Just trying to learn here... Hopefully, this post does not classify myself as a "Delta Sierra"... Whatever that is.
Since the QRH is specified, I assume you're in flight, tony64peter????
Again, you have not specified the aircraft, only a partial engine name, so you may be drawing flack because of the lack of information in your post.
The QRH does not specify the reason for starting the APU and my B767 Maintenance and Training Manuals don't go into enough detail to tell me what happens in flight if you disconnect the IDG and (then) start the APU. With the IDG disconnected (prior to APU start), the automatics have told the good generator to provide power to the whole aircraft. Does the act of just starting the APU cause the good generator BTB to trip and the APB to close? Or is the APU started just to provide a backup should the other generator fail?
Thanks.
Rgds
NSEU