insulator

An insulator is a material that does not conduct electrical current. Insulating materials include paper, plastic, rubber, glass and air. Vacuum is also an insulator, but is not actually a material. Most electrical conductors are covered by insulation. Magnet wire is coated with an extremely thin layer of insulation so that more turns or larger wire may be used in the winding of transformers etc. Insulators are generally rated at hundreds of volts, but some that are used in power distribution are rated as high as hundreds of thousands of volts. Insulators support and/or keep electrical conductors from making unintended contact with each other.

 Best material of insulator

 The best insulator in the world right now is most probably aerogel, with silica aerogels having thermal conductivities of less than 0.03 W/m*K in atmosphere. of aerogel preventing ice from melting on a hot plate at 80 degrees Celsius! Aerogel has its amazing properties because it's mostly made out of air.

(1)  pin type insulator

(2). Suspension type insulator

(3). Strains insulator

(4). Shackle insulator

(5). Over head line insulator

(1) Pin Type Insulator

• it consists of a single or multiple units.
• it is secured to the cross-arm on the pole.
• it used only to 33 KV.
• beyond operating voltage of 33 KV, it becomes to bulky and uneconomical.
• there is a groove on the upper end of the insulator for housing the conductor.
• the conductor passes through this groove and is bound by annealed wire of the same material as the conductor.

(2) Suspension Type Insulator

• it used for voltages above 33 KV.
• it consists of a number of disc units mounted one above each other to form a string.
• the conductor is suspended at the bottom end of the string while the other end of the string is secured ti the cross-arm of the tower.
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• Advantages of Suspension Type Insulator are
• cheaper than pin type for voltages beyond33 KV.
• number of units in the string depend on working voltage as they are connected in series.
• each disc of the string is designed for low voltage 11 KV.
• if any disc is damaged, it can be replaced easily.
• string has a high flexibility.
• additional insulation can be provided by adding the desired number of discs to the string.
• Disadvantages of Suspension Type Insulator are
• costlier than pin type for voltages under 33 KV.
• it requires more height of supporting structure than pin type need which is uneconomical.
• the amplitude of free swing of conductors is larger in suspension type, hence, more spacing between conductors should be provided (by making the arms of the tower more long).

(3) strains insulator

A  strain insulator is an electrical insulator that is designed to work in mechanical tension (strain), to withstand the pull of a suspended electrical wire or cable. They are used in overhead electrical wiring, to support radio antennas and overhead power lines. A strain insulator may be inserted between two lengths of wire to isolate them electrically from each other while maintaining a mechanical connection, or where a wire attaches to a pole or tower, to transmit the pull of the wire to the support while insulating it electrically. Strain insulators were first used in telegraph systems in the mid 19th century.

(4) shackle insulator

Shackle type insulator  is  an insulator  of  generally cylindrical form,  the other  name of  shackle insulator is  butterfly  insulator,  because porcelain shackle type insulator  has  two  or  four  big  sheds  looks  like butterfly,  so we  some place calls  it    butterfly insulator. 

The  application of  shackle insulator 

Shackle insulator  like spool  insulator,  both  of  them have a hole，a  D-iron  bracket  with  bolts  cross  the hole of  shackle insulator.  Shackle insulator’s  main function is  combinations  with D-iron  bracket  fixed on telegraph pole and  insulate the conduct  wire. 

The color of  shackle insulator Shackle  insulator  has  different colors  to  meet different  countries’  demand .

•  Grey 
•  White   
•  Brown   
•  Blue 
•  Green   

(5) overhead line insulator

  An overhead line  may  be  used to  transfer  or  distribute  electric  power.  The  proper overhead line  operation  depends  to a  big  extent  upon  its  mechanical  design.  While constructing  an  overhead line,  it  has  to  be verified  that  line  mechanical  strength is  such so as  to provide  against  the  most  probable  weather  conditions.  Typically,  the main elements  of  an overhead  line  are:   

- - Conductors  which  transfer  power  from  the sending  end station to the receiving end  station.

 Supports  which may  be poles  or  towers.  They  keep  the  conductors  at an appropriate  level  above the  earth. 

- Insulators  that  are  connected  to supports  and  insulate the conductors  from  the earth.

- - Cross arms which  give  support  to  the insulators.

Miscellaneous  elements  such as  phase  plates,  danger  plates,  surge  arrestors, etc.


The overhead  line  operation  continuity  depends  upon  the judicious  selection  of  above elements.  Hence,  it  is  beneficial  to  have detailed discussion on them.


Material of insulator

(a) Polyethylene is a thermoplastic material that combines unusual electrical process, high resistance to moisture and chemical, easy processability and low cost. It has got high resistivity and good dielectric properties at high frequencies and therefore, is widely used for power and coaxial cables, telephone cables multi-conductor control cable, TV lead-in wire etc.

(b) Fibre glass. It is made from material, which is a free alkali metal oxide, which may form a surface coating that may attack the glass silicates. Glass does not absorb moisture volumetrically, but may attract it by capillary action between the fine filaments. Tapes and clothes woven from continuous filament yarns of glass have high resistivity, thermal conductivity and tensile strength and from a good class B insulation. Glass is used as a cover and for internal support in electric bulb, electronic valve, mercury arc switches, X-ray, equipment capacitors and as an insulator in telephone.

(c) Porcelain. It is made from china clay used as insulating supports for overhead lines, and also in making spark plugs. Inferior porcelain is used in low voltage switches and fine gears.

(d) Asbestos. It is a mineral substance, fibrous in structure and is of white or brown colour. It is an incombustible and fire-proof materials good insulator of sound, heat and electricity widely used in steam pipe joints, making fire proof roofing materials, partition wall, Electric Iron and Heating Oven etc.

(e) Bakelite. It is a synthetic product. Insulating properties are good and so is used in making all kinds of small electrical fitting, terminal boards, lamp holders, switch covers etc.

(f) Empire Cloth. It is made by varnishing cotton cloth, silk or paper and is used as wrapper of armature coils.

(g) Mica. It is a mineral product and occurs as crystals. It is easily split into thin sheets and is not much used in pure form, it is fire proof and does not absorb moisture. It is used as an insulating material for Heaters, Irons, Commutator of DC motor etc.

(h) V.I.R. Insulation. It is prepared by mixing with pure rubber about 5% of sulphur, Zinc Oxide, Whiting and some colouring matters. The mixture is then heated to about 150 degree centigrade and in vulcanised form, rubber is made tough which does not absorb moisture. It forms a good insulating and protective covering for low and medium voltage cable conductors.

(i) Ebonite. It is vulcanised rubber containing about 30% to 50% of sulphur which is subjected to prolonged heating at about 150 degrees C. It is a hard substance and can be moulded into different shapes and widely used for making containers of lead acid battery, small Instrument panels and terminal mountings.

(j) Gutta Percha. It is a chemical product whose properties resemble to those of rubber. It is not affected by water even immersed in it. It is suitable insulation for submarine cables.

(k) Bitumen. A natural product, obtained from lakes. Bitumen refined with petroleum compound is used for preparing varnishes and for filling joint boxes of underground cables. Vulcanised bitumen used as an insulating covering for cable conductors, meant especially for damp places.

(l) Press Han. It is a form of paper prepared from hemp, rags, and wood pulp chemically treated. It is widely used for lining in armature slots insulating coil sides for low and medium voltage machines.

(m) Leathered Paper. It is a tough material prepared chemically from cotton rags-which is unaffected by grease and oil and used for slots and coil insulation and also in transformer core covering etc.

(n) Shellac. It is made from the product which grows on some varieties of trees in India. It is used in making insulating varnishes.

(o) Silk & Cotton Cloth. This insulation is used on conductor required for low voltage. The conductor may have a single or double layer covering according to the types of work mostly such wires are used for instrument and armature winding.

(p) Gar Film. It is a good insulator and plastic product used for slots and coil insulation.

(r) Air is a free gift from God. It has got application everywhere in spite of our consent. It is used as insulation in transmission line. It has high insulation resistance but drops due to moisture. When using air, there is very less dielectric loss. At high voltage corona effect takes place. Dielectric strength- 30 KV/cm