One of my favorite "gotchas" is on the old CSD units (as fitted to the old GTCP 85-98 APUs);
If the frequency is moving past 390 hz without any voltage indication and the starter still engaged you would be wise to immediately terminate the start, unless you want to wreck your APU starter.
But I digress, if you're interested, though, here's a diagram of the CSD, the Generator and the Generator Protection. I'm sure there are differences between this and the 747, but the general idea is the same.
