as the subject says, have had an interesting discussion that goes something
like this.

assume the main power utility line is dead, that is, the local power utility
is "down" and your service line is dead.

turn off your house main switch, right next to your utility service meter
(also called "the can").

if the power utility turns power back on the line, this energizes the "can"
(but keep in mind that the main switch is still OFF), the claim is that
despite the main switch being turned off, you can still have 120v (not 240v)
leaking through to your breaker panel and if there is a generator feeding
power, it can cause a shutdown or worse.

can this be even remotely true?

if not, can the condition occur somehow due to the way the power utility has
wired the neutral line connection from the main line to the house?

the main question here is: Does the house remain safe, ie. power off, if the
Main switch is Off, even if the main utility line is restored to power?

Posted by daestrom on July 4, 2008, 8:12 pm

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Tom wrote:

> as the subject says, have had an interesting discussion that goes

> something like this.

> assume the main power utility line is dead, that is, the local power

> utility is "down" and your service line is dead.

> turn off your house main switch, right next to your utility service

> meter (also called "the can").

> if the power utility turns power back on the line, this energizes the

> "can" (but keep in mind that the main switch is still OFF), the claim

> is that despite the main switch being turned off, you can still have

> 120v (not 240v) leaking through to your breaker panel and if there is

> a generator feeding power, it can cause a shutdown or worse.

> can this be even remotely true?

> if not, can the condition occur somehow due to the way the power

> utility has wired the neutral line connection from the main line to

> the house?

> the main question here is: Does the house remain safe, ie. power off,

> if the Main switch is Off, even if the main utility line is restored

> to power?

Of course. With the main switch off, the power stops at the switch and goes
no further.

daestrom

Posted by hubops on July 5, 2008, 10:37 am

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>> as the subject says, have had an interesting discussion that goes

>> something like this.

>> assume the main power utility line is dead, that is, the local power

>> utility is "down" and your service line is dead.

>> turn off your house main switch, right next to your utility service

>> meter (also called "the can").

>> if the power utility turns power back on the line, this energizes the

>> "can" (but keep in mind that the main switch is still OFF), the claim

>> is that despite the main switch being turned off, you can still have

>> 120v (not 240v) leaking through to your breaker panel and if there is

>> a generator feeding power, it can cause a shutdown or worse.

>> can this be even remotely true?

>> if not, can the condition occur somehow due to the way the power

>> utility has wired the neutral line connection from the main line to

>> the house?

>> the main question here is: Does the house remain safe, ie. power off,

>> if the Main switch is Off, even if the main utility line is restored

>> to power?

>Of course. With the main switch off, the power stops at the switch and goes

>no further.

>daestrom

I wonder if he is talking about the momentary situation -
when a phase conductor is re-energized, after a trip,
while in contact with the neutral ?
John T

Posted by Neon John on July 5, 2008, 1:40 am

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>>Of course. With the main switch off, the power stops at the switch and goes

>>no further.

>>daestrom

> I wonder if he is talking about the momentary situation -

>when a phase conductor is re-energized, after a trip,

>while in contact with the neutral ?

No, this idiocy started a couple of years ago when someone who knew just
enough to be dangerous ran his mouth. His allegation was that the utility
power could come back 180 deg out of phase with the generator power (true, so
far) and that the main breaker could have a differential voltage across it of
480 volts (also true). The idiocy starts with his next claim, that since the
main breaker is only "rated for 240 volts", the double voltage could flash
over. I mean, after all, "480" is a much larger number than "240", right?
Sheesh.

This guy was totally ignorant of what kind of impulse tests such apparatus has
to withstand to get the UL and other stamps of approval. I don't recall the
exact numbers but the voltage is in the thousands of volts. Dissecting even
the cheapest stuff on the market like Murray breakers quickly shows just how
much arc/creep distance there is inside.

I thought that we beat this one down pretty well when it made the Usenet
rounds but it sounds like this same idiot is now promoting his zany idea in
the fora world.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
If stupidity hurt then there'd be Aspirin in the salt shakers.

Posted by on July 6, 2008, 6:51 pm

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|
|
|>>Of course. With the main switch off, the power stops at the switch and goes
|>>no further.
|>>daestrom
|>
|>
|> I wonder if he is talking about the momentary situation -
|>when a phase conductor is re-energized, after a trip,
|>while in contact with the neutral ?
|
| No, this idiocy started a couple of years ago when someone who knew just
| enough to be dangerous ran his mouth. His allegation was that the utility
| power could come back 180 deg out of phase with the generator power (true, so
| far) and that the main breaker could have a differential voltage across it of
| 480 volts (also true). The idiocy starts with his next claim, that since the
| main breaker is only "rated for 240 volts", the double voltage could flash
| over. I mean, after all, "480" is a much larger number than "240", right?
| Sheesh.

That depends on where the flashing is expected to take place. Assuming the
neutrals are already connected (e.g. the switch does not disconnect neutral)
then as the utility has +240 (x sqrt 2) volts and the generator has -240
(x sqrt 2) volts, there would be a difference of 480 volts (x sqrt 2).

This is assuming a 240 volts relative to ground system (e.g. UK, AU).

| This guy was totally ignorant of what kind of impulse tests such apparatus has
| to withstand to get the UL and other stamps of approval. I don't recall the
| exact numbers but the voltage is in the thousands of volts. Dissecting even
| the cheapest stuff on the market like Murray breakers quickly shows just how
| much arc/creep distance there is inside.

Of course the tests already take into account the (x sqrt 2). So a test for
240 volt AC is a test for 340 volts peak. A test for 480 volts AC is a test
for 680 volts peak.

It is quite normal for UL tests to test for voltages at least double that for
which the device insulation is to be rated, and perhaps well more than that.

| I thought that we beat this one down pretty well when it made the Usenet
| rounds but it sounds like this same idiot is now promoting his zany idea in
| the fora world.

Sounds like he is wrong. But it is more a social question than a technical
question that he is wrong about. If UL had done tests exactly for 240 volts
AC (340 volts peak) and no more than that, and the device under test was
designed for the minimal capability that would pass, it could be that such
a device could fail a little above 240 volts AC.

Fortunately, UL is not so foolish. At least I hope not.

But that begs the question of manufacturing tolerance. If the device is
tested at merely a 480 volt AC (680 volt peak) level, variations of say
10% in manufacturing could result in some devices that could break down
at 432 volts AC (612 volts peak). Such a device would work to protect a
240 volts circuit under normal circumstances, yet fail when faced with the
scenario described wherein 480 volts AC (680 volts peak) is applied to
the terminals of the breaker pole.

I just don't know at what level the UL actually tests these breakers, and
what aspects the tests apply to. For example, it might test for two times
the voltage between conductors and the frame outside the breaker (e.g. the
voltage withstand of the breaker casing) and test for four times the voltage
between terminals (e.g. the contact gap). UL testing procedure documents
are available, but have a high cost, so I have not purchased them. If you
really want to know for sure, you will have to part with some coin to see.

I'm far less worried about double voltage jumping across an open breaker
contact than I am about the idiots that might close the breaker when the
utility linemen are working on the wires. The scenario of double voltage
across an open breaker should not be allowed to be possible, regardless of
whether the breaker can withstand it or not.

--
|WARNING: Due to extreme spam, googlegroups.com is blocked. Due to ignorance |
| by the abuse department, bellsouth.net is blocked. If you post to |
| Usenet from these places, find another Usenet provider ASAP. |
| Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) |

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