Shut down switch for Electric engines?

Isn’t that really a Minefactory Reloaded question? Since it’s not really the power source you want to shut off, but the machines?

Since there’s no such thing as an “electric engine” in Gregtech, I’m guessing you’re using a Forestry Electric Engine to convert EU to RF. Which you don’t actually need to do, since Gregtech power sources will drive RF machines directly.

In any case, the single-block power sources (Combustion Engine, Steam Turbine, Gas Turbine) in Gregtech shut down if the source isn’t demanding power. I’m not sure whether your Minefactory Reloaded machines continue to draw power after they’re full. I’ve never used it. Most machines I’ve used from most mods stop drawing power if they’re full, but there are exceptions.

If you really do need to shut off the machines, you’ll need some way of detecting whether they’re done. I doubt you can test whether internal storage is full. Gregtech has methods of doing that, but only for Gregtech machines.

One possibility, again assuming it’s necessary, is to put the output in a chest and test whether the chest is full with a comparator.

Gregtech power sources can be shut off with a Machine Controller cover, though again you usually don’t need to. You can also place a shutter module on a power cable, and use a machine controller cover to control that via redstone.