BOAC..
Hi there..
Quite common after a ground air start, and often can be avoided by selecting ground power OFF before connecting the engine gens after start.
Works the same if you select the Generator of the donkey you've just started onto the bus, then select the ground power switch off to drop that off the other bus - before the ramp crew disconnect the GPU.
Otherwise doesn't it go dark and dump everything you've put in to the FMC??
99% of the time the Elec light is due to Stby Bus transfer fault when the Ground power drops off instead of being selected off.