[Physltest] [Phys-L] Re: ISOLATION TRANSFORMER

[John Denker <jsd@AV8N.COM>]

Bernard Cleyet wrote:

> 2) The white wire (which for various reasons can sometimes be gray), the
> silver screws (commonly called the white screws), the wide slot on power
> outlets and the threaded sleeve of the light bulb sockets are all the
> neutral connection.

I agree.  I very much regret showing non-standard coloring
in my note earlier today.

> This is insulated from ground everywhere but at
> the service entrance where it is ALSO tied to the ground bus.   Yes,
> green and white/grey are tied together at the service entrance.   The
> hot wire is the power source, the white/grey is the power return.   As
> with the green wire safety ground above, the white/grey neutral wire
> should never be interrupted by any disconnecting mechanism (but I've
> seen too many residential bedroom ceiling light circuits wired with the
> switch in the neutral lead).   Note that while the white/grey wire is at
> zero volts, it is NOT considered as or referred to as "ground".

That's all true.

> Any color except green or white/grey is a HOT wire   I will repeat
> myself in fewer words and with emphasis:   ALL OTHER COLORS ARE HOT.
> Engrave that in your brain.   Deeply.   It may save your life or that of
> a family member, friend or co-worker.

The mnemonic of color versus lack-of-color obviously doesn't
work for me, or I wouldn't have made a fool of myself this
morning.  From the point of vue of almost any theory of color,
white has no hue, but neither does black.  Maybe a better
mnemonic would be to think of white as being the color of
innocence and purity, while black and red are not.

  -- The place where it matters most is lamps.  When somebody
is changing a bulb, it is vastly more likely that they will
touch the threaded part, as opposed to reaching all the way
down into the socket and touching the "hot button" at the
bottom.

  -- Also when there's a simple switch, you're better off
switching the hot side rather than the neutral side.

  -- You can find other examples where hot-versus-neutral
matters, but it's a stretch.

In most cases other than lamps, nobody is going to touch
either side of the line unless there's a fault, and assuming
faults are equally likely, it doesn't matter which wire is
hot and which wire is neutral ... you're faced with the
choice of betting on heads versus betting on tails, and it
just doesn't matter.  This is reflected in the fact that the
cord on my cell-phone charger is unpolarized, i.e. it has
no wide prong, so you can plug it into the outlet either
way around.

Also, there are perfectly code-compliant situations where
the white wire can be hot.  The "three-way" switches for
hall lights are a common example.  In the junction box at
each end you're supposed to wrap an open helix of black
tape around the white wire in question, to remind people
it is hot half the time ... but if you cut into the wire
at some random point halfway down the hall, you can find
a hot white wire with no distinguishing marks.

==============

My advice:  Take the belt-and-suspenders approach.
   1) Follow the color code in particular and the National
Electrical Code in general.  You can get a copy of the
NEC in bookstores.
   2a) Don't touch any wire, *regardless of color*, unless
you have shut off the circuit, double-checked that it is
powered down, and made sure it's going to stay that way.
   2b) When designing apparatus, make sure *both* sides of
the line are insulated from the outside world.
_______________________________________________
Phys-L mailing list
Phys-L@electron.physics.buffalo.edu
https://www.physics.buffalo.edu/mailman/listinfo/phys-l