(5)  The overfrequency and underfrequency circuits are set to operate at 425 to 435 cps and 360 to 370 cps respectively. A time delay of 0.25 to 0.60 second is provided to override frequency transients from the CSD. The circuit also provides for a lockout for undervoltage protection during normal shutdown of the system. The function of the circuit is to prevent application of power to loads at frequencies outside of the above limits.
G.  Transfer bus control is automatic with manual override provided. The transfer bus relays (R3) and (R 4) are magnetically held in either normal or alternate positions with a center-off position. The transfer bus control has a preferred source, designed to minimize transfer of power under operating conditions. A time delay of 0.1 to 0.2 second is incorporated to override normal switching operations. 
H.  Control power or the necessary dc to operate a generator and its connecting channel is derived from that generator, backed up by the airplane battery system. With an engine running or the APU operating, the generating system is capable of being started and normally operated without the use of the battery system. Current drain on the battery system with the generating channel de-energized should not exceed 0.25 ampere, not considering indicating lights. The battery charging voltage will be 26 to 32 volts and between 23.5 and 18 volts on discharge. DC power from the GCU T-R is capable of tripping two contactors when the ac voltage is 115 for the system. 
I.  The electrical power annunciation panel is provided to indicate the position of the generator field relay and the source of the tripping signal. The annunciator lights are located on the P6 panel along with the lamp test switch (S1) and the annunciator erase switch (S2). This module M238 contains the indicator lights necessary for monitoring major portions of both the ac and dc electrical systems. (See figure 5.)
6.  Static Inverter
A.  The static inverter (refer to 24-54-0) is a transistorized power supply for changing 28 volts dc to single-phase 400 cycle 115 volts used for critical communication and navigation equipment. The inverter is secured in a standard ATR tray on the E3-1 shelf of the electronic racks in the lower forward section. All of the components in the electronic circuits of the inverter are the static type except the startup relay (K1). The output rating may be either 250 or 410 volt-amperes. A static inverter of the same rating as the unit removed should be installed when replacements are made.
B.  The inverter power circuit breaker (C30) on the P6-5 panel is the standby dc input connection to the inverter from the battery bus. However, ac output is dependent on an additional dc supply, which closes relay K1, inside the inverter, energizing the remaining portions of the circuits necessary for ac output. Placing the STANDBY POWER SWITCH on the forward overhead panel in the OFF position (Fig. 3) opens the turn-on signal to K1 only. To completely de-energize the inverter, the source from the battery must be opened or the battery bus de-energized, either by having the battery switch in the OFF position or opening the inverter circuit breaker (C30).
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24-21-0 Page 14  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Aug 15/72 

C.  A connector attached to the rear of the inverter enclosure disconnects all connecting wires when the two forward hold downs are loosened and the unit is pulled out. The forks on the front hold downs are adjustable.
7.  Test Module
A.  The power system test module M400 is a switch panel with current transformer selecting relays and portable meter jacks, for testing the airplane power system. It permits measuring dc voltages and current values, and ac voltages, current values and frequency at various electrical system component connections throughout the electrical system. The module is located at the top of the load control center panel (P6). Jacks are provided so that calibrated portable meters may be connected to check the accuracy of meters on the pilots' overhead panel (P5).
B.  The system test operation consists of selecting various combinations of settings on the two, eight position, multiple wafer, rotary switches (S1 and S2) mounted in M400, and observing the parameter on the airplane meters on P5. See the table in Fig. 5A for the voltages and currents made available by the rotary switches. The dc and ac rotary switches on the pilots' overhead panel are turned to the TEST position for meter readings of dc volts, ac volts, and frequency (Fig. 3). Generator ammeter readings are only dependent on the position of the S2 switch. The above tests are made by using only airplane facilities, but for additional flexibility for a more extensive test of the system, the generator control units (GCU's) and bus protection panel (BPP) are pulled out and a multiple T-connection is made to breakout boxes.
　
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