couple of 737NG questions
 

1- wheel well fire detection system is powered by which transfer bus?
This question comes up because older FCOMs say it is transfer bus 1 and newer ones say it is bus 2.
2- what happens if the ground power is switched on bus while the bus transfer switch is in the Off position? Which buses are energized?

Based on what you wrote its xfer Bus 2.It needs 115 AC in any case.My manual shows Xfer bus 1.

That is purely a theoretical question as the transfer bus must be on the auto position on the ground...
The ground power powers both AC transfer buses,when the transfer switch is in the OFF position,the transfer buses are isolated from one another but still powered by the GPU.

I've seen NG wiring schematics showing Bus 2. As de facto says, refer to your latest manuals. There may have been a wiring change or a typographical error discovered which caused the difference between your old and new FCOMs.

There are a few variables to consider. It depends on what power was on the busses prior to switching the Transfer Switch to OFF. If External Power was on both busses to start with, and the Transfer Switch is put to OFF, the Bus Tie Breakers (BTB's) remain closed and both AC Transfer Busses remain powered.

Note: The Bus Tie Breakers (BTBs) are magnetically latched in their last position. They need power to both open and close them (not like normal relays which relax into a certain position). Left/Right BTB open/close power comes from their respective Generator Control Units (GCUs). The GCUs must be powered to change the state of the BTBs.

Rgds
NSEU

NSEU，
can I conclude what u just say as : as long as GCU is not powered , the BTB will remain closed?
and also , is IDG the only power for GCU?
THX

According to my manuals, apart from generator power, GCU 1 can be powered by the Battery Bus and GCU 2 can be powered by the DC Standby Bus.

Airplane logic will not allow the BTBs to open if the Transfer Switch is selected to OFF when the aircraft is on External Power. With the application of external power, both BTB's are commanded to close (if not already closed).

If the GCUs are not powered when Ext Power is applied (and, for some strange reason, the BTB's were open prior to power being removed from the aircraft), the BTBs cannot reclose. This situation, however, seems unlikely to me.

Rgds
NSEU