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Lecture 42
Electrical insulating materials are used in various forms to provide insulation for the apparatus.
The insulating materials may be solid, liquid, gas, or even a combination of these such as paper
impregnated with oil. These materials should possess good insulating properties over a wide
range of operating parameters, such as a wide temperature range (O0C to 11O0C) and a wide
frequency range (d.c. to several MHz in the radio and high frequency ranges). Since it is
difficult to test the quality of an insulating material after it forms pan of an equipment, suitable
tests must be done to ensure their quality in the said ranges of operation. Also, these tests are
devised to ensure that the material is not destroyed as in the case of high voltage testing.
These tests are mainly done to assess the electrical properties, such as the resistivity (d.c.), the
dielectric constant, and loss factor over a wide frequency range. In the high voltage apparatus,
the quality of insulation is assessed by measuring the loss factor at high voltages and also by
conducting partial discharge tests to detect any deterioration or faults in the internal insulation
of the apparatus.
These tests may be conducted at a desired temperature or over a temperature range by keeping
the test specimen in controlled temperature ovens. A knowledge of the variation of electrical
properties over the operating range can be obtained from these tests and this will help the design
engineer to take into account such variations in the design of electrical insulation for equipment.
Specimens and Electrodes
The specimen shape and the electrode arrangement should be such that the resistivity can be
easily calculated. For a solid specimen, the preferable shape is a flat plate with plane and
parallel surfaces, usually circular. The specimens are normally in the form of discs of 5 to 10cm
diameter and 3 to 12mm thickness.
Dept. of EEE, NIT-Raichur
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Lecture 42
If the electrodes are arranged to be in contact with the surfaces of the specimen, the measured
resistance will be usually greater due to the surface conductivity effectts. Often, a three
electrode arrangement shown in Fig. 9.1 is used. The electrode which completely covers the
surface of the specimen is called the "unguarded" electrode and is connected to the high voltage
terminal. The third electrode which surrounds the other measuring electrode is connected to a
suitable terminal of the measuring circuit. The width of this "guard" electrode must be at least
twice the thickness of the specimen, and the unguarded electrode must extend to the outer edge
of the guard electrode. The gap between the guarded and guard electrodes should be as small as
possible. The effective diameter of the guarded electrode is greater than the actual diameter and
is given as follows. Let TI, r2, and r be the radii of the guarded electrode, guard electrode
including the gap, and the effective radius of the guarded electrode. Let the gap width = g and
the specimen thickness = t. Then from references 5 and 6,
Dept. of EEE, NIT-Raichur
Page 2