MIL-DTL-28778A
4.6.2.5 Electrostatic shielding. With all windings short-circuited and those on the same side of the
electrostatic shield connected together, using the circuit shown on figure 2 the voltage of the signal generator at the
specified frequency shall be set to give a definite indication on the detector, with switch "S" open. With switch "S"
closed, the generator voltage shall be increased so as to yield the same indication on the detector, and the ratio of
the generator voltages shall be computed. The detector shall have a minimum input impedance of 1 megohm.
4.6.2.6 Wave shape. With the source and load conditions as specified, the wave shape of the output
shall be determined by means of an oscilloscope. The wave if sinusoidal.
4.6.2.7 Turns ratio. The turns ratio shall be determined by the volt-meter method or any other
suitable means.
4.6.2.8 The following tests shall be made:
(a) Exciting current (see 4.6.2.11)
(b) Core loss (see 4.6.2.12)
(c) Impedance measurements (see 4.6.2.13)
4.6.2.9 Short circuit. The transformer shall be undamaged after a full secondary short circuit for 6 full
cycles when fed from a 460 VRMS source having an impedance of 10 percent on a 92 kVa base. Such operation
occurs during normal operation due to crowbarring of the power supply.
4.6.2.10 Voltage impulse.
4.6.2.10.1 Secondary number 1. The winding shall not be damaged when tested for impulse at a
level of 60,000 volts.
4.6.2.10.2 Secondary number 2. The winding shall not be damaged when tested for impulse at a
level of 15,000 volts.
4.6.2.11 Excitation current. With primary winding connected in series (terminal 4 connected to
terminal 19, terminal 5 connected to terminal 20, terminal 6 connected to terminal 21) apply 460 volts, 3-phase, 60 Hz
across terminal 1 and 23, 2 and 24, and 3 and 22. The input line current shall be measured and recorded.
4.6.2.12 Core loss. With primary windings connected in series (terminal 4 connected to terminal 19,
terminal 5 connected to terminal 20, terminal 6 connected to terminal 21) apply 460 volts, 3-phase, 60 Hz across
terminal 1 and 23, 2 and 24, and 3 and 22. The core loss shall be measured and recorded.
4.6.2.13 Impedance measurements. Short circuit secondary high voltage windings and then with
the primary connected, increase the current 3-phase until the current read is equivalent to the normal load current. At
this time record the voltage and the current and the impedance will be the voltage divided by the current. Record this
value in ohms.
4.6.3 Terminal strength (see 3.7). Transformers shall be tested as specified in 4.6.3.1.1. After each
test, the terminals shall be examined for loosening and rupturing, and other mechanical damage. Unless otherwise
specified, all terminals on each test sample shall be subjected to the following test, up to a maximum of four identical
terminals per sample.
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