I am a long time reader but have finally made the plunge to join your extremely informative group. As you can see from my name, I am more familiar with cameras and photography than electrical matters, but have done some electrical wiring around the house. Yes, I even pulled permits for the work and had it approved!
I have a very basic, but nagging question, however: Since most appliances and tools are A/C today, what is the real purpose of the wider prong that is supposed to meet with the Neutral connection in an electrical receptacle when it's plugged in. Along that same vein, what are the consequences of having the not/neutral reversed at the receptacle? Since the current alternates this doesn't make total sense to me, aside from the grounding issues at the main box. Could the tools be damaged some how, for instance?
The first example that comes to mind would be the screwshell of a lamp. The 'Hot' wire is meant to connect to the contact down inside the socket. If wires are reversed the 'Hot' would be connected to the shell itself and might be easily contacted by someone turning on the lamp or changing the bulb.
I found the earlier chat threads from Janruary of this year dealing with "Reversed Polarity", and "Wrong Polarity" and although they proved interesting, I still didn't find an answer to what I'm curious about. Let me put my question another way: I have a Sawzall that is double insulated and runs on AC, of course. It has one wide prong and one narrow prong on the plug guaranteeing it gets plugged in only one way - with the neutral at the larger slot in the receptacle. Since the tool needs no ground (in fact has no ground terminal) and the current being alternating, what's the difference if it gets plugged in one way or another to the receptacle as far as safety or function, etc. goes? I see lots of power equipment and appliances with that wide prong on the plug. How come?
Like Bill said in the first response. The light bulb socket has two contacts. The little round metal on the very bottom of the light bulb and the metal shell that is the base of the light bulb. The base should connect to the neutral of the 120V circuit and the little round metal on the bottom of the light bulb should connect to the hot wire of the 120V circuit. If the receptacle that you pluged the extension cord into has reverse polarity then the large metal base of the light bulb would be energized with 120 volts and that has potential for personnel injury.
That, computer equipment, and digital equipment need to have correct polarity. The light bulb would still work don't get me wrong. It's just dangerous. So would motors like your sawzall. But when it comes to computers and digital equipment, that is another story.
Polarity observation is a safety, not functional issue. Keep in mind that it is unsafe to touch the black (hot) conductor while you are in contact with grounded surfaces (concrete, metal piping, frames of grounded metal appliances, etc.) It is usually relatively safe to touch the white conductor, because it is grounded at the source and thterfore there is no potential difference to push current through you. When you turn offf a switch, it should be the hot conductor which is interrupted, thereby rendering the equipment safe to "put your fingers in". If however the polarity is reversed, the equipment will be de-energized but the neutral and not the hot will be interrupted, thereby exposing you to the hot conductor. Also, under certain fault conditions, if a piece of equipment is wired with reverse polarity, it may suddenly start-up by itself without the switch being operated.
...computer equipment, and digital equipment need to have correct polarity. The light bulb would still work don't get me wrong. It's just dangerous. So would motors like your sawzall. But when it comes to computers and digital equipment, that is another story.
Cameramanbob, it makes no difference whether the source is AC or DC. Unless the system is totally isolated (floating), one lead will be referenced to ground. A million negative volts can kill you just as surely as a positive million volts.