FWIW,

On my 727-100 I cannot engage EP if I have closed one of the generator breakers.

Engines are running and EP is available. I have to trip the generator breaker (MGR) first, then I can close the EP switch. This is not the way the real 727 works. Also, when the EP switch drops to the off position the A.C. meters drop to zero even though the EP is still connected to the aircraft - also not how it is in real world. If the external power is plugged into the plane and the volts and freqs are OK the meters will show the voltage and frequency regardless of the EP switch position.

When you move any of the power source selectors - APU, engine driven generators or EP, the other power source will trip off. This is the way the plane was designed. You cannot have two different sources of power at the same time. Again, it seems there is a problem in how the trip system is working in the 727 sim.

Lou