On Wed, 23 Jun 2004 22:31:15 -0700, "Harold & Susan Vordos"
wrote:
The panel of which I spoke is a real simple device, must be switched
manually, and I even have to plug in the generator, although it would be
left plugged in and in position if desired. It has a couple breakers
mounted back to back with a slide that doesn't permit both the generator and
the line to be connected to the structure at the same time. That way it
can't back feed into the entire grid system.
Harold,
Does that switch transfer BOTH the hot and
neutral wires? If not, there are circumstances
where you COULD send power back into the grid.
Any method you use to connect a generator MUST
transfer all wires except the ground off the
grid. You might work something up with switches
intended for 3-phase that would allow a proper
transfer.
Here is something I wrote in rec.boats in Sept 99:
[I asserted that it was possible to feed power back
into the grid if only the hot lead was transferred.
Another reader said the breaker would trip first.]
A short of that magnitude would trip the breaker on the generator,
but perhaps not quickly enough to prevent someone getting a nasty
shock.
As I pointed out earlier, breakers don't trip all that fast if
the overload is not great. Resistance in the local "loop" is
one way that the overload may be fairly low.
The 60V at the transformer "neutral" (and anywhere between the
transformer and the house) could be a problem if a few things
failed simultaneously. But there is no 2nd wire to return current
(if the mains are disconnected at the "hots"), so there could be
no step-up at the transformer. As far as I know, the transformer
secondary has windings only between hot and neutral, and there is
no winding between neutral and earth, as that would impress a
voltage on neutral with respect to ground. The neutral is
connected to a stiff earth ground somewhere, either at the breaker
panel itself, or somewhere closer to the transformer. This stiff
ground would have to fail before any significant voltage would be
seen at the neutral up at the transformer.
Neutral-to-ground (N2G) bonding is not always a good connection.
When it is made at the breaker box, it is usually made with a single
screw from the neutral buss-bar to the shell of the breaker enclosure.
If the screw is loose or has failed to cut through the paint in the
pre-tapped hole (common, in my experience), then the N2G bond may
have fairly high resistance.
The local (demand side of transformer) loop can take many paths.
When a generator is connected across neutral and disconnected
hot leads, a hot to ground connection (short in an appliance,
damaged insulation, etc) will try to raise the neutral to 120v
against whatever bonding exists. A high-resistance bond (or low
current-carrying capacity such as a tiny bit of thread cutting
through paint) can make this possible without overloading the
breaker. Now, all that is needed is a path to ground on the
hot lead between the transformer and the disconnected mains
OR ON ANY HOT LEAD STILL CONNECTED TO THE TRANSFORMER. If there
is another dwelling on the same transformer, then any appliance
or load that is still connected provides a path to ground through
the neutral-to-ground bonding in THAT dwelling. Other paths to
ground could include fallen drops (transformer to house wires)
that let the hot lead connect to ground, fallen branches or
other power lines across the drop -- anything that connects
the transformer side of the hot wire (before the disconnected
main breakers) to ground.
The loop then becomes: generator-hot - hot wire on load side of
breaker - damaged appliance - ground - (say) fallen drop hot
wire - hot terminal of transformer - transformer secondary
winding - neutral terminal of transformer - neutral buss in
breaker box - generator-neutral. There are plenty of places in
this loop likely to be high enough resistance that the breaker
does not trip immediately (or even at all). These things will
all tend to reduce the current in the secondary and thus the
current induced in the primary which is being fed back into
the transmission lines. Nonetheless, with a step-up of 120v
to 4000v or more, the induced voltage is going to be quite
high. Even 12 volts across the secondary can make 400 volts
on the primary.
If the transmission grid was all in working order, it wouldn't
be enough to cause a dangerous voltage on the transmission lines,
but the nature of storm damage is that many sections of the
transmission lines are isolated either deliberately or by
automatic disconnections with little or no load on them.
It is precisely these sections that are likely to have a
water-soaked lineman working on them.
No quibbling -- it is just extremely bad practice to connect a
generator to a dwelling that is not COMPLETELY isolated from
the transformer by a transfer switch that is designed to
do exactly that job with no failure modes that can bypass it.
Here is a google-groups url that will take you
to the original post; you can get to the whole 47
message thread from the
http://www.google.com/groups?q=hot+n...n.com& rnum=1
If you have trouble cutting and pasting that long
line try searching google groups using advanced
search for:
all these words: hot neutral generator
author: peter meek
--
--Pete
"Peter W. Meek"
http://www.msen.com/~pwmeek/