Chosen Solution
So, ive got this old turntable and i want to see if it works but it has no plug. I bought a standard plug at the hardware store and when i tried to put the copper wire into the plug i realized it has another concentric copper wire covered. The question is can i just put it into the plug the way i normally would do with a regular copper wire with no risk of exploding or something?
Jorge Yépez the third wire is indeed the ground wire.
You cannot connect power to the ground,so you will need to open your Record Player and check to see how the wires connect. If unsure at all, post some images of what you are seeing with your question. For that use this guide.
