MIL-PRF-27G
(2) Grades 5 and 6 - Upon completion of step 6 of the final cycle, transformers or inductors shall be
removed from the humidity chamber, insulation resistance shall be measured at any temperature
above 20°C and at ambient room humidity, but rejections shall be based on measurements at
25°C +10°C, -5°C and at a relative humidity not greater than 80 percent.
g.
Visual examination - Transformers and inductors shall be examined for any visible damage including
corrosion and obliteration of marking.
4.7.21 Overload (see 3.23). The overload test shall be performed for a period of 48 0, +2 hours for qualification
inspection. When transformers and inductors attain an operating temperature of less than the maximum specified for
the class during the temperature-rise test specified in 4.7.13, the ambient temperature for the overload test shall be
increased above that specified to a value that results in an operating temperature equivalent to the maximum specified
for the class (see 6.7). The overload shall be applied as specified in 4.7.21.1 and 4.7.21.2, as applicable. At the
option of the manufacturer, the test may be performed at 60 Hz for transformers and inductors rated at 50/60 Hz and at
400 Hz for transformers and inductors rated at 360/400 Hz provided that the primary voltage is increased to 1.2 times
the rated voltage for 50 Hz units and 1.1 times the rated voltage for 360 Hz units and the secondary currents are
maintained at rated current before applying the overload as specified in 4.7.21.1 and 4.7.21.2. At the conclusion of the
test, all transformers or inductors shall be examined for leakage or other visible damage. Transformers or inductors
shall be allowed to cool for approximately 8 hours at standard test conditions (see 4.3) before any additional tests are
performed.
4.7.21.1 Transformers.
4.7.21.1.1 Output of less than 0.8 watt. Transformers rated at less than 0.8-watt output (except grade 4) shall
be tested with 112 percent of rated ac and dc voltages applied, with no load, and at an ambient temperature equivalent
to the maximum operating temperature for the class. The test shall not be performed on grade 4 transformers of this
electrical rating.
4.7.21.1.2 Output of 0.8 watt or greater. For transformers rated at 0.8-watt output or greater, the rated voltage at
the minimum frequency of the specified frequency range shall be applied at the rated duty cycle to the primary winding
and with rated load connected to the secondary to set the load impedances. The input voltage shall then be raised to
112 percent of the rated voltage. When applicable, rated dc current shall also be applied during the test.
4.7.21.2 Inductors.
4.7.21.2.1 Rated power less than 0.2 watt. Inductors for which the product of the dc resistance and the square of
the rated current is less than 0.2 watt (except grade 4) shall be tested with rated ac and dc voltages applied at an
ambient temperature equivalent to the maximum operating temperature for the class. The test shall not be performed
on grade 4 inductors of this electrical rating.
4.7.21.2.2 Rated power 0.2 watt or greater. Inductors for which the product of the dc resistance and the square of
the rated current is equal to, or greater than 0.2 watt at the minimum frequency at the specified frequency range shall
have 112 percent of all rated dc currents and ac voltages applied at the rated duty cycle.
4.7.21.3 Saturable core devices. Saturable core devices shall be tested as required by 4.7.21.2.1 or 4.7.21.2.2, as
applicable, except that the load requried shall be adjusted to 112 percent of rated current output.
4.7.22 Flammability (grade 5) (see 3.25). Transformers and inductors shall be tested in accordance with method
111 of MIL-STD-202. The following details and exceptions shall apply:
a.
Point of impingement of applied flame - One of the lower free corners, so that the flame is just in
contact with the transformer or inductor. The free corners of the transformer or inductor are those
corners which are the greatest distance from the mounting brackets. However, the flame shall be
applied so that it will impinge upon the corner or area containing the encapsulating compound.
b.
Allowable time for burning of visible flame on specimen - 3 minutes maximum.
c.
Examinations during and after test - Transformers and inductors shall be examined for evidence of
violent burning which results in an explosive-type fire, dripping of flaming material, and visible burning
which continues beyond the allowable duration after removal of the applied flame.
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