I know what your talking about. I actually was playing with her and did the whole bite inhibition thing and left the room, which was for not even 20 seconds. When I looked back I noticed she was not in my site and trying to rip the cord out of the wall.
I have actually done exactly that. Told her NO! then gave her a toy and said "this is yours". Doesnt seem to get through to her. Im not sure if she is registering anything I say or do. Its only been a week. But no I unplug and hide the cord when shes in the room.
If the dog continues to chew on electrical cords, you may not have to worry about it.
Seriously, that is VERY dangerous. It can kill a puppy.
So, you need to ONLY let the puppy have access to ANY dangerous area when you are paying attention and then you need to divert him to something appropriate. Saying EH! or NO! MINE! and then showing him something he can have and saying in an easier voice, Yours, and give it to him.
If necessary, keep a lead on the dog and do not allow him access to the cords, though it is best to teach him to not mess with the cords.