Specifications

7
chokes and transformer coils etc. Silk is more expensive than cotton but takes up less space
and is therefore used for windings in fractional horse power machines.
Silk is less hygroscopic and has a higher dielectric strength than cotton, but like cotton it
requires impregnation. The operating temperature of cotton and silk is 100
0
C and the material
may catch fire above this temperature.
Rubber : Rubber is obtained by vulcanizing raw rubber [natural or synthetic]. Ordinary electrical
insulating rubbers, have the following electrical characteristics under normal conditions :
The electric strength of organic rubbers strongly depends on the kind of current involved,
the degree of stretch, and the time during which the voltage remains applied. When left
unstretched and subjected to a short-time 50 Hz test voltage, rubber will have an electric strength
within the following limits, depending on the pure-rubber constant.
For a 20 to 25 % rubber content………..20 to 30 kV/mm
For a 30 to 35 % rubber content………..30 to 45 kV/mm
The di electric strength of rubber is 2 to 2.5 times the electric strength at 50 Hz.
Although rubber is practically water and gas tight its electrical characteristics are affected
by moisture, especially for rubbers compounded with considerable quantities of the substances
which increases the sensitivity to moisture. Only specially compounded rubbers can maintain
their electrical characteristics nearly unchanged when kept continuously in contact with moisture.
Normally used as seals, gaskets and washers.
For normal rubbers the maximum operating temperature is usually 55
0
C, for rubbers of
great heat resistance it is 65
0
C. For butyl rubbers the working temperature can be as high as
90
0
C. Rubbers possess a limited post resilience and at sufficiently low temperatures become
brittle.
Silicon rubbers : They have high electrical insulating properties, heat resistance, frost resistance,
moisture resistance, as well as resistance to ozone and light. These rubbers can be produced
as adhesive tapes [lined with a layer of vulcanized rubber] suitable for insulating the windings of
high-voltage electrical machines. Those tapes also serve to insulate the terminal leads of electrical
machines designed for high temperature rise. Silicon rubbers retain their flexibility at temperature
as low as -10
0
C. One of their drawbacks is relatively low mechanical strength, another is high
cost.
Insulating fabrics : Base materials for insulating fabrics include natural fibres such as cellulose,
cotton and silk; synthetic organic fibres of, for example cellulose derivatives, polyamides [nylon],
polyethylene tarepathalates; and inorganic fibres, chiefly glass and asbestos. Non-woven synthetic
organic fibres are usually bonded into a fabric by use of a bonding resin or by fusion. They find
electrical use chiefly as a base for resin-impregnated insulation.
Unimpregnated woven fabrics find some limited use in electrical insulation. The electric
strength of such fabrics generally does not exceed the breakdown strength of an equivalent air
gap and indeed be less. Their chief use, therefore, is to provide mechanical strength, abrasion