MIL-PRF-17773C(SH)
b. When the source (see 6.4.14) is not within preset levels. The ABT shall transfer to the alternate source
provided the alternate source is available. The ABT shall provide two automatic transfer modes: in-phase transfer
and random transfer as specified by configuration type (see 1.2.3). The ABT shall allow for operation with a
preferred source (see 6.4.18), in which case the unit shall preferentially power the load from that source, provided it
is available. This is called Normal Seeking. However, when the ABT is Power Seeking, it shall re-transfer (see
6.4.19) to the initially active source (see 6.4.3) following restoration of the initially active source to an available
status, only if the presently active source becomes unavailable.
3.4.2.1.1 In-phase transfer mode. ABTs shall be provided with in-phase monitoring in accordance with
configuration type (see 1.2.3) to allow completion of transfer when two sources are within 60 electrical degrees of
each other.
3.4.2.1.2 Random transfer mode. When the alternate source is available, the ABT shall initiate a transfer when
the normal source voltage and/or frequency specified by configuration type (see 1.2.3) falls below the random
transfer setpoint for a time period longer than that set by the control circuit time delay. Random transfer will occur
regardless of the phase angle difference between the sources.
3.4.2.2 Manual operation of MBTs. A manually operated handle, lever, or wheel located on the front of the
MBT shall be used to transfer power from one power source to another. A sustained off position between energized
power sources/closed positions shall be incorporated. White indicator lights shall provide power available
indications for each source.
3.4.2.3 Manual operation of ABTs. To ensure fail-safe mode of operation, the ABT shall have a manual mode
for selecting power sources. During the manual mode of operation, the ABT automatic functions are disabled.
Manual transfer shall be performed by activating a switch handle or wheel (see 3.4.8). Manual transfer of an ABT
can only be initiated when both power sources are de-energized.
3.4.2.3.1 Selector switch. ABTs shall be provided with a selector switch that controls manual or automatic
ABT operation (see 6.4.2).
3.4.2.3.2 Preferred source selector switch and control circuit.
3.4.2.3.2.1 Two-way switches. Two-way switches with normal (ship's service) and alternate (ship's service)
supply sources shall be provided with a preferred source selector switch that controls the selection of the preferred
source of power.
3.4.2.3.2.2 Three-way switches. Three-way switches with normal (ship's service), alternate (ship's service)
and emergency (service) supply sources shall be provided with a preferred source selector switch that controls the
selection of the preferred source of power to be either the normal (ship's service) or alternate (ship's service).
3.4.2.3.3 Operating voltages. Switches shall operate with tolerances in accordance with MIL-STD-1399-300 as
3.4.2.3.4 Drop-out voltages. Drop-out voltages for configuration types (see 1.2.3) shall be in accordance with
Table II. A tighter tolerance may be specified (see 6.2). For configuration types CT03, CT05, and CT10 with in-
phase monitoring, the in-phase transfer shall operate between 80 and 85 percent to between 20 and 30 percent.
Below 20 and 30 percent random transfer shall occur.
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