42
Materials
and
their applications
in the
electrical
industry
increases with temperature (7
R
increases). As ò increases, the loss
increases, giving rise to more heat and further temperature rises. It
can be envisaged that this could lead to failure if the temperature
were allowed to rise too far. This is particularly important in very
high voltage cables where the heat produced by this effect becomes
very nearly as great as that due to conduction losses in the conductor
(PR losses). See also page 50, 'Heating of cables'.
Breakdown voltage If a sufficiently high voltage is applied to any insulating material it
will break down and become conducting. Particular materials have
uses as dielectrics for a particular range of voltages. The breakdown
strength of a dielectric depends on its thickness but is not pro-
portional to it. This means that if a sample of insulating material is
subjected to an increasing voltage it will eventually puncture and <