MIL-PRF-27G
4.7.9.2 At barometric pressure. Transformers and inductors designed for operation above 10,000 feet shall be
tested as specified in 4.7.9.1 and in accordance with method 105 of MIL-STD-202. The following details and
exceptions shall apply:
a.
Test condition or altitude in feet if below 30,000 feet - As specified (see 3.1).
b.
Magnitude of test voltage - As specified in table XII. A greater potential may be used at the option of the
supplier.
c.
Examination during and after test - Transformers and inductors shall be examined for evidence of
arcing, flashover, breakdown of insulation, and damage.
TABLE XII. Dielectric withstanding voltage at reduced barometric pressure.
Working voltage 1/
Test voltage
Volts
Volts, rms
≤ 50 - - - - - - - - - - - - - - - - - -
100
> 50 - - - - - - - - - - - - - - - - - -
300 or 1.25 x working voltage:
whichever is greater
1/ The working voltage is defined as the maximum instantaneous (peak) voltage stress that may appear under
normal rated operation across the insulation being considered. This insulation may be between windings or
between a winding and the case or core.
4.7.9.2.1 For special designs. When the induced-voltage test is used (see 4.7.9.1.1), it shall be performed at an
atmospheric pressure equivalent to the altitude for which the transformer or inductor is rated, and the test voltage shall
be sufficient to cause 125 percent of rated voltage to appear across the winding (see 3.1 and 4.7.10).
4.7.9.3 At reduced voltage. Transformers and inductors shall be subjected to the dielectric-withstanding voltage
tests specified in 4.7.9.1, except that the test voltages shall be 75 percent (65 percent for life test) of the values shown
in table XII and shall be applied for a period of 5 seconds.
4.7.10 Induced voltage (see 3.12) (this test is applicable when any winding has a rated voltage in excess of 25
volts rms). Transformers and inductors shall be subjected to the tests specified in 4.7.10.1 through 4.7.10.3, as
applicable. During this test, the transformers and inductors shall be examined for evidence of continuous arcing,
breakdown of insulation, and abrupt changes in the input current. Means shall be provided to indicate fluctuations of
input current or changes in Q, as applicable.
4.7.10.1 All transformers and inductors except pulse, sonar and saturating core power (see 3.1). Transformers
and inductors shall be subjected to a voltage sufficient to cause twice the rated voltage to appear across any winding.
The test voltage shall be applied to any winding. Windings should be grounded as they would be in service. The test
frequency shall be as selected by the manufacturer and shall be remote from any resonant frequency. The test
potential shall be applied for 7,200 ±200 cycles, or 5 ±1/2 seconds, whichever is greater for qualification testing. For
quality conformance inspection, the test potential shall be applied for 5 ±1/2 seconds.
4.7.10.2 Pulse and sonar transformers and inductors. A test pulse voltage shall be applied for 1 minute to any
winding at the specified rated repetition rate (see 3.1) sufficient to induce a voltage across any winding between 25 and
50 percent of the rated pulse width, in accordance with table XIV, and shall be performed in air. For grade 4
transformers and inductors, this test shall be repeated for units rated at greater than 10,000 volts at twice rated voltage
with the transformer or inductor terminals under oil. At the option of the manufacturer, the test in air for grade 4
transformers and inductors may be made at twice the rated voltage, in which case the second test under oil will not be
required. During the test, the transformer and inductor shall be loaded as specified and fitted with specified protective
devices (see 3.1). The operation of any specified protective device shall not be a cause for failure. In the case of a
magnetron load, a resistance equivalent to the static resistance of the magnetron can be used.
TABLE XIV. Induced voltage for pulse and sonar transformers and inductors.
Max. voltage rating on highest voltage winding
Induced voltage
≤ 10,000 - - - - - - - - - - - - - - - - - - - - - - - - - - -
Twice rated voltage
> 10,000 to 20,000 incl. - - - - - - - - - - - - - - - -
1.666 x rated voltage
> 20,000 to 35,000 incl. - - - - - - - - - - - - - - - -
1.333 x rated voltage
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