[monkeyshine]

So you’re saying that the device that generates power needs a separate power supply to operate the controls for the power generating device?

[blackdog]

Uhh....yes! Pretty dangerous if you don't! Think Fukushima.

Controls voltage is what controls the (Machine name here). If there is a problem with the generator, reactor, or turbine, and systems need to open/close valves, produce compressed air for pneumatic valves controls, pump lubricants, pump hydraulic fluids, and control speeds, you're in deep doo-doo without control voltage.

Or here's an easier way to understand. "How do you start a generator turbine when the turbine isn't running"? How do you open and close circuits, run pumps, and governors, when the turbine hasn't started yet?

[George from New England]

I believe that nuclear plants need external power to start up and run too.

[TexasGator]

“So you’re saying that the device that generates power needs a separate power supply to operate the controls for the power generating device?”

At nuclear power plants there is an independent source of incoming power AND diesel generators for back-up.