MIL-PRF-17773C(SH)
6.4 Definitions. For the purpose of the following definitions, the ABT is assumed to be in automatic mode with
the normal source supplying the loads unless specifically indicated otherwise. Note that in the manual mode the
ABT will remain lined up to the operator-selected power source regardless of the availability of either source.
6.4.1 Automatic bus transfer switch (ABT). The ABT is a self-acting device for transferring one or more load
cable connections from one power source to another.
6.4.2 ABT operation. ABT operation is the transfer or the re-transfer of the connected load from one power
source to another.
6.4.3 Active source. The active source is the source that is connected to the load. The active source is
indicated on the front panel.
6.4.4 Continuous duty. Operation at substantially constant load for an unlimited period of time is considered
continuous duty (100 percent).
6.4.5 Control circuit time delay. The control circuit time delay is the time a condition exists as specified by the
configuration type, which must be continuously present in order for the unit to recognize a condition and initiate a
transfer. (see 3.4.2.3.8)
6.4.6 Control circuits. Control circuits are solid-state (semiconductor) circuits that sense and control the
transfer from normal to alternate power sources and re-transfer or otherwise control operation of the ABT.
6.4.7 Fault. A condition whereby a power source voltage or frequency falls outside its preset limits, causing it
to become not available.
A fault of the type described above will always trigger a transfer or a transfer inhibit. Most faults of the type
described above can be cleared when conditions external to the ABT change (i.e., restoration of a source from not
available to available status).
6.4.8 Full phase protection. Full phase protection occurs when an ABT device monitors all phase voltages of
one power source and initiates transfer of the connected load from the active power source to another when voltage
of the active power source drops below a predetermined value. The device initiates transfer of the connected load
back to the preferred power source when all voltages of the preferred source return to within specified limits.
6.4.9 In-phase monitoring. In-phase monitoring for sensing and control is a device used to sense the phase
angle between the normal and alternate sources prior to load transfer. It is used with a two way instantaneous type
ABT switch to initiate transfer only when the two sources are nearly synchronized thereby limiting the motor inrush
current to below normal starting current levels and avoiding inadvertent circuit breaker tripping.
6.4.10 Instantaneous. A qualifying term indicating that no delay is purposely introduced in the action of the
ABT switch.
6.4.11 Interlock. Interlock is a device actuated by the operation of some other device with which it is directly
associated, to govern succeeding operations of the same or allied devices. An interlock system is a series of
interlocks applied to associated equipment in such a manner as to prevent or allow operation of the equipment only
in a prearranged sequence. Interlocks are classified into three main divisions: mechanical interlocks, electrical
interlocks, and key interlocks, based on the type of interconnection between the associated devices.
6.4.12 Line circuit time delay. A designated time delay specified by configuration type (see 1.2.3) between the
opening of the main contacts and the closing of the other set of main contacts.
6.4.13 Manual bus transfer switch (MBT). A manual (non-automatic) bus transfer switch is a device by which
a human operator manually transfers the connected load from one power source to another.
6.4.14 Normal source. This establishes an arbitrary convention to distinguish between the two sources. Which
source is normal and which is alternate depends entirely on which set of bus bars the cables from the two available
power sources are connected to inside the ABT and hence is independent of which source should supply the load
under normal operation. The term "normal" should therefore not be confused with "preferred".
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