Hi-Potting is the most dangerous of all acceptance and general maintenance tests
performed. It must be performed by qualified, experienced personal. This test is
however, one of the most important to be performed. The tester must be well trained in
the use of his own equipment.
Southern Substation, Inc., technicians use only Hypotronics or Associated Research
Test equipment. Each man is well trained in the use of this equipment and takes all
necessary safety precautions before applying high voltage to the equipment under test.
A) A High Potential test is a voltage applied across an insulation at or above the DC
equivalent of the 60 Hertz operating crest voltage. This test can be applied as a
dielectric absorption test, or a step voltage test. When applied as a dielectric absorption
test the maximum voltage is applied generally over a period of from sixty to ninety
seconds. The maximum voltage is then held for five minutes with leakage current
readings being taken each minute. When applied as a step voltage test, the maximum
voltage is applied in a number of equal increments, usually not less than eight with each
voltage step being held for an equal interval of time. The time interval between steps
should be long enough to allow the leakage current to reach approximate stability
usually one or two minutes. A leakage current reading is taken at the end of each
interval before the voltage is raised to the next level. A plot of the test voltage versus
insulation resistance is drawn as the test progresses. After the maximum test voltage is
reached, a dielectric absorption test may be performed at that voltage, usually for a five
minute period.
NOTE: The step voltage method is the best, as a lot may be drawn during each test.
If this test is performed properly over a period of years, the cable deterioration will be
monitored and a failing cable can be removed from service during regular maintenance
periods. This will eliminate a cable fault during a production period saving lost time and
large sums of money.
B) The maximum permissible test voltage for acceptance tests performed on cables
listed in the Insulated Power Cable Engineers Association (IPCEA) standards for rubber,
thermo-plastic and varnished cloth insulation and in the Association of Edison
ILLuminating Companies (AEIC) standards for solid type impregnated paper insulation.
Also each cable manufacturer supplies its own recommendation for testing of their
respected cables. Ordinarily, routine maintenance tests are conducted with a maximum
test voltage at or below 75% of the maximum test voltage permitted for acceptance
testing.
C) Extreme care must be taken in choosing the appropriate test voltage for routine
maintenance tests on cables which have been in service for long periods. If the level
selected is too low, marginal weak spots may not be revealed, if the level selected is too
high, damage to the insulation may result. Further, if it is inconvenient or impossible to
disconnect switchgear, instrument transformers or cutouts from the test circuit, it may be
necessary to reduce the maximum voltage to the level that this equipment can withstand
without damage. Once again, I must say please use only the highest qualified people for
this test.
Prior to testing, Lighting arresters must be removed from the test circuit and wherever
practical, all instrument transformers, switches, cutouts and switchgear, so that if
significant leakage currents are encountered during the test it will be known that these
currents represent losses in the cable, not in the associated equipment. The test
voltage should be applied from phase to phase and phase to ground on each
conductor with the other conductors, the shields, and the metallic jackets also
connected to the ground. The DC and AC (RMS) test voltage ratios ordinarily used are
as follows:
TYPE CABLE INSULATION RATIO
Rubber, or Rubber-like, Ozone Resisting 3.0 to 1
Rubber, or Rubber-like, other Ozone Resisting 2.2 to 1
Impregnated Paper, Solid Type 2.4 to 1
Varnished Cloth 2.0 to 1
Polyethylene 3.0 to 1
D) When the step voltage type testing is used, the condition of the cable is evaluated
on the basis of:
1. The absolute values of insulation resistance
2. The slope of the curve voltage versus insulation resistance
3. Whether or not a significant downward "knee" appears in the curve at higher levels of
test voltage. Once again I must state that the step voltage method is the best to use.
AC TESTING
AC High Potential Tests are made at voltages above the normal system voltage for a
short time, such as one minute. The test voltages to be used vary depending upon
whether the device or circuit is low or high voltage, a primary or control circuit and
whether tested at the factory or in the field. In a test such as this, only the
manufacturers instructions and his applicable standards should be used to obtain the
proper values. Southern Substation, Inc., does not perform any AC High voltage testing
in the field and does not recommend that it be done.
High Potential Testing
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