MIL-PRF-17773C(SH)
4.6.1.4 Preferred source selection. The preferred source selector switch shall be switched to each position and
the ABT shall transfer to that preferred source.
4.6.1.5 Timing. Timing, time delay, and operating transfer time, as specified by the bus transfer switch
configuration type (see 1.2.3), shall be measured five times to determine conformance with the required time periods
as specified in 3.4.2.3.6. Adjustable time delay devices shall be operated over their adjustable range to determine
that the specified time range can be achieved. When instantaneous is specified, the voltage shall be reduced quickly
to approximately five equal intervals from 20 percent rated voltage to specified setpoint voltage including maximum
and minimum setpoint. The ABT shall pick up the non-preferred source within 0.05 seconds after initial time delay
is completed. The switch shall not transfer at the maximum setpoint.
4.6.1.6 Re-transfer to normal. ABTs shall be tested for re-transfer (normal to alternate and back to normal) as
specified by 3.4.2.3.9, 3.4.2.3.10, and configuration type (see 1.2.3). Normal rated power shall be adjusted to
reduced and zero voltage and returned to normal at the start, middle, and end point of the test. Alternate and/or
emergency power shall be maintained at rated power throughout the test. No additional delay in time other than
4.6.1.7 Test switch. The ABT's test switch shall be operated to determine that it simulates a voltage failure and
a transfer action occurs.
4.6.1.8 Full phase protection. When specified (see 6.2), the ABTs shall be operated to determine the operating
voltage of each phase. Not less than five transfers and five re-transfers per phase voltage test shall be made under
conditions of constant temperature, frequency, and rate of change of voltage.
4.6.1.9 In-phase monitoring. ABTs furnished with in-phase monitoring shall be operated five times in each
direction including the low voltage setpoint (minimum of 80 percent voltage or as specified in 3.4.2.3.13). The
frequency of the two sources shall be a nominal 60 Hz. After these operations are completed the voltage shall be
rapidly reduced to below random transfer setpoint for instantaneous random transfer verification. When frequency
sensing is specified, the switch shall be operated five additional times in each direction. The voltage shall be set at 5
points including 114 percent or greater and 85 percent or less. These tests shall be conducted by one of the two
following methods:
a. The tests shall be performed with a load consisting of a motor of sufficient size to maintain a voltage
greater than 30 percent for minimum of 0.50 second after the power is suddenly disconnected.
b. The tests shall be performed with any convenient load. A calibrated phase angle sensing instrument shall
be used to record the phase angle displacement between the normal and alternate sources at the completion of
transfer. In each operation, except the instantaneous random transfer when voltage is reduced to below random
transfer setpoint, the phase angle difference (see 6.4.16) at the completion of transfer shall not exceed 60 degrees.
4.6.1.10 Frequency sensing. The frequency as specified (see 6.2) shall be tested five times at the high and low
frequency pickup points to determine compliance with the frequency requirements as specified (see 3.4.2.3.11).
4.6.1.11 Overload transfer inhibit. When transfer inhibit on overload has been specified (see 6.2), a transfer
operation shall be initiated by failing voltage on the connected source to determine that transfer shall not occur if the
load current is above overload inhibit value specified in 3.4.2.3.14.
4.6.1.12 Power available indicator. Switches shall be operated to determine that each power available indicator
operates properly when power is supplied and removed from each supply line.
4.6.2 Line-to-ground impedance. Line-to-ground impedance tests shall be conducted in accordance with
Method 302, test condition "B" of MIL-STD-202. Test conditions shall be as follows:
a. Points of measure - Between each electrically isolated circuit and all other circuits connected together to
ground (frame, chassis or enclosure).
b. Electrification time - 60 seconds minimum for insulation suitability test, and only sufficient time to take
resistance readings for all other tests.
c.
Temperature at time of test shall be in accordance with NSTM Chapter 300.
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