MikeM said:
If you have an open ground, how do you tell that the "polarity" is
reversed?
In the US, the 120V line is distributed on two wires, called "Line"
(Black wire), "Neutral" (White wire), with an optional "Ground"
(Green/Bare wire).
On newer style outlets, the ground is the round fat pin.
The Neutral slot is located clockwise from the ground pin.
It is usually slightly wider (or T shaped). The Line slot is
located counterclockwise from the ground pin.
In the main circuit distribution panel, the Neutral line
is supposed to be connected to the same "earth ground" as the
the ground pins on the outlets.
In other words, all of the white wires from the respective
outlets come together to a common (insulated from the panel)
screw-terminal bus inside the panel. All of the green/bare
wires from the respective outlets come together to a bare
screw-terminal (connected directly to the panel box). There
is a "jumper" which ties the ground bus to the neutral bus
in the panel...
There are lots of old houses with outlets which only have two
identical slots. The outlet is not "polarized", so you have
no knowledge of which slot is which.
In these old houses, there where only two wires inside the wall
box (Line and Neutral). The third wire (ground) doesn't exist.
What happens over time is the two slot outlet gets replaced
with a new outlet, so the ground pin is left unconnected.
If the installer knew the color code, they may have connected
the Line to correct pin; or not!
MikeM
It is quite easy to test which wire is hot or not, use a volt pen, available at any "Home
Depot". Bring it near the wire in question, and if voltage is present, it will either
beep or light up (depending on the model, some do both), the neutral wire will not do
this. No physical connection is required to the wire, just hold it up near it. It is
inductive.
You mention that ground is optional in the US? hehe, it certainly isn't in Canada. I
believe you mean that older homes may or may not have ground running up to conventional
outlets.
And the T shaped style U-ground receptacles are designed for 15A/20A nothing else. They
accept a standard 15A 120V U-ground, and a 20A 120V U-ground which is designated by one
blade on a horizontal plane.
BTW: if you observe a standard 120V 15A plug, the zinc plated screw is Neutral, the brass
screw is Hot.
The biggest problem with reversed hot/neutral wiring in a home is if you "think" you've
switched off power to lets say a lamp fixture, and you are servicing it, you "hope" that
you've switched off the hot, if it were reversed, you'll be working on a live circuit.
Another poster suggested that he would have designed 120VAC to be made uo of two hots (60V
lets say each leg), the problem with that is, you would need to switch both lines at a
switch, complicating everything, (3-way switches for example), fusing would have to be
doubled (as in 220VAC single phase), so simply put, a hot/neutral setup isn't such a bad
thing. Same with HV (Canadian industrial is 347/600VAC 3 phase), you can supply an entire
row of HV 347 flourescent lamps with one hot, but if you had to do it with two hots, this
would complicate matters, unbalanced loads, etc....
The whole moral of the story, a properly wired outlet is important.