Hi mfor,
I am not really sure what the frequency of a full bus failing is, but Boeing provided dedicated abnormal procedures for some of them, so I guess it happens. All you need is a wire chafing and subsequently shorting.
The highest probability of an electrical problem is most likely one involving moving parts - like the engine generators (breaking) or the breakers ("glueing"). But I also heard of total electrical failures with the battery charger not working and the battery getting totally drained and dropping the generator buses offline in flight,
I myself haven´t had any problems with the electrical systems in 10 years of flying the 737. We dispatched with an engine generator inop one day (MEL allows that), and I had a generator drop off the bus during single-engine-taxi in one day, but thats it.
Cheers, Jan