There are two basic principles of operation for the 737 electrical system:
• There is no paralleling of the AC sources of power.
• The source of power being connected to a transfer bus automatically
disconnects an existing source.
BUS TRANSFER Switch
AUTO (guarded position) – BTBs operate automatically to maintain power to AC
transfer busses from any operating generator or external power
• DC cross tie relay automatically provides normal or isolated operation as
required.
OFF – isolates AC transfer bus 1 from AC transfer bus 2 if one IDG is supplying
power to both AC transfer busses
• DC cross tie relay opens to isolate DC bus 1 from DC bus 2.
OFF- causes both BTB's to open, isolating the transfer busses. APU power is supplied to the tranfer busses via the BTB's. If the BTB's are open, no APU power is supplied to any transfer bus.
If IDG1 was powering both Transfer busses and the BUS TRANSFER Switch was selected off, then Tranfer bus1 would remain powered, while TB2 would lose power.