what is meant by dielectric breakdown ? where we using HV test kit & what

what is meant by dielectric breakdown ? where we using HV
test kit & what purpose?

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what is meant by dielectric breakdown ? where we using HV test kit & what purpose?..

High-voltage transformers contain oil as their insulating
dielectric. When a critical electric field is exceeded,
conduction paths grow at microsecond speeds through the oil,
in the form of branched trees, called streamers. These can
lead to destructive breakdown.

Above a particular electric field, known as the dielectric
strength Eds, the dielectric in a capacitor becomes
conductive. The voltage at which this occurs is called the
breakdown voltage of the device, and is given by the product
of the dielectric strength and the separation between the
conductors

The high voltage test (also called dielectric strength test
or hipot test) can be made in AC or DC. If the high voltage
test is made in DC, it is then combined with insulation ;
if the high voltage test is made in AC, it is then, this is
then, more stressful for the sample and made according to
the sketch below.

Measurement of high voltage test under alternating current
is performed using an alternating voltage (50Hz) adjustable
to an effective 50V to 1,500V. As is the case with direct
current, the high voltage test detects any sudden rise of
current up to a programmed threshold.

http://www.sefelec.com/en/high-voltage-test.php

purpose
to test the insulators for their effectiveness. normally
used for transformer oil testing purpose.