MIL-DTL-28778A
4.6.8.2 Secondary number 2. Test for intra-winding insulation with the following sequential
conditions, increase output voltage to 1530 VRMS. If no corona is present, then the unit is acceptable. If corona
occurs, reduce output voltage until corona is extinguished. Extinction voltage shall be 1310 VRMS minimum. The
test shall be performed 3 times, one with each of terminals S1, S2 and S3 grounded in turn.
shall be performed on transformers rated at more than 0.8-watt average output. The temperature rise of each
winding shall be based on the change-in-resistance method and shall be computed by the following formula:
T = (R-r)/r * (t + 234.5) (T-t)
T = Temperature rise (in ēC) above specified maximum ambient temperature (see 3.13)
Where:
R = Resistance of winding (in ohms) at temperature (T + T)
r = Resistance of winding (in ohms) at temperature (t).
t = Specified initial ambient temperature (in ēC).
T = Maximum ambient temperature (in ēC) at time of power shutoff. T shall not differ from t
by more than 5ēC.
The transformers shall be conditioned for at least 8 hours at temperature (t) in a location free from drafts before
resistance (r) is measured. For transformers, rated voltage shall be applied to the primary with the specified loads
across the secondaries. Transformers shall be operated until two consecutive resistance readings on the highest
resistance winding, taken 30 minutes apart, are the same. If the power is required to be shut off, the resistance
measurements (R) shall be made as soon as possible. The transformers shall then be examined for evidence of
physical damage. At the option of the supplier, the test may be performed at 60 Hz for transformers rated at 50/60
Hz provided that the primary voltage is increased to 1.2 times the rated voltage and the secondary currents are
maintained at rated current.
4.6.9.1 Oil leaks. After enduring the temperature rise test, the transformer shall be checked for any
oil leaks.
4.6.10 Winding continuity (see 3.14). All windings of transformers shall be tested for electrical
continuity by any suitable means.
4.6.11 Switches. The switches shall be tested in accordance with the following:
4.6.11.1 Thermostatic switch. The thermostatic switch shall be tested to assure opening at the
specified temperature.
4.6.11.2 Oil level switch. The oil level switch shall be tested to assure that it opens when oil level
drops below the specified level.
4.6.11.3 Overpressure switch. The overpressure switch shall be tested to assure opening at the
specified pressure.
5. PACKAGING
5.1 Packaging. For acquisition purposes, the packaging requirements shall be as specified in the contract or order
(see 6.2). When packaging of materiel is to be performed by DoD or in-house contractor personnel, these personnel
need to contact the responsible packaging activity to ascertain packaging requirements. Packaging requirements are
maintained by the Inventory Control Point's packaging activities within the Military Service or Defense Agency, or within
the military service's system commands. Packaging data retrieval is available from the managing Military Department's
or Defense Agency's automated packaging files, CD-ROM products, or by contacting the responsible packaging
activity.
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