Mark McIntyre said:
Perhaps it could start with "this standard expects its readers to have
common sense, if you're too stoopid to wire up a 3-pin plug, then stop
now".
Wiring a 3-pin plug _today_ is easy, because recent standards define what
each of them should be and what color wires to connect where. In the olden
days, it was undefined which of the two original prongs was which.
[1] It is Christmas time so a perfect example. Strings of lights used
to have fuses on each lead. Why?
Because if the cable broke half way along, both halves would be carrying
current and you could get fried off each half.
Wrong. One wire would be carrying current but the other wouldn't -- but you
wouldn't know which. Even today, most two-wire plugs can be inserted either
way. If it matters to the device, one prong will be larger than the other
(a so-called polarized plug) so it can only be inserted one way and/or the
plug will have a ground pin. However, to deal with older sockets that can't
take the larger prong and/or extra pin, adapters are easily found at
hardware stores, so there's still no guarantee -- even if the user inserts
the adapter correctly, the older outlet might be wired backwards (using
today's rules; at the time it was installed, it was okay).
(Yes, the "correct" solution is to install modern outlets, but if you do
that, everything between each outlet and the transformer must be brought up
to current code and an electrician has to verify it, which is a very
expensive undertaking and would affect millions of homes. Because of that,
when new codes come out, all preexisting wiring is grandfathered in _as long
as it's not modified_.)
Euh? Do you guys use DC in the states? If not, it doesn't matter which is
live, they're both carrying 240/110 V. Follow 'em back to the pole where
the phases split out....
You misunderstand American AC wiring. In a typical residential setup, the
transformer provides three wires: two hots (each 120VAC, 180 degrees out of
phase) and one neutral (via a center tap). Neutral is _not_ ground except
at the transformer and/or fuse/breaker box; inside the house, the neutral
wire can vary quite a ways from ground and it can shock you even if you're
grounded (or start a fire).
For standard 120V circuits, the fuse/breaker will be connected to one of the
hots and to neutral, and the outlets connected to the breaker. The wiring
between the fuse/breaker and the outlet was originally not standardized,
which meant you never knew which of the two prongs was the hot one and which
was neutral. Later, the left prong (facing the socket) was defined as hot,
color coded black in the wiring, and the right was neutral, color coded
white. Also, a bit later, a pin was added below for earth ground, color
coded green (or sometimes bare).
For the 240V outlets that are used for clothes dryers, air conditioners, and
heaters, there are special breakers that connect to both hots but not
neutral. Obviously in that case it doesn't matter which side is which, but
recently they've been color coded as red and black (to make it obvious
they're not 120V circuits, which are white and black). Again, a third wire
for earth ground, color coded green, was later added for devices that needed
it.
S