a gap of 0.04 to 0.08 in. (1–2 mm) with a rate of voltage rise of 500 V/s. The disk
cup is used for mineral-based oils and synthetic liquids such as askarel,
silicone, and others. It has a gap of 0.1 in. with rate of rise of 3000 V/s. The
step-by-step procedures for conducting these tests are described next.
Dielectric test ASTM D-877 (disk electrodes): Portable oil dielectric testers are
usually used for making dielectric tests on oils in the fi eld. Units with a variable
high voltage of 40 kV or greater between the electrodes and which have
Bakelite test cups are considered satisfactory. Instructions and procedures
are as follows:
The electrodes and the test cup should be wiped clean with dry, calendered
tissue paper or with a clean, dry chamois. The spacing of the electrodes
should be checked with a standard round gauge having a diameter of
0.1 in. (2.5 mm) or with fl at steel go and no-go gauges having thicknesses of
0.0995 and 0.1005 in., respectively; the electrodes should be locked in position.
It is important to avoid touching the electrodes or the cleaned gauge
with the fi ngers or with portions of the tissue paper or chamois that have
been in contact with the hands.
The electrodes and test cup should be rinsed with dry, lead-free gasoline
or other suitable solvent until they are entirely clean. To avoid any possible
contamination, care should be taken to avoid touching the electrodes or
the inside of the cup after cleaning.
After a thorough cleaning, the test cup is fi lled with a sample of the cleaning
fl uid; voltage is applied and uniformly increased at a rate of approximately
3 kV/s (rms value) until breakdown occurs. If the breakdown is not less than
the established value of the oil being tested, the test cup should be considered
in suitable condition for testing. If a lower value is obtained, the cup
should again be thoroughly cleaned and the test repeated. A cleaning fl uid
whose breakdown is not less than the established value of the oil being tested
must be used.
At the beginning of each test, the electrodes should be examined for
pitting and carbon accumulation and the electrode spacing should be
checked. The test cup should be thoroughly cleaned and tested as described
previously. It should then be fl ushed with a portion of the sample to be tested
before it is fi lled for the test.
If the test of a sample is below the breakdown value being used by the operator
as a minimum satisfactory value, the cup should be cleaned and prepared
before testing the next sample. Evaporation of the cleaning fl uid from the electrodes
may chill them suffi ciently to cause moisture to condense on their surfaces.
For this reason, after the fi nal rinsing with cleaning fl uid, the cup must
immediately be fl ushed with the oil to be tested and then fi lled for the test.
The dielectric strength of liquid dielectrics may be markedly altered by the
migration of impurities through the liquid. To obtain representative test
specimens, the sample container should be gently tilted or inverted and the
oil swirled several times before each fi lling of the test cup, in such a way that
any impurities present will be thoroughly mixed with the liquid dielectric.