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|  MIL-T-15108C(NAVY)
4.6.11.2.1 Test voltage. Test voltage shall approximate a true sine wave. Insulation
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of the high voltage winding shall withstand a dielectric test voltage (rms) of 4,000 volts
a.c., 60 Hz, for a period of 1 minute with the low-voltage winding and case and core grounded.
The low-voltage winding insulation shall withstand a dielectric test voltage (rms) of 4,000
volts a.c., 60 Hz, for a period of 1 minute with the high-voltage winding and case and core
grounded. In addition, all other windings other than the one being tested shall be tied to
the core and case which are grounded.
4.6.11.2.2 Measurement of test voltage. The voltmeter method shall be used. In
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measuring the voltage, the voltmeter shall derive its voltage from the high-voltage circuit
either directly or through an auxiliary ratio transformer placed across the testing trans-
former.
4.6.11.3 Induced potential. The test voltage shall be applied to either winding and
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shall approximate a true sine wave. Insulation of the low-voltage and the high-voltage
windings shall withstand a voltage applied to either winding of twice normal operating
voltage at a frequency of not less than 120 Hz or a period of not less than 7,200 cycles
at the frequency of the applied test voltage. The frequency of the voltage shall be such
that the exciting current is not in excess of 30 percent of rated-load current.
4.6.12 Inrush currents. Oscillograph measurements of the maximum inrush currents to
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the primary winding shall be obtained with rated voltage at rated frequency applied to the
primary winding to determine conformance with table II. The secondary winding shall be
open-circuited. The following procedure shall be used in conducting this test:
(a) Apply direct current (d.c.) to primary winding. The value of d.c.
current shall be equal in amperes to the crest value of the a.c.
exciting current.
(b) Gradually remove d.c. current from primary winding.
(c) Apply rated value of a.c. voltage to primary winding. Provision
shall be made to Insure that the voltage is applied when the voltage
wave is passing through zero (plus or minus a maximum of 15 degrees
is permissible) in the direction that adds to the residual.
(d) Record inrush current, primary voltage, and secondary voltage waves
on oscillograms. Copies of the oscillograms shall be in the test
report.
4.6.13 Short circuit. Rated voltage and frequency shall be applied to one winding,
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for transformers rated below 15 kVA, the other winding shall be short-circuited. For
transformers rated 15 kVA and above, the short-circuit current and duration of short-circuit
test shall be as specified in table VI for the corresponding impedance.
Table VI - Short circuit test.
Minimum values of rms
Minimum
symmetrical short-
Percent
time
circuit current to
impedance
period
be withstood
Seconds
2
4 or less
25 times rated current
3
5
20 times rated current
4
6
16.6 times rated current
5
7 or more
100 divided by percent
impedance times rated
current
Intermediate valves may be determined by interpolation. There shall be no evidence of
shifting of coils or cores or damage to the insulation or loosening of the coil supports.
4.6.14 General examination. Each transformer shall be subjected to a thorough exam-
ination to ascertain that the material, workmanship, and design are in conformance with
this specification.
4.7 Inspection of preparation for delivery. The packaging, packing, and marking shall
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be inspected to ensure compliance with section 5 of this document.
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