1.14 FUNCTIONAL DESCRIPTION.
1.14.1 208/120V, 400Hz Main Power Circuits.
Generators 1 and 2 (figure 1-7) provide three-phase 208V/400Hz power to the four bus bars L1, L2,
L3, and N in the EPP III power distribution unit. Prom the bus bars, all phase lines are directed to
power contactors K3 to K6 by pass through current transformers T1 to T12. The pass through
current transformers transform the phase currents in the ratio 150:1. The resulting analog voltages
are delivered to the monitoring circuits of the PDU (paragraph 1.14.2). A 120V/400Hz power supply
is made available at receptacle J11.
Main power circuit control system
The main power circuits are switched on with the ECS ON (figure 1-8) and/or RS ON switches on
the ECS control panel; this energizes relay K10 and/or K11. Contact K13/14 of K10 enables the
main power circuit for power contactor K6, and contact 3/4 of K11 enables the main power circuit of
power contactors K3 to K5. The power circuits are identical, and will be described below using the
example of K6.
Relay contact 13/14 of K10 is closed. The +24 VDC signal passes by the ECS interlock loop J4-D/B of
the power cable, and break contact 22/21 of relay K19, to power contactor K6. The contactor is
energized. This opens its contact 31/32 and reduces the holding current of the coil via 7.4-ohm
protective resistor R6. At the same time, POWER ON-J4-ECS lamp H8 on the control panel lights
If current transformer T10, T11, or T12 (figure 1-7) indicates an overload, PDU electronics group
N6 sets time pulse relay K19 (figure 1-9). Break contact 22/21 of K19 (figure 1-8) opens, and the
main power circuit is shut down as power contactor K6 de energizes.
Once the fault is remedied, OVERLOAD RESET pushbutton switch S4 (figure 1-9) on the control
panel must be pressed to reset relay K19 and turn the main power circuit back on. If an overload
cutout occurs, OVERLOAD-J4-ECS lamp H4 (figure 1-8) on the PDU control panel lights up.
Activation of power contactors K3 to K5 requires that all three power cables be connected to J1 to J3
in order to close interlock loop J1-D/B, J2-D/B, and J3-D/B.
The control and overload cutout systems for power contactors K3 to K5 are similar to those for K6.