Sort of. The GCU protects against under/over voltage and frequency. I cannot remember the percentages off the spec values of 115 Vand 400 Hz, been too long since I had to memorize them.
When you power up as you described, the GCU will keep power off the plane until the undervoltage and underfrequency values are passed. What you'll see/hear is a lower voltage than normal, but it's not like you will see the voltage rise from 0 to 115 V. Probably more like 95 V to 115, but that's a quick second. Same thing if you shut off the APU first then the generator. You'll hear the electric flow and see the lights dim until the GCU disconnects to save the avionics and such.
In operation, the GCU senses a power drop, allows more voltage to pass from the PMG (Permanent Magnet Generator) built into the generator back into the GCU and then into the windings of the generator, which then increases voltage output of the generator to meet the demand. That's how we are able to maintain volts and hertz at such a small range of fluctuation as loafs change.
Hopefully that clears up any previous confusion I may have caused.
And as for the previous poster who said that the green light means power is good after the GCU... not quite. It just means there is power at the external power monitor panel. As in available for the GCU to check it when you press the switch in. Push the switch with "bad" power, and the light stays green, but the GCU won't allow power to travel thru it to the plane. Similar to the light on a power strip when you plug it into the wall; it means that power is there, but unless you plug in your device, no power is flowing anywhere.
Originally Posted by
Happyflyer
Ballsy move, your saying on the 200 if you turn on the battery then flip the gen switch to on, then start the APU you won't get get an AC until the gen is on line?
I think the GCU just won't transfer form GPU to the APU but if you didn't have AC power already on it would try to power the plane.
I knew a guy who shutdown the APU with the gen switch on and both engine gen switches off and AC power wasn't immediately lost power remained while the gen spoiled down and he was able to get an engine gen switch before everything went out.
According to you and I'am not positive the GCU would have isolated the apu gen killing AC power immediately.
Either way his post is relying on the premise that the engineers knew he wouldn't care or wouldn't understand and idiot proofed it for him, so now he's at peace with himself because he's been made aware of said idiot proofing, which is still ignorant.