What buses are powered when the battery master is selected on?
- Left/right essential buses
- closes 3 contactors and connects the 3 batteries to the essiential DC buses.
What buses are powered when the battery master and main BATT switches are selected on?
- Left/Right essential buses
- Right Main Bus
What buses are powered when the battery master I MAIN BATT I AUX BATT I AND STBY BATT switches are selected on?
- Left/Right essential buses
- Right Main DC Bus
- Left Main DC Bus
What buses are powered with the APU generator power?
- Left/Right Essential
- Left and Right Main
- Left and Right Secondary
What buses are powered with DC external power?
- Left and Right Essential
- Left and Right Main
- Left and Right Secondary
What buses are powered with AC external power?
- Left or Right AC bus
- Left or Right Galley bus
What controls the 4 bus ties?
The Electrical Power Control Unit (EPCU)
What is max DC generator load?
1.0
What is the maximum battery load for take-off?
0.1
What does a battery load indication of +1.05 indicate?
It indicates that the battery is in overcharge at 5% over the maximum rate of charge.
What is the maximum output rating of the DC generators?
400 amps max
What is the maximum output rating of the TRU’s?
300 amps max
When does automatic load shedding of the galley buses occur?
If one of the AC Generators fail. The functioning generator will power both AC buses, but the load shedding relays will not allow power to the galley buses.
When does emergency mode of operation occur?
The EPCU operates in emergency mode if:
- The airplane is in the air and
- Both DC generators are not available and
- At least one TRU is not available
What will cause the EPCU react to a main bus fault?
- Transient nature of power or a voltage surge
What does the EPCU do when it detects a main bus fault?
If the EPCU confirms an actual Main Bus Fault it will automatically take offline all of the DC components associated with the faulty bus (I.e., DC Starter-generator and batteries), leaving the faulty bus I powered and isolated. The QRH will instruct the crew to cycle the Bus Fault Reset switch in an attempt to reset the EPCU’s bus fault protection logic.
If a main bus fault occurs, the EPCU prevents main bus tie, and the main to secondary bus ties from closing, isolating the bus from other buses & power sources. The DC BUS caution light comes on to warn of the fault impeding condition.
- If the fault persists, after approximately 5 seconds, the EPCU sends a TRIP signal to the GCU, isolating the affected generator. The EPCU will also open and lock-out the contactors connecting the batteries to the affected main bus. The #1 DC GEN or #2DC GEN caution light comes on, and the MAIN BATTERY or AUX and STBY BATTERY caution light(s) come on. The EPCU continues to monitor the operating buses. All main DC services on the failed bus side will not operate.
Will all buses be powered if a DC generator fails? How?
Yes, the left to right main bus tie closes and the other DC generator then powers both main feeder buses.
Will all buses be powered if both DC generators fail? How?
Yes, both the Left and Right main to secondary bus ties close, and the TRU’s supply power to all the DC buses.
Will all buses be powered if a TRU fails? How?
Yes, the left and right Secondary bus tie closes, and the other TRU then powers both secondary buses.
Will all buses be powered if an AC generator fails?
No, the galley bus will not be powered.
Which buses will be powered if both DC generators and one TRU fail?
This would be emergency mode;
- Left/Right Essential buses (by the batteries for approx 30 min only)
- Left/Right Secondary DC buses by the 1 remaining TRU
Note: Should an air start attempt be necessary while the EPCU is operating in the emergency mode, it will allow the batteries to be reconnected to the Main buses momentarily to assist in the start, but only for the duration of the start attempt.
Which battery is the hot battery bus (battery power bus) connected to?
Standby Battery
Can the EPCU disconnect the main bus tie if the main bus tie switch is on?
Yes, manual operation of the main bus tie through the MAIN BUS TIE switch is not possible once the EPCU has reacted to a fault.
General electrical system of the Q400.
The DC power system is supplied by:
- 3 Ni-Cad batteries
- 2 engine driven starter/generators
- 2 transformer rectifier units (TRU’s)
- an optional Auxiliary Power Unit (APU)
The TRU’s supply 28 VDC and are powered by two engine driven 115 VAC generators.
There are both DC and AC external power receptacles for Ground power unit (GPU) connection.
Power is distributed by an electrical bus system. It re-configures for individual power source and bus failures, by automatically closing and opening bus tie contactors.
All AC and DC aircraft services can be powered from the AC generators or the AC external power alone.
