During ground operation, the external DC power system supplies all aircraft electrical loads under normal mode and partial aircraft electrical loads during ground service mode.

The external DC power is controlled via a PBA on the electrical panel in the cockpit. It provides normal DC power to all buses via a bus tie contactor and has priority over the aircraft generator power.

To supply external-power to the aircraft electrical loads, the procedure that follows must be done:

• A 28 VDC power source must be connected to the external-power receptacle
• On the flight-compartment ELECTRICAL control panel, the external-power control switch must be set to on.

When external power is used, the voltage supplied by the external power source is continuously monitored to make sure that it is satisfactory. If the voltage supplied by the external power source is unsatisfactory, it is rejected. This protection comes from a DC power center protection module found in the left side DC power center (DCPC). The module rejects external power when an over-voltage, under-voltage, or reverse polarity condition is sensed.

The external power control panel is installed on the belly fairing of the aircraft on the right side. The external power control panel contains a standard receptacle (three pins) for aircraft connection to a power unit. It also has a ground service mode PBA to apply a limited DC power source to the aircraft through a service bus relay, an external DC annunciator to indicate the status of the external DC power source (such as available or in use), an APU shutdown PBA, and a lamp test switch.

External DC power system monitoring is performed via the AFD electrical synoptic page which provides voltage indication.

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The external-power control panel is installed on the belly fairing of the aircraft, on the right side, at FS683.00 (behind the ground-power access door 182FR). It is attached to the aircraft structure with screws that go through the front plate.

The external-power control panel contains a standard receptacle (three pins) for aircraft connection to a power unit. The two large pins are the positive and negative DC power terminals. The remaining pin is used for a control function.

The external-power control panel also contains the indicator and the switches that follow:

• A ground-service pushbutton annunciator (PBA) identified as GND SERVICE
• An external DC indicator identified as EXT DC
• An auxiliary power unit (APU) shutdown switch identified as APU SHUT DOWN
• A lamp test switch identified as TEST LAMP

The GND SERVICE PBA is a momentary action pushbutton. When the PBA is pushed in, it lets external-power energize the ground-service bus. To de-energize the ground-service bus, external-power must be removed.

The GND SERVICE PBA has an ON lamp. When the ON lamp is on, it is an indication that the ground-service bus is energized.

The EXT DC indicator has two annunciation lamps: the AVAIL and the IN USE annunciation lamps. When the AVAIL annunciation lamp is on, it is an indication that DC power is available at the external power receptacle. It is also an indication that the voltage supplied by the ground power unit is satisfactory. When the IN USE annunciation lamp is on, it is an indication that external-power supplies the aircraft electrical loads.

The APU SHUTDOWN switch is a momentary action pushbutton. When the pushbutton is pushed in, it causes an APU shutdown (it sends a ground signal to the APU electronic control unit (ECU)).

The APU SHUTDOWN switch has a protective cover that has to be lifted to get access to the switch. This property helps prevent accidental APU shutdown.

The LAMP TEST switch is a momentary action pushbutton. When the pushbutton is pushed in, the annunciation lamps (the ON lamp on the GND SERVICE PBA, and the AVAIL and IN USE lamps on the EXT DC indicator) come on if they are serviceable.

During the lamp test, if one of the annunciation lamps does not come on, it is an indication that one of the fault condition that follows has occurred:

• The lamp is unserviceable (burned filament). This is the most frequent fault condition, or
• There is a fault condition in the lamp's electrical circuit wiring.

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There are two protection modules (PCBs) installed in the their respective DCPC and include the following functions:

• Internal power supplies
• Battery protection
• Feeder fault protections
• Bus fault protection (related to overload conditions of power sources)
• External DC power protection
• Electrical system built-in test functions
• Interface communication

The PCB provides for over-voltage, under-voltage, and reverse polarity monitoring of the external DC.

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The external DC contactor (EDCC) is located in the LEFT DCPC and provides a connection between the external power and the aircraft buses through the bus tie contactors (LBTC and RBTC). The EDCC receives control power from the logic PCB in the left DCPC.

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The ground service contactor (GSC) is located in the LEFT DCPC. The function of the GSC is to connect the external DC power to the ground service bus. The GSC receives control power from the logic PCB in the left DCPC.

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The service bus relay is located on the right side of the forward cabin fuselage .Under normal operation with external DC power selected from the cockpit, the service bus relay is de-energized.

With the ground bus selected from the external DC control panel, the service bus relay becomes energized and allows ground service bus circuit breaker power 1 and 2 on the left DCPC to supply DC bus 3 and 4 in the cabin compartment. The service bus relay receives control power from the logic PCB in the left DCPC whenever the GSC contactor is energized.

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Normal Mode

When external-power is connected to the aircraft external-power receptacle, an external-power protection module (found in the left DCPC) makes sure that the quality of the supplied voltage is satisfactory. To do so, the external-power protection module makes sure that there are no condition of over-voltage, under-voltage or reverse polarity in the supplied voltage. If the quality of the supplied voltage is satisfactory, the annunciation lamps that follow come on.

• On the external-power control panel, the AVAIL annunciation lamp that is part of the EXT DC indicator, and
• On the ELECTRICAL control panel, the AVAIL annunciation lamp that is part of the EXT PWR PBA switch.

To let external-power supply the aircraft electrical loads, the EXT PWR PBA on the ELECTRICAL control panel must then be pushed in. When this is done, contactors in the left and right DCPC close to let external-power supply the aircraft electrical loads. At the same time, the indications that follow occur.

• On the external power control panel, the AVAIL annunciation lamp that is part of the EXT DC indicator extinguishes and the IN USE annunciation lamp (also part of the EXT DC indicator) comes on
  
  
• On the ELECTRICAL control panel, the AVAIL annunciation lamp that is part of the EXT PWR PBA switch extinguishes and the ON annunciation lamp (also part of the EXT PWR PBA) comes on.

The list that follows shows the contactors that are automatically closed when the EXT PWR PBA on the ELECTRICAL control panel is pushed in.

• The external DC contactor (EDCC)
• The left bus tie contactor (LBTC)
• The right bus tie contactor (RBTC)
• The left essential contactor (LESSC)
• The right essential contactor (RESSC)
• The left auxiliary contactor (LAUXC), and
• The right auxiliary contactor (RAUXC).

Ground Service Mode

When external-power is used, it is shown on the ELECTRICAL synoptic page of the EICAS.

When external-power is used, it has priority on the aircraft generator power.

To energize the ground service bus, the conditions that follow must be met.

• External-power must be connected to the aircraft external-power receptacle
• The quality of the supplied voltage must be satisfactory (on the external-power control panel, this will cause the AVAIL annunciation lamp that is part of the EXT DC indicator to come on. On the ELECTRICAL control panel, this will cause the AVAIL annunciation lamp that is part of the EXT PWR PBA to come on)
  
  
• On the external-power control panel, the GND SERVICE PBA must be pushed in.

When the GND SERVICE PBA is pushed in, the ON lamp that is part of the PBA will come on to show that the ground-service bus is energized.

The mating surfaces between the external-power control panel and the aircraft structure must be flat and clean.

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The external dc power system has interfaces with the aircraft components that follow:

• ELECTRICAL Control Panel
• Engine Indication and Crew Alerting System (EICAS)
• Electronic Control Unit (ECU).

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External DC power is monitored using the electrical synoptic on the multifunction displays (MFDs).The list that follows gives more details about the type of protection given by the external-power protection module.

• Over-voltage protection: If an over-voltage condition is sensed, the external-power protection module will not let the EDCC close. If the EDCC is already closed, the external-power protection module will open it.
  
  

  Note: For voltages between 31.5 VDC and 60.0 VDC ± 2%, there is a small time delay between the moment an over-voltage condition is sensed and the moment the EDCC is opened. The quantity of time delay is inversely proportional to the quantity of over-voltage For example, when the quantity of over-voltage increases, the quantity of time delay before the EDCC is opened decreases.

• Under-voltage protection: If an under-voltage condition is sensed, the external-power protection module will not let the EDCC close. If the EDCC is already closed, the external-power protection module will open it. The under-voltage protection circuit operates when external-power is less than 23.0 ± 1 VDC for 2 ± 0.2 sec.
  
  

  Note: Under-voltage protection does not work during an auxiliary power unit (APU) start.

• Reverse polarity protection: The external-power protection module will not let the EDCC close when external power is sensed to be of reverse polarity.

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Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
PL40	EXTERNAL-POWER CONTROL PANEL	PANEL(S) 182FR	24-41-05

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