From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Campground connectons wired wrong
Date: Wed, 27 Jun 2007 16:05:17 -0400
Message-ID: <tgf58357264kujlm5dnlgu94rdm01h7569@4ax.com>

On Wed, 27 Jun 2007 11:26:41 -0500, Jud Hardcastle
<I5i5changethistodash5rbo@xemaps.com> wrote:


>I am a bit confused about the "feature" listed for all the 50amp models:
>"Accidental 220V protection: If 220 volts is detected when plugging into
>source power the EMS will not allow power to the RV." I kinda think
>you're going to detect 220/240 at most 50amp RV outlets :-) even if the
>coach only uses it as two 110 circuits. What are they referring to?

It'll be looking at each hot lead to ground which should be 120vac even on a 50 amp
connection.  If there is a screw-up and 240 to ground ends up on one power lead then
it will not connect.

One common way this happens is when the utility doubles the primary voltage to
increase the power handling capacity of an overloaded distribution line.  Around here
that would be from 7200 volts to 14,400.  Each transformer has a switch on the side
that is flipped to keep the output 120/240.

The problem is that careless line crews sometimes miss transformers.  In that case,
what comes out is 480/240.  Not a good thing.

I'm a GIS consultant to a rural utility, hired to help stop those happenings.
Basically, I go around, map each resource on the proposed conversion circuit using
good old Street Atlas, convert the GIS data into work sheets and checkoff lists and
then on the day of the conversion, sit on the radio checking off each resource as it
is converted.  Only when I have a green screen (uncoverted tags are red, converted
ones are green), do I give the OK to turn the power back on.

Knock on wood, so far we've not burned down a single house since I've been involved.
They would routinely start one or two house fires before.

The problem would arise if you happened to be the first person to plug into a 50 amp
outlet after the conversion.  If the conversion had been done in the winter and
especially if the park closes for the winter, the conversion could have been done
months ago.

The places to watch out for this are areas with extreme growth where the power
distribution branch has become suddenly overloaded.  The utility can double the
branch's power handling capacity by simply doubling the primary voltage and changing
out any hardware that isn't compatible.

This is a relatively rare occurrence but as long as the engineer is designing the
instrument, might as well watch out for it.

Another infrequent occurrence is when the line crew grabs the wrong pole transformer
to replace one that is storm damaged.  Again, either 240/480 or 60/120, depending on
which way the error went.  Crews are supposed to check the output voltage before
connecting the customer drop(s) but in the heat of storm recovery, they sometimes
don't.

Yet another still-infrequent but a little more common occurrence is when a 7200 volt
distribution line passes under a 14,400 line and something (storm, falling limb, car
wreck, etc) knocks the higher line down onto the 7200 volt line.  If the high line
doesn't touch the ground then its protective relaying probably won't de-energize it.
The low line's A breaker back at the Substation will open but the line will remain
energized with double the normal voltage.  Usually the insulation on something on the
7,200 volt line eventually fails and 14,400 volt line trips but not before death and
destruction is wreaked on the downstream customers, again sending 240/480 into their
houses.

When you hear of a fire being started because someone's breaker panel or appliance
suddenly burst into flame, this is the usual cause.

John

From: John De Armond
Newsgroups: alt.energy.homepower
Subject: Re: can utility power jump main power cutoff switch?
Date: Mon, 07 Jul 2008 21:12:56 -0400
Message-ID: <fte574hol0a5s4omur6srqan84h749sa5q@4ax.com>

On Mon, 07 Jul 2008 17:24:18 -0400, hubops <tabbertj@perth.net> wrote:


>  There are cases where the sub-transmission feeder falls
>down into the rural underbuild  ...
>  44 kv  sitting on a 8 kv phase conductor
>and gets re-energized  ...
> I'm not sure what blows-up first, in these cases  -
>the homeowners breaker, appliances, generator ,
>or the pole top transformers  ...
>  .. but rest assured - it's messy.
> ..  maybe Phil will petition the NEC to insulate
>all lines to the maximum voltage that might be
>strung over-top  ?  (couldn't resist)

Far more common is the condition that occurs when the utility upgrades the
voltage on the feeder to accommodate more load.  A common occurrence is to up
the primary from 7,200 volts to 14,400.  All the pole pigs are of dual-voltage
switched primary style.  Old pigs are replaced with switched pigs before THE
DAY.

On THE DAY, the feeder is killed and a wave of linemen assault the system,
flipping the switch on every pig, while back at the sub, either the
transformer is changed out or a tap is changed.  When the sub breaker is
closed again, 14,400 volt power flows on the feeder and everything is
wonderful.

Except for the few pigs that they invariably miss because of their sloppy
procedures (I'm thinking of one of my client utilities in particular now).
When power is reapplied, 240/480 is applied to each load (house) attached to
the pig.  The pig can handle it for awhile - long enough to start a 4th of
July show at each house.

At a minimum, everything electronic in the house is smoked.  If the water
heater and heat pump are on (almost always the case since the power has been
off for a few hours), they are smoked too.  For the lucky homeowners, that's
the end of it.  For the unlucky ones, their houses catch on fire and, being
that this is a rural area, the houses burn to the ground.

Regardless of where you live, if you ever get a post card or email or whatever
from your utility advising that the power is going to be off on a certain date
for "upgrades to the system", pull the main breaker that morning and leave it
off until AFTER the power has been restored and you know that everything is
back to normal.  That way, if yours is one of the unlucky houses, all that
will happen is that the meter will be blown off its base.  If you have a
whole-house surge suppressor, it'll be blown up too but it probably won't
start a fire.

John