What is dielectric strength?
Dielectric Strength is defined by the
ASA as the maximum potential gradient that the material can withstand
without rupture. Practically, the strength is often reported as the
breakdown voltage divided by the thickness between electrodes,
regardless of electrode stress concentration.
Breakdown appears to require not only
sufficient electric stress but also a certain minimum amount of
energy. It is a property which varies with many factors such as
thickness of the specimen, size and shape of electrodes used in
applying stress, form or distribution of the field of electric stress
in the material, frequency of the applied voltage, rate and duration
of voltage application, fatigue with repeated voltage applications,
temperature, moisture content, and possible chemical changes under
stress.
The practical dielectric strength is
decreased by defects in the material, such as cracks, and included
conducting particles and gas cavities. As will be shown in more
detail in later sections on gases and liquids, the dielectric
strength is quite adversely affected by conducting particles.
To state the dielectric strength
correctly, the size and shape of specimen, method of test,
temperature, manner of applying voltage, and other attendant
conditions should be particularized as definitely as possible.
ASTM standard methods of dielectric
strength testing should be used for making comparison tests of
materials, but the levels of dielectric strength measured in such
tests should not be expected to apply in service for long times. It
is best to test an insulation in the same configuration in which it
would be used.
Also, the possible decline in
dielectric strength during long-time exposure to the service
environment, thermal aging, and partial discharges (corona), if they
exist at the applied service voltage, should be considered. ASTM has
thermal life test methods for assessing the long-time endurance of
some forms of insulation such as sheet insulation, wire enamel, and
others.
There are IEEE thermal life tests for
some systems such as random wound motor coils. The dielectric
strength varies as the time and manner of voltage application.
With unidirectional pulses of voltage,
having rise times of less than a few microseconds, there is a time
lag of breakdown, which results in an apparent higher strength for
very short pulses. In testing sheet insulation in mineral oil,
usually a higher strength for pulses of slow rise time and somewhat
higher strength for dc voltages is observed.
The trend in breakdown voltage with
time is typical of many solid insulation systems. With ac voltages,
the apparent strength declines steadily with time as a result of
partial discharges (in the ambient medium at the conductor or
electrode edge). These penetrate the solid insulation.
The discharges result from breakdown of
the gas or liquid prior to the breakdown of the solid. Mica in
particular, as well as other inorganic materials, is more resistant
to such discharges. Organic resins should be used with caution where
the ac voltage gradient is high and partial discharges (corona) may
be present.
Since the presence of partial
discharges on insulation is so important to the longtime voltage
endurance, their detection and measurement have become very important
quality control and design tools.
If discharges continuously strike
the insulation within internal cavities or on the surface, the time
to failure usually varies inversely as the applied frequency, since
the number of discharges per unit time increases almost in direct
proportion to the frequency. But in some cases, ambient conditions
prevent continuous discharges.
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