737NG Electrical "Source Off" light annunciator
 

So the "TRANSFER BUS OFF" light will illuminate when the associated Transfer Bus is not powered.

But at the same time "SOURCE OFF" light will also illuminate if the associated Transfer Bus is not powered.


To me the, the SOURCE OFF light seems "redundant" since the TRANSFER BUS off lights seems to convey the same info.


But I'm sure there is a good reason to have the SOURCE OFF light.

Anyone know why it's there?

I'm guessing the SOURCE OFF light serves to let us know if we're trying to connect a power source whose Frequency/Volts are not in safe limits?

Hi PosClimb,

The Source Off light is necessary to show when the last manually selected source is no longer powering the Transfer bus, but the Transfer bus is still powered.

For example if a generator was lost, providing the bus transfer switch is in Auto, the Bus Tie breakers will automatically close allowing the offside generator to power the affected side's Transfer bus. In this case as both Transfer busses are still powered, the Transfer Bus Off lights will not be lit.

Hope this is of some help.

Regards,
MHA.

Exactly - it serves as a warning that the automatic transfer prevented the bus from becoming unpowered due to a failure or disconnection of the previously selected source (be it APU or engine driven generator).