Ron Schmittou wrote:
> I have never really understood the mania on this third wire or "Ground" wire
> in the US since 99.9% of all electrical panels have the supposedly "Neutral"
> wire and the "ground" wire connect to the same buss. In the UK where all
> the two wire plugs are actually both hot (two phases of 120) 220 volt - it
> makes good since to have a separate ground wire - but in the US each outlet
> already has a ground wire, I don't see the reason you need two - It's even
> sillier now on the 220 appliances because you now have to have 4 wires
> instead of three - 2, 120 wires and 1 ground and now lets add another ground
> wire.
>
> Just hook up a wire form the ground screw on the back of the outlet to the
> neutral (white side) on the back of the plug - and you have the exact same
> circuit as if you had hooked the ground wire to the white wire at the panel.
> Stick in one of those fancy three light testers and you'll see what I mean.
Wow.
Do NOT do this.
Although the original description is partially correct, in that the
white and ground wires are connected, they are connected PRECISELY one
time and only inside the panel. (ie: they don't make a circuit, it's
like a giant star)
However, this does NOT make the white wire and ground the same or
interchangeable.
White is the return path for the current the device is using. It has
the same current flow as the black wire.
Ground is chassis ground only. It should have no current flow, or at
least just sort of a "background noise" of current flow. Some devices
like fridges apparently leak to ground, I've never measured this but I
know for this reason you can't put a fridge on a GCFI circuit because it
will pop it leaking to ground. But basically, no current flows on the
black wire, it's for safety.
The most important ramification of this is that the chassis of the
unit, which is grounded via the ground wire, is not part of the circuit.
Current is not flowing through the box of your computer, or the body of
your dishwasher. It's flowing through the black and white wires only.
If you connect the ground pin in the box to the neutral wire, then
the chassis of the device is now part of the circuit that carries the
full current of the device.
(if you are lucky, it's part of the circuit you hoped, but depending
on your luck and what is touching what, it might decide to be part of
another circuit as the current finds a different way back to the panel
than down the wire you expected it to... current might come down the
black wire into your computer box, go through the chassis, and back down
the ground of the fax machine your computer is connected to...)
As well, now when you draw current through the device (by turning it
on) then the voltage of the chassis is no longer 0V held to ground. It
acquires a voltage depending on the current flowing through the circuit
and the resistance of the wire feeding the box from the panel.
Now turn on another device in the same room, it has a voltage too,
possibly out of phase with the first one if it's on the other half of
your panel. Now you have two items with exposed metal that have
different voltages, just waiting for somebody to touch them both at once.
As well, if these devices are touching, then one of them might become
the return path for the other if the circuit has lower resistance. Which
might put more current down the ground wire than it was intended for,
and cause a fire. Not to mention that all of the electrical boxes all
the way back to the panel are now part of the circuit as well and have
current flowing through them.
--
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/
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