An insulator is a substance possessing such enormous resistance that practically it does not permit electricity to flow through it. Electrons can be removed from the atom of any insulator with great difficulty. The outer electrons can be detached from their parent atoms when, in comparison with a conductor, considerably high amount of voltage is applied.
Amongst the good insulators are cotton, silk, polyvinyl chloride, dry paper, dry glass, dry air, paraffin wax, mica, vulcanite, shellac, India rubber, sulphur, sealing wax, porcelain, ebonite, asbestos and certain oils of the paraffin family.
As a protection for electric wires and cables vulcanised India rubber, which is the rubber cured with sulphur at a high temperature, is largely used. To avoid the bad effect of sulphur on the copper conductor, a layer of pure rubber is always put between them. But at present polyvinyl chloride is widely used in place of rubber insulation. For house wiring every where P.V.C. wire is now preferred over V.I.R. wire or C.T.S. wire. P.V.C. is a kind of plastic insulation. It is not corroded by acid, alkali, ozone, or oil like rubber.
Mica has the advantage, for certain purposes that it can be obtained in very thin sheets. It is used as insulation in commulator, electric iron etc.
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Paper, impregnated with insulating oil, is largely used in cable insulation. But now-a-days tropodur cables, insulated with P.V.C. tape, has larger field of application than paper insulated cable.
Certain mineral oils are good insulators and many electrical appliances such as switches, transformers, etc. are oil-immersed to obtain high insulation. Distilled water is also an insulator, but the slightest impurity makes it a conductor.
Cotton and silk insulating tapes are applied over enamelled copper and aluminium conductors which are used for the manufacture of armature and field coils of generators, motors, electric calling bell, etc. In order to make the cotton insulation non-hygroscopic, the insulating tape is coated with shellac varnish. Porcelain and glass are extensively used as overhead line insulators required in transmission and distribution, systems.
Insulators become worse insulators when they are heated, i.e. the resistance they offer to the flow of electricity becomes less, and some, if strongly heated, may even become fairly good conductors. Further, a body may be an insulator when the applied p.d. is low but a conductor when the p.d. is sufficiently high.
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The size of an insulator depends on the following factors:
(i) The size depends on applied p.d. The higher the applied voltage, the greater should be the thickness of insulation.
(ii) The size also depends on the working temperature of the machines or apparatus. With the rise of temperature the resistance of the insulator decreases. The size of insulator should be such as to prevent unusually high leakage of current through it. Also it should not be burnt out or melted at this high working temperature.
(ill) The insulator must be capable of withstanding stress and strain applied on it at the time of use.
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(iv) The insulator must not be corroded by the surrounding atmosphere (such as acid fume).
(v) The insulating materials must have good heat conductivity. The heat generated in the conductors due to flow of electric current should be readily transferred through insulation to the outer surface where there is ventilating air to carry away the heat.