MIL-PRF-27G
4.7.9 Dielectric withstanding voltage (see 3.11 and 6.4). Transformers and inductors shall be tested in accordance
4.7.9.1 At atmospheric pressure. Transformers and inductors shall be tested in accordance with method 301 of
MIL-STD-202. The following details and exceptions shall apply:
a.
Magnitude of test voltage - As specified in table XI for transformer and inductor windings not grounded
internally. However, where there is a high-voltage center-tap winding and another adjacent winding
operated at a dc potential, the peak voltage that may be present between the windings should be
considered in computing the test voltages. Test voltages greater than 1,000 volts rms shall be applied
gradually at a rate not exceeding 500 volts rms per second.
b.
Nature of potential - AC.
c.
Duration of application of specified test voltage - Minimum of 5 seconds for quality conformance
inspection, 1 minute for qualification inspection.
d.
Points of application of test voltage:
(1) Winding to case or core - Between each winding and the case or core with all windings not under
test grounded to the case (if cased) or to the normal mounting means (if uncased) and to the
core (if accessible).
(2) Between windings - The voltage shall be applied between each winding and each of the other
windings with all windings not under test grounded to the case (if cased) or to the normal
mounting means (if uncased) and to the core (if accessible). These tests need not be made if
the winding-to-case or -core test voltage of either winding under consideration is equal to, or
greater than, the winding-to-winding test voltage. The method used to perform the
between-windings dielectric-withstanding voltage test shall consist of two sources of test voltage,
so proportionated and phased, that the winding-to-winding test voltages shall be according to
table XI. One terminal of each source shall be grounded to the case (if cased), or to the normal
mounting means (if uncased), and to the core (if accessible). The test voltages applied shall not
exceed the test voltages required for each of the windings to ground, and shall be applied so that
the required test voltage appears between the windings. Multiple-section windings designed for
operation only in series or parallel shall be considered as a single winding. In no case shall the
test voltage applied between the windings exceed the sum of the test voltages for each of these
windings to the case (if cased) or to the normal mounting means (if uncased), and to the core (if
accessible). An acceptable alternate test method is to connect one voltage source terminal to a
winding and the other terminal to the remaining winding(s).
e.
High voltage source shall have a minimum of 1 kilovolt-ampere capacity for voltages over a kilovolt.
f.
Examination during and after test - Transformers and inductors shall be examined for evidence of
arcing, flashover, breakdown of insulation, and damage.
TABLE XI. Dielectric withstanding voltage at atmospheric pressure.
Working voltage 1/
RMS test voltage (at 28 to 32 inches of
mercury)
≤
100
50 - - - - - - - - - - - - - - - - - - - -
>
300
50 to 100 incl. - - - - - - - - - - - -
>
500
100 to 175 incl. - - - - - - - - - - -
2.8 x working voltage
>
175 to 700 incl. - - - - - - - - - - -
1.4 x working voltage, +1,000
>
700 - - - - - - - - - - - - - - - - - - - -
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.
4.7.9.1.1 For special designs. Transformer and inductor windings internally grounded or having any part of the
winding designed for operation at or near ground potential shall be subjected to the induced-voltage test or a
combination of the dc dielectric withstanding voltage on the low-voltage terminal together with induced voltage, as
applicable (see 4.7.10). Windings with special dielectric features (e.g., graded insulation) shall be subjected to the test
voltage specified (see 3.1), or to the induced-voltage test (see 4.7.10). Dielectric withstanding voltage tests for pulse
transformers shall be as specified (see 3.1).
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