At 01:49 PM 7/11/2012, David Beierl wrote:
>Open neutral between the breaker box and the service entrance is
>much less common but more dangerous, and will NOT give the readings
>I said above.  It will give these instead:
>
>H-N 0-240 V; H-G 120 V; N-G 0-120 V.  The closer N-G is to 0, the
>closer H-N will be to 120.  Voltages may change abruptly as loads
>shift throughout the house/camp.
>
>Explanation to follow.

US domestic wiring comes into the house on three wires, two hots and
a neutral.  Each hot wire is 120 VAC with respect to ground/neutral,
but they are in opposite phase; in other words they cross zero volts
at the same time, but then one goes negative while the other goes
positive.  As a result, though they each carry 120 V to ground, they
carry 240 V between themselves.

At the circuit breaker box, the breakers are mounted on a tree such
that each successive breaker going down a column is energised by the
other hot wire from its neighbors.  That way, to get a 240 V circuit
for stoves etc it's only necessary to mount a twinned breaker with
two wires to the appliance.  The other breakers each have one wire to
the circuit they feed, and a return wire comes back to the neutral
bus inside the box which is connected to the neutral service feed
(which is at some point grounded).

So in normal operation, half the 120 V circuits are out of phase with
the other half, but nobody cares.  Both phases return to the neutral
bus which is grounded, and each appliance is connected between one of
the phases and neutral, and sees 120 V.

But if the neutral bus should be disconnected from the neutral
service wire, everything changes.  Now all the (operating) appliances
on one phase are connected through all the ones on the other
phase.  The voltage each set sees is in inverse proportion to the
amount of load on that phase.  So if you've got let's say only a
ten-amp toaster connected on one phase and only a 1/10-amp compact
fluorescent light operating on the other phase, the toaster will be
cold and the light will suddenly discover the thrills of operating at
twice its rated voltage.  Usually the imbalance won't be that
extreme, but you get the idea.  I've seen this happen once, after
hours at an electronics company I worked for.  I was one of two
people in the place and we figured it out, but not in time to save a
couple of smoking radios.  Fortunately most of the fancy gear was turned off.

Yours,
David