# ELECTRICAL - DC
### TRU 1 and TRU 2 TOGGLE
**AUTO:** Automatic operation of the electrical system. Connects TRU 1 (2) to the respective DC bus
**OFF:** Isolates TRU 1(2) from the respective DC bus
### TRU ESS SWITCH
**AUTO:** Automatic operation of the electrical system. Connects TRU ESS to the ESS BUS 3
**OFF:** TRU ESS is isolated from ESS BUS 3
NOTE: DC GPU supplies power to only APU START BUS. It does not charge the batteries.
### BATT 1 SELECTOR KNOB
**ON:** Connects Battery 1 to DC ESS BUS 1
**OFF:** Power is only supplied to HOT BATT BUS 1
**HOT BATT BUS 1 powers:** Engine 1A and 2A fire extinguisher, engine fuel shutoff valve 1, hydraulic system shutoff valve 1, engine 1 oil level indication, Courtesy/Service lights fwd, DC ceiling lights.
### BATT 2 SELECTOR KNOB
**AUTO:** Connects Battery 2 to DC ESS BUS 2. Allows Battery 2 to power the APU START BUS during APU start.
**OFF:** Power is only supplied to the HOT BATT BUS 2
**HOT BATT BUS 2 powers:** Engine 1B and 2B fire extinguisher, engine fuel shutoff valve 2, hydraulic system shutoff valve 2, engine 2 oil level indication, refueling panel, Water and Waste controller, and CMC.
NOTES:
* Supply essential loads for 10 minutes.
* On ground if powered only by batteries for more than 5 minutes a horn will sound.
* Wait to turn the batteries off when leaving the aircraft until the APU off and APU FUEL SOV CLSD EICAS messages are displayed (approx.. 80 seconds).
### DC BUS TIES
**AUTO:** Allows automatic operation of the electrical system.
**OFF:** DC bus ties open.