(4)  
On airplanes with a standby bus, relay, M328:

(a)  
Set STANDBY POWER switch to BAT and check that charger stops pulsing as in (2).

(b)  
Set STANDBY POWER switch to AUTO to restore normal operation.

(5)  
Remove electrical power if no longer required (AMM 24-22-0).

C.  Test Battery Thermal Switch
(1)  
Place battery switch to OFF and remove external power contactor from airplane.

(2)  
Remove battery charger and check that resistance between pins 10 and 12 of battery charger tray disconnect is not greater than 5 ohms.

(3)  
Heat battery thermal switch until it activates. Check that resistance between pins 10 and 12 is now infinite (open circuited).

CAUTION:  DO NOT ALLOW HEAT TO REMAIN ON THERMAL SWITCH LONGER THAN 5 SECONDS AFTER SWITCH ACTUATION TO PRECLUDE POSSIBLE DAMAGE TO SWITCH.

(4)  
After switch cools (and closes), check that resistance goes back to 5 ohms or less.

(5)  
Install the battery charger.

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AC EXTERNAL POWER - DESCRIPTION AND OPERATION
1.  General

A.  The aircraft is fitted with electrical components and circuits including protective devices to enable external power to be used when engine or APU generators are inactive. External power receptacles provide for use of external 3 phase 400 cycle power in lieu of engine generated power. (See figure 1.) A ground service switch is provided which energizes a portion of the electrical power system for ground maintenance purposes. The switch is on the forward attendant's panel. A transformer-rectifier unit located within the bus protection panel converts a portion of the 400 cycle ground supply to dc for control of the electrical system and to supply power to the dc system through the ground service switch on the forward attendant's panel. The GROUND POWER switch on the overhead panel is used to trip or close the external power contactor. The selector switch is used to connect the voltmeter and frequency meter to the external power source. (See figure 2.)
B.  A bus protection panel (G7) is located adjacent to the generator control units in the load control center (P6). The panel functions to protect aircraft circuits and equipment against the following external power system abnormal conditions: overvoltage, undervoltage, negative sequence and anticycling. Overvoltage would be considered some potential of 130 (+ 3) volts line to-neutral. At this voltage the basic sensing circuit in the protection panel applies a trip signal to the external power contactor (R8). For undervoltage the panel uses an average 3-phase voltage sensing device which applies a trip signal to the external power contactor (R8) within 7 (+ 2) seconds after the voltage has dropped to 100 (+ 3) volts line-to-neutral. The negative sequence protection actuates a circuit which trips (R8) instantaneously. Anticycling is a condition in which bus protection prevents the control system from cycling when fault conditions exist and the switches in the control cabin are manually held in the ON position. Power required to operate the bus protection panel is derived from the ground power transformer-rectifier in the panel, backed by the aircraft battery system. The control system is capable of normal operation without relying on the battery system. The bus protection panel also controls the external power contactor, and the bus tie breakers when power is supplied to the load busses from external power, and contains tripping interlocks necessary to prevent paralleling of independent power sources. The bus protection panel supplies approximately a 5-ampere load for ground fueling and lighting circuits when external power is connected and the battery is de-energized or disconnected. The voltage of this power source is 28 (+ 2) volts with a 3-phase input to the aircraft of 115 volts line-to-neutral.
500 
Nov 15/67  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  24-41-0 Page 1 

G39641H77634
External Power System Schematic  500 
24-41-0  Figure 1  Feb 15/73 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

505  Extense or Ground Power Component Location 
Oct 20/84  Figure 2  24-41-0 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 
　
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