The BU generator is a 20kva frequency wild (i.e. it turns at engine speed no CSD). It is quite big, not like a Dedicated generator. It is tested every engine start and then sits there waiting for an engine or IDG failure when it powers the Transfer Bus. The BU gen convertor sits in the MEC and converts the power to Constant Frequency. Only one BU generator can be on line at a time. It comes on line just before the engine reaches idle, just like any other generator.