SYSTEM DESCRIPTION (cont)
(k) The current sensing transformers are monitored by the current limiting sensor. If an overcurrent
condition exists, the ac regulator provides feedback to the generator exciter field to reduce the voltage. If the
overcurrent condition persists, the voltage drops enough to trip the undervoltage detector. The switching unit then
deenergizes the CCR, disconnecting the generator from the bus.
TO RH FAB
DC BUS 1 OR 2
28 VDC FROM
TO BUS TIE RELAY
TO PIN D
TO PIN E
GCU Functional Block Diagram
(l) The ac contactor (fig. 975) connects the ac generator to the appropriate ac essential bus, provides
bus tie operation if one generator fails, prevents application of external power if one of the generators is operating,
and prevents operation of the generators if external power is applied.
(m) The ac contactor contains two relays (generator contactor and bus tie). A deenergized GCR
provides a ground path for the bus tie relay through pins 5 and 6. Pins 7 and 8 complete a path to light the GEN 1
indicator on the pilots caution/warning panel. Pins A3, B3, and C3 are disconnected from the bus. Pins 9 and 10
complete a path for external power.
(n) When the GEN switch on the pilots ELEC PWR panel is placed in the GEN position and the
generator and GCU are operating normally, the generator contactor relay is energized from voltage supplied by
the GCU through the GEN switch to relay coil A. Ground for the circuit is provided through pins 11 and 12 of the
bus tie relay. Pins 5 and 6 open which remove ground from the bus tie relay coil. Pins 7 and 8 open which remove
the GEN 1 indicator pilots caution/warning panel path. Pin A3 mates with A2, B3 mates with B2, and C3 mates
with C2 applying power to the bus. Pins 9 and 10 open which remove the external power interlock.