no smoking says:
As long as the engine and the alternator is running you still have electrical power available.
and then sotiras says:
The fire in flight drill does not include switching the Alternator off, so with the alternator switch still made I presume that electrical power is still available. (I don't know how the alternator is driven)
Yes, the alternator will continue to power the bus so as long as the engine is running everything will be powered normally. However as no smoking said, if you lose the engine or alternator, then you lose a lot.
Many years ago I did some testing to see whether the alternator would continue to produce enough electricity as the engine was failing to allow the low RPM warning horn to sound. Sadly, it did not. As the engine died I got a very brief sound from the warning system, however it didn't sound enough like the regular horn for you to necessarily know what it was.
Still, I would expect someone with a fire on board to be paying attention and would realize the engine had failed even without the warning systems...