On Sat, 17 Sep 2011 09:30:34 -0400, Doug Miller
wrote:

On 9/16/2011 11:02 PM, John Gilmer wrote:


Consider two simple circuits, A and B, each supplying no loads other
than a single 100W light bulb, each of which is turned on -- but,
unbeknowst to you, their neutrals are cross-connected. Well and good.
Now suppose you need to replace the fixture on circuit A. Knowing that
it's on circuit A, you shut off that breaker, and disconnect the hot
lead to the fixture. Now when you go to disconnect the neutral lead, you
get a shock, because -- and I think this is the point you are missing --
electricity doesn't "follow the path of least resistance" as many people
believe, it follows *all possible paths*, including the one that you've
just made with your body by touching that energized lead.

Huh?

The reason you don't use the "wrong" neutral is just the subtle effect
of poor cancellation of the magnetic field generated by current carrying
conductors. If the conductor and it's neutral are close to each other
their magnetic fields cancel.

I know that. But that's completely unrelated to the shock danger.

You *can* get shocked by the neutral.

The main time neutrals can produce a dangerous situation is when,
somehow, they get dis-connected from the power source

Even if the neutral is not disconnected, if you make, with your body, a
parallel conductive path from the neutral to ground, *most* of the
current in that neutral will indeed return to ground through the neutral
conductor -- but *some* of it will return through your body, as well.

Electricity follows *all possible paths*. Don't use your body as one of
them.

IF the neutral connection is "adequate" - less than 1/2 ohm, let's
just say for arguement's sake, and your body resistance is lower than
normal (let's just pick 500 ohms out of thin air) and the load current
is 15 amps (anything more would trip the supply side breaker) what
current would flow through your body???? The voltage drop across the
..5 ohm ground connection would be 7.5 (15 ampsX.5 ohms)volts. The
current through your 500 ohm body would be less than 15 ma.

A .5 ohm ground connection would show up as an obvious problem with a
7.5 volt drop and 112 watts of heat dissipation at the ground
connection. A much more common (or let's say POSSIBLE situation is a
..05 ohm ground connection - with a 0.75 volt drop, dissipating 11
watts of heat at the bad connection (still very noticeable as a bad
connection) where the current flow through a 500 ohm body resistance
would be less than 1.5 ma.

Now substitute a much more realistic body resistance (with wet skin)
of 10,000 ohms and you can see how rediculous your arguement is. 7.5
volts across a 10,000 ohm resistance is 0.75 ma of current.

Go to a DRY SKIN situation, with body resistance of 450,000 - 495,000
ohms, and you see where we are headed??????? We are aproching 1.5 e-5
amps. that's something like 0.0015ma if my exponential math is
correct. And that is with the VERY BAD 0.5 ohm ground.

(either at the
service/breaker panel or in an intermediate junction box.) Any kind of a
load will make a white wire (your neutral, HOT, HOT, HOT!

And that makes your body the *only* path for the current to flow
through. Your mistake is in thinking that this is the only dangerous
condition. It's not.