fault conditions. Two faults that occur (which support
that does not allow external ac power from a MEPP to
equipment could cause) are voltage and frequency
be applied to the aircraft's electrical system unless it is
faults. Voltage control was discussed in the preceding
of the proper phase sequence.
paragraphs; however, you need to know more about an
The most common cause of improper phase
ac voltage fault that could be a problem--improper
sequencing of MEPP equipment is incorrect
phase sequencing of the MEPP's ac generator output.
connection of the MEPP's ac power output cable. If the
The normal phase sequence of the MEPP's and
output cable leads are not properly connected at the
aircraft's ac generator is A-B-C; therefore, if the
unit's ac output terminals, the phase sequence will not
sequence of the MEPP's ac power was B-A-C, it must
be A-B-C. Therefore, any time the ac power from a
not be allowed to enter the aircraft's electrical
MEPP is rejected by an aircraft, the receptacle end of
circuitry. Improperly phased voltages applied to the
the power cable should be checked with a phase
aircraft electrical systems could cause serious damage
sequence indicator, especially if the power cable has
to the three-phase instruments, motors, and
just been replaced. The AN/USM 128 (fig. 7-31), when
servomechanisms by trying to reverse their rotation.
used with an adapter or an ac electrical load bank, can
This damage, aside from twisting, breaking, and
test for proper phase sequence at the cable end.
binding, could result in overheating and fire.
When correcting a problem of improper phase
A phase sequencing protector, normally located in
sequence, keep in mind that a generator does not
the aircraft, contains a phase sequence-sensitive relay
PHASE "ABC"
INDICATOR
LAMP
AC, DC,
METER
FUNCTION
SELECTION
INDICATOR
FUNCTION
SELECTOR
SWITCH
PHASE
"ABC"
INDICATOR
LAMP
ASf07031
Figure 7-31.--AN/USM-128A electrical test set.
7-22
