Discussion on answer by Sandro: Should I be restrictive in my son's interest in electricity?

I don't think that at 12 he knows enough of electrical safety to be able to determine what is dangerous or not when it comes to high power (and he will be naturally biased towards minimizing the risks if he wants to do the experiment). So in order to keep rules simple, I suggest no high voltage whenever he is alone. As I said, I see no problem to go into higher voltage/power in presence of a knowing adult able to judge if there is danger or not (not everything above the limits I suggested is dangerous, but you need a good knowledge to know what is or isn't)