www.easa.com
August 2003
A Closer Look At High Potential Testing
Of Rotating Electrical Machine Windings
By Tom Bishop, P.E.
EASA Technical Support Specialist
A frequent question that arises during repair or
maintenance is how much voltage to apply when
performing a high potential test. The test voltage
for a new winding will be higher than that of a
winding that has been in service, but what is the
appropriate test voltage? Seeking answers to this
question can sometimes lead to confusion. Our
goal in this article is to clarify high potential testing and test voltages.
What is meant by high potential (hipot) testing
is not always clear. The term “high potential testing” is defined by NEMA (MG1-1.57) and IEEE
(IEEE Standard 858) as a test that “consists of the
application of a voltage higher than the rated voltage for a specified time for the purpose of
determining the adequacy against breakdown of
insulating materials and spacings under normal
conditions.” For example, a 5000-volt hipot test
on a motor rated 4000 volts would be considered
a high potential test.
Before proceeding into the selection of a test
voltage we should consider a critical step in hipot
testing. That is the determination of the suitability
of the winding to be hipot tested. An insulation resistance (“megohmmeter”) test is the first step
prior to the hipot process. EASA AR100 and
IEEE 43 recommend the test voltages in Table 1
for determining the insulation resistance of motor
and generator windings.
Table 1-Insulation resistance test voltages
versus machine rated voltages.
Winding rated
voltage
(AC or DC)
Insulation resistance
test DC voltage
<1000
500
1000-2500
500-1000
2501-5000
1000-2500
5001-12000
2500-5000
>12000
5000-10000
Most random and
form coil windings rated
below 1000 volts should
have a minimum insulation resistance of 5
megohms; and for most
form coil windings rated
1000 volts or more the
minimum is 100
megohms. All insulation
resistance values must be corrected to 40º C, and the
reading taken after applying the voltage for 1
minute. In addition to measuring the winding temperature to determine the correction factor, the
humidity should also be measured.
Appy Voltage Gradually
Hipot test voltage should be applied gradually
until the final voltage is attained. The final voltage
should be held for 1 minute before reducing the voltage gradually to zero. An alternative to gradually
increasing the test voltage would be to apply the test
voltage in steps until the final voltage is attained.
The number of steps and length of time to hold the
voltage at each step will vary with machine rating
and is seldom used on machines rated less than 600
volts. A 4000-volt machine might be tested in 10
steps whereas a 13 kV machine might be tested in 30
steps; a 200 hp motor might have the voltage steps
held for 30 seconds while an 8000 hp motor might
have voltage steps held for 2 minutes each. If there is
a current indicator on the test set, the steps should be
held until the current stabilizes.
A Pass-Fail Test
A winding that withstands the final voltage, and
return to zero voltage, is considered to have passed
the hipot test. There are no standards for minimum
insulation resistance for a hipot test. It is a pass-fail
test. An AC hipot does not indicate insulation resistance, and although a DC hipot can indicate
insulation resistance, it is not an acceptance parameter for the test. After performing any hipot test, for
safety purposes, make certain that the winding is
discharged to ground through a resistance circuit
(e.g., the hipot) for a long enough period that the
winding does not recharge. IEEE 43 recommends a
minimum discharge time of 4 times the length of
time the test voltage was applied.
Continued on Page 2
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EASA CURRENTS
High Potential Testing . . . Continued From Page 2
note that all of the above hipot tests apply to new
windings that have been impregnated and fully
cured. Untreated random or form (“green”) coils
must be tested at reduced levels. For form coil windings make certain that the coil supplier agrees to the
test voltages that will be applied to untreated “green”
coils. There are no industry standards for hipot testing of untreated coils. At most, a level of 60% of the
equivalent treated winding test value is suggested.
What test levels do we use for windings that
are no longer new? EASA AR100 states that a
hipot test on a reconditioned winding should be at
65% of the new winding test value. The 4000-volt
machine would be hipot tested at 5850 volts AC,
or 9945 volts DC; or 5750 volts AC and 10000
volts DC in more practical values. It is suggested
in AR100 that windings that have not been reconditioned be insulation resistance tested, and not
hipot tested. That suggestion applies to service
center work, where the subject winding would be
reconditioned prior to a hipot test.
According to NEMA MG1 and EASA AR100, a
new winding should be AC hipot tested at twice
rated voltage plus 1000 volts. If a DC hipot is used,
the AC test voltage is multiplied by 1.7. A new
winding rated 4000 volts would be AC hipot tested
at 9000 volts (2 x 4000 + 1000), or 15300 volts DC
(9000 x 1.7). It is usually more practical to read the
display of a hipot in 250 volt increments, making
15250 a more practical value than 15300. This ultimate test voltage must only be applied once to a
winding. Therefore, the winding should be impregnated and fully cured for this ultimate test.
If a subsequent hipot test is desired or required, the test level should not exceed 85% of the
new winding test. This test is sometimes termed
an acceptance test and is also a one-time test. It is
often applied to the new winding after the motor
or generator is assembled. Using the 4000-volt
rating example, the 85% AC hipot test level
would be 7650 volts, and the DC equivalent
would be 13005 volts. Note that some calculated
values are not practical, and should be modified to
a value that can be read on the hipot. Thus, the
15300 value may be modified to 15250, 7650 to
7750, and 13005 to 13000.
Maintenance Hipot Test
Machines installed at the end user location can
be hipot tested, if desired or required, at 125150% of rated voltage in accordance with IEEE
432. This test is often referred to as a maintenance
hipot test. The 125-150% level applies to an AC
hipot test. The DC hipot equivalent would be 1.7
times the 125-150% levels, or 215-255% of rated
voltage. For the 4000-volt machine used in the
earlier examples, the AC hipot level would be
5000-6000 volts, and the DC hipot level would be
8600-10200 volts (more practical values would be
8500-10250 volts).
Conducting Subsequent Tests
Subsequent hipot tests on a new winding, per
NEMA MG1, should be at 75% of the initial new
winding test value. The 4000-volt machine in the example above would subsequently be AC hipot tested
at 6750 volts, or DC hipot tested at 11475 volts or a
more practical 11500 volts. Note that this test (75%
level) applies to all tests after the initial (100% level)
test and the one subsequent (85% level) test. Also
Table 2: Hipot tests and test levels for rotating electrical machines.
Type of test
AC test level
DC test level
Test limitations
New winding
Ultimate test
(2 x rated voltage)
+ 1000
1.7 x [(2 x rated
voltage) + 1000]
One time
Acceptance test
.85 x ultimate test
.85 x ultimate test
One time
Subsequent tests
.75 x ultimate test
.75 x ultimate test
Can be repeated
Reconditioned winding
Reconditioning test
.65 x ultimate test
.65 x ultimate test
Can be repeated
In-service winding
Installation test
1.25-1.50 x rated
voltage
2.15-2.55 x rated
voltage
Can be repeated
Continued on Page 3
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August 2003
High Potential Testing . . . Continued From Page 2
Table 3: Hipot tests and test levels, modified to practical values, for a 4000-volt machine.
Type of test
AC test level
DC test level
Test limitations
New winding
Ultimate test
9000
15250
One time
Acceptance test
7750
13000
One time
Subsequent tests
6750
11500
Can be repeated
Reconditioned winding
Reconditioning test
5750
10000
Can be repeated
In-service winding
Installation test
5000-6000
As all of the above stated test values suggest,
the appropriate test level determination takes
some thought and understanding of the type of
hipot test that will be applied. For simplicity and
consistency, we chose a 4000-volt machine rating
and used it in each hipot example. Table 3 further
illustrates this point by indicating the test voltages
for each type of test. Whenever you are considering a specific hipot test, calculate the test voltage
to be applied for the rated voltage of the motor or
generator to be tested. Then calculate the test level
using a 4000-volt rating as a check to verify that it
matches the example given here. That will confirm that your hipot test is at the correct level,
before the test is actually applied. If the planned
test level is too low, the test will have to be repeated, and if the test level is too high, the
winding may fail due to the overpotential.
What test levels do we use for DC motors and
generators? The nameplate rated voltages,
whether AC or DC, for the windings to be tested
are used in calculating the hipot test level. For example, an armature rated 500 volts DC would be
8500-10250
Can be repeated
AC hipot tested at an ultimate test level of 2000
volts (2 x 500 plus 1000).
A Note Of Caution
There is a precaution to keep in mind about
hipot testing. The main purpose of a hipot test is to
detect weaknesses in the insulation structure that
could lead to an in-service failure. The application
of overpotential carries the inherent risk that the
winding may fail the hipot test, and would probably need to be rewound. The owner of the motor
or generator to be tested should be willing to accept
that risk; otherwise, hipot testing is not advisable.
As one hipot testing expert commented after a
winding failed while under test: “The winding was
faulty and we proved it.” Table 2 indicates that
some hipot tests can be repeated; however, the frequency of repetition should be extended as far as
possible. Hipot testing is a circumstance where
more is not better. Overpotential testing can leave
residual dielectric stresses in the winding that could
lead to premature failure. Hipot test as necessary to
verify winding suitability for continued service, but
don’t “overtest.”
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