wires and cables isnowadays done by passing them through an extruder.Formerly, materials used for insulation included treatedcloth or paper, and various oil-based products. Since themid-1960s, plastic and polymers exhibiting propertiessimilar to rubber have predominated.

Extrusion is a process used to create

objects of a fixed cross-sectional profile. A

material is pushed or drawn through a die

of the desired cross-section.

Wires

• Wire is grouped by gauge number, running from 0000 toNo. 40. The smaller the number, the thicker the wire. Forhome use, the most common gauges are between 10and 20.

• Larger wire carries more current. Forcing too muchcurrent through a wire will cause it to overheat and tripa breaker.

Selection Of Wire Size

• Several factors must be considered in selecting the sizeof wire to be used for transmitting and distributingelectric power. Wires can carry only a limited amount ofcurrent safely. If the current flowing through a wireexceeds the current-carrying capacity of the wire, excessheat is generated. This heat may be great enough toburn off the insulation around the wire and start a fire.

• Factors Governing The Current Rating

• Conductor Size

• Wire Location

• Insulation

• Safe Current Ratings

Stranded Wires• Solid wire, also called solid-core or single-strand wire,

consists of one piece of metal wire. Stranded wire iscomposed of a bundle of small-gauge wires to make alarger conductor.

• A stranded conductor is a conductor composed of a group ofwires or of any combination of groups of wires. The wires ina stranded conductor are usually twisted together and notinsulated from each other.

Stranded copper wire

• Stranded wire is more flexible thansolid wire of the same total cross-sectional area.

• Solid wire is cheaper to manufacturethan stranded wire and is usedwhere there is little need forflexibility in the wire.

• Stranded wire is used whenever ease of bending orrepeated bending are required.

• Such situations include connections between circuitboards in multi-printed-circuit-board devices, where therigidity of solid wire would produce too much stress as aresult of movement during assembly or servicing; A.C.line cords for appliances; musical instrument cables;computer mouse cables; welding electrode cables;control cables connecting moving machine parts; miningmachine cables; trailing machine cables; and numerousothers.

Solid versus stranded

Conductors

• Conductors are stranded mainly to increase theirflexibility. The wire strands in cables are arranged in thefollowing order:

• The first layer of strands around thecenter conductor is made up of sixconductors. The second layer is madeup of 12 additional conductors.

• The third layer is made up of 18additional conductors, and so on. Thus,standard cables are composed of 7, 19,and 37 strands, in continuing fixedincrements.

• The overall flexibility can be increasedby further stranding of the individualstrands.

Construction of Stranded wires

Number of strands

• The more individual wire strands in a wire bundle the more flexible, kink resistant, break resistant, and stronger the wire is. But more strands cost more.

• The lowest number of strands is 7. One in the middle, and 6 surrounding it.

• The next level up is 19, which is another layer of 12 strands on top of the 7. After that the number varies, but 37 and 49 are common, then in the 70 to 100 range (the number is no longer exact). Even larger numbers than that are typically found only in very large wires.

What is a Cable?

• A cable is two or more wires running side by side andbonded, twisted or braided together to form a singleassembly.

• In mechanics cables, otherwise known as wire ropes, areused for lifting, hauling and towing or conveying forcethrough tension.

• Electric cables discussed here are mainly meant forinstallation in buildings and industrial sites. For powertransmission at distances greater than a few kilometressee high voltage cable, power cables and HVDC

Classification of a cable

• Classified according to the number of wires it containsand their size or gauge.

• All cables are marked with a series of letters followed bya number, a dash and another number. The lettersindicate the type of insulation (cord, wire andinsulation). The first number indicates the resistance ofthe wires in the cable, and the number following thedash indicates the number of individual conductors inthe cable.

• Cables can be securely fastened and organized, such asby using trunking, cable trays, cable ties or cable lacing.Continuous-flex or flexible cables used in movingapplications within cable carriers can be secured usingstrain relief devices or cable ties.

• At high frequencies, current tends to run along thesurface of the conductor. This is known as the skineffect.

Electrical cables

• Figure shows a typical cross section of a 37-strand cable. It also shows how the total circular-mil cross-sectional area of a stranded cable is determined.

Stranded conductor.