INSULATION RESISTANCE METER TESTS
15. General.
- The insulation resistance
meter test method for determining the
condition of electrical insulation has been
widely used for many years as a general
nondestructive test method. A serious
limitation of this test is that its operating
voltage of 500 to 1,000 volts will not always
detect insulation punctures, whereas the
higher voltages used by the high-voltage, dc
testers will detect these punctures.
The insulation resistance meter test will
show (a) the relative amount of moisture in
the insulation, (b) the leakage current over
dirty or moist surfaces of the insulation, and
(c) winding deterioration or faults by means
of insulation resistance versus time curves.
condition of electrical insulation has been
widely used for many years as a general
nondestructive test method. A serious
limitation of this test is that its operating
voltage of 500 to 1,000 volts will not always
detect insulation punctures, whereas the
higher voltages used by the high-voltage, dc
testers will detect these punctures.
The insulation resistance meter test will
show (a) the relative amount of moisture in
the insulation, (b) the leakage current over
dirty or moist surfaces of the insulation, and
(c) winding deterioration or faults by means
of insulation resistance versus time curves.
16. Description of Test.
- A dc voltage of
500 or 1,000 volts is applied to the insulation
and readings are taken to the insulation
resistance versus time. Data should be
recorded at the 1-and 10-minute intervals
and at several other intermediate times.
and readings are taken to the insulation
resistance versus time. Data should be
recorded at the 1-and 10-minute intervals
and at several other intermediate times.
17. Test Equipment.
- The hand-cranked
insulation resistance meter has been the
standard instrument for many years for
checking insulation resistance. The hand-
cranked instrument is satisfactory for "spot
checks" but is not recommended for routine
dielectric absorption tests, because very few
men can continue cranking for 10 minutes
without tiring and slowing up the cranking
speed toward the end of the period. Motor
driven or electronic insulation resistance
testers operating from a 115-volt, ac source
or a self-contained battery are available and
should be used for this purpose. Because
the value of insulation resistance varies with
applied voltage, it is important that the test
instrument have sufficient capacity to
maintain its rated output voltage for the
largest winding being tested, and the output
voltage be constant over the 10-minute test
period. For this reason, some of the smaller
test instruments may not be suitable for
tests on large generators or transformers
which draw a large dielectric absorption
current. For occasional checks on the
calibration and proper function of insulation
test instruments, it is recommended that a
standard instrument for many years for
checking insulation resistance. The hand-
cranked instrument is satisfactory for "spot
checks" but is not recommended for routine
dielectric absorption tests, because very few
men can continue cranking for 10 minutes
without tiring and slowing up the cranking
speed toward the end of the period. Motor
driven or electronic insulation resistance
testers operating from a 115-volt, ac source
or a self-contained battery are available and
should be used for this purpose. Because
the value of insulation resistance varies with
applied voltage, it is important that the test
instrument have sufficient capacity to
maintain its rated output voltage for the
largest winding being tested, and the output
voltage be constant over the 10-minute test
period. For this reason, some of the smaller
test instruments may not be suitable for
tests on large generators or transformers
which draw a large dielectric absorption
current. For occasional checks on the
calibration and proper function of insulation
test instruments, it is recommended that a
resistor in the 100-megohm range be
attached to the inside of the instrument
cover for use as checking standard. It is
recommended that the same test instrument
be used for each periodic test on a certain
piece of equipment, as differences in
instrument output characteristics may affect
the shape of the dielectric absorption
curves, especially at the lower end.
attached to the inside of the instrument
cover for use as checking standard. It is
recommended that the same test instrument
be used for each periodic test on a certain
piece of equipment, as differences in
instrument output characteristics may affect
the shape of the dielectric absorption
curves, especially at the lower end.
18. Dielectric Absorption Curve.
-
Insulation resistance is not a definite
measure of the voltage an insulation will
withstand, but when properly interpreted
affords a useful indication of the suitability of
the winding for continued service. It should
be remembered that values of insulation
resistance, even on identical machines and
for identical conditions, may vary over a
wide range. Changes occurring in insulation
resistance are more significant than certain
absolute magnitudes. This is shown by the
slope of the curve plotted between
resistance readings and time, as further
discussed under
measure of the voltage an insulation will
withstand, but when properly interpreted
affords a useful indication of the suitability of
the winding for continued service. It should
be remembered that values of insulation
resistance, even on identical machines and
for identical conditions, may vary over a
wide range. Changes occurring in insulation
resistance are more significant than certain
absolute magnitudes. This is shown by the
slope of the curve plotted between
resistance readings and time, as further
discussed under
paragraph 10
, "Effect of
moisture." This curve is called the curve of
dielectric absorption. The test voltage should
be applied for a standard period of 10
minutes, with readings taken at intervals of 1
minute or less. Any such curve which
reaches a constant and lower than normal
value in about 3 minutes or less, indicates
high leakage current (due to the leakage
current being large in proportion to the
absorption current), and the winding should
be thoroughly cleaned and retested or
further investigated. Such cleaning should
preferably precede all insulation resistance
tests. In case of very damp insulation, the
dielectric absorption curve may start upward
and then droop to a value lower than at the
start of the test.
dielectric absorption. The test voltage should
be applied for a standard period of 10
minutes, with readings taken at intervals of 1
minute or less. Any such curve which
reaches a constant and lower than normal
value in about 3 minutes or less, indicates
high leakage current (due to the leakage
current being large in proportion to the
absorption current), and the winding should
be thoroughly cleaned and retested or
further investigated. Such cleaning should
preferably precede all insulation resistance
tests. In case of very damp insulation, the
dielectric absorption curve may start upward
and then droop to a value lower than at the
start of the test.
19. Minimum Values of Machine In-
sulation Resistance
sulation Resistance
.-"Recommended
Practice for Testing Insulation Resistance of
Rotating Machinery," IEEE Standard No. 43,
November 1974, indicates the
recommended minimum insulation
resistance R
Rotating Machinery," IEEE Standard No. 43,
November 1974, indicates the
recommended minimum insulation
resistance R
m
for armature and field
windings of ac and dc machines can be
determined by the equation:
determined by the equation:
7 (FIST 3-1 12/91)