Page 143 - WCS - Electrical

WORKSHOP CALCULATION & SCIENCE - CITS

Insulating materials

Description
These are the materials which offer very high resistance to the flow of current and make current flow very negligible
or nil. These materials have very high resistance - usually of may megohms (1 Megohm = 106 ohms) are centimetre
cubed. The insulators should also posseses high dielectric strength. This means that the insulating material should
not break down or puncture even on application of a high voltage (or high electrical pressure) to a given thickness.
Properties of insulators

The main requirements of a good insulating material are:
• High specific resistance (many megohms/cm cube) to reduce the leakage currents to a negligible value.
• Good dielectric strength i.e. high value of breakdown voltage (expressed in kilovolts per mm).
• Good mechanical strength, in tension or compression (It must resist the stresses set up during erection and under
working conditions.)
• Little deterioration with rise in temperature (The insulating properties should not change much with the rise in
temperature i.e. when electrical machines are loaded.)

• Non-absorption of moisture, when exposed to damp atmospheric condition. (The insulating properties, specially
specific resistance and dielectric strength decrease considerably with the absorption of even a slight amount of
moisture.)
Products and insulators

Insulators Uses in electric field
Mica In elements or winding (Slot insulation)
Rubber Insulation in wires
Dry cotton Winding

Varnish Winding
Asbestos In the bottom of irons and kettles, etc.
Gutta parcha Submarine cables
Porcelain Overhead lines insulators

Glass -do-
Wood dry Cross arms in overhead lines
Plastic Wires insulation or switches body
Ebonite Bobbin of transformer

Fibre Bobbin making and winding insulation
Empire cloth Winding insulation
Leathroid paper -do-
Millimax paper -do-
P.V.C. Wire insulation

Bakelite switch etc. making, for insulation
Shellac -do-
Slate Making panel board
Paraffin wax Sealing

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CITS : WCS - Electrical - Exercise 11