Two phases to house - loss of neutral

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I have to phases of power supply to my house - so three power lines, two
phases plus neutral.

I've on occasion wondered what would happen if we lost the neutral line.
It seems to me that we'd then have the voltage between the two phases
across two sets of appliances, one set attached to one phase, and the
other set attached to the other phase, with the two sets in series as a
result of their common connection to the neutral wire. Since the two
sets are unlikely to represent equal loads, the net result would be a
large overvoltage on one set of appliances.

My electrician says it's not an issue, but I can't see why.

Any thoughts?

Sylvia.

Re: Two phases to house - loss of neutral


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You are both right and wrong as in the absence of Neutral line, a lot
of the neutral current (imbalance) would probably flow via your earth
stake back to the transformer neutral or to a neighbours neutral via
their earth stake and while its not a perfect situation, imbalance
probably would not be as bad as you might think ?

Of course, if there is a long run through earth (terra firma) back to
the nearest neutral, the soil is really dry, the earth stake is sub-
standard etc etc it might not work very well.





Years and years back I once saw the result of this at a 3 phase
installation at a carnival, we were called out to fix

They would have with them, and take from site to site portable fuse
boxes with a 3 phase plug, going to a box with breakers feeding rows
of power points going to each phase (like in a normal building
installation, but portable). They would plug in all their gear to
these sockets, and plug the 3 phase into the 3 phase sockets provided
at the venues.

Of course, a lot of the carnival workers would probably just plug
things in at random, and who knows what load would be on the end of
each lead.  There could also be very large numbers of flashing lights
on one lead too, so the load would be less than perfect, and surely
not anywhere near balanced or stable ;)

One case, the neutral lead broke off.  On one phase there was a lot of
damage to computer gear / arcade games (same sort of thing -
switchmode power supplies)  etc.  Some was just blown fuses, some
worse.
Note, they didn't have the earth and neutral bonded inside the box,
and didn't have a separate earth stake, where in a home fuse box they
would.

2 phase situation like yours would be a similar result, unless the
load was really well balanced, and consisting of mostly NON-switchmode
power supply type devices.  The phase with the lowest load would
suffer overvoltage.

This load imbalance and voltage difference would change all the time
as various appliances automatically switch off and on, like fridges,
air cons, electric HWS etc.  Light bulbs may blow from overvoltage,
this would also change the balance



Try connecting 12v bulbs of different wattages in series across 24v
and see what happens.  The voltage will not be the same across each.
Much the same thing.

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I actually had the neutral fail (break) a few years back during a
winter wind-storm. No deleterious effects. The imbalance (neutral)
current was carried by the ground, a copper water pipe that's probably
at least 10 feet below gound level and 20 0r so feet long.

    Jerry

Re: Two phases to house - loss of neutral


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Sometimes, just to make things really exciting, some of the neutral
current flows over alternate paths to a neighbor's house.  These
alternate paths can include things like cable TV shields.

Re: Two phases to house - loss of neutral



"Stupider than Anyone  Else"

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** Loss of the neutral conductor in a 3 phase ( or 2 phase) AC supply system
can easily be a DISASTROUS  event.

For example:

In the live entertainment productions -  high powered lighting and sound
systems operate from 3 phase outlets via a hefty 5 pin plug and socket.  If,
due to wear and tear or damage to the plug or socket, one of the phases
drops out  -  that is an annoyance but not a disaster.

However, if the pin carrying the neutral fails to connect or as is often the
case *disconnects* during a show because of overheating   -  ALL HELL
BREAKS  LOOSE !!

Typically, some of the lamps will glow very brightly for a second or two and
fail, then other previously dim lamps will follow suit.  The AC fuses in
amplifiers will pop instantly and some equipment will be seriously damaged
when the PSU sections are destroyed. It all happens in a few seconds, while
the operators stand there with their mouths open  !!!

The MEN system:

In Australia, AC power delivery uses a system called " Multiple Earth
Neutral "  -  which requires that the neutral conductors in a premises be
connected to the plumbing system at the distribution board (ie power box).
It also requires that an earth stake be installed for the same purpose, but
some older premises may not have this.

Because EVERY premises has MEN installed, the ground itself and underground
metal plumbing act as an alternative neutral, should the supply conductor
feeding a given premises fail. The disastrous event sometimes experienced in
live entertainment systems is hardly possible in domestic or commercial
premises.

So the sparky is right.



....  Phil





Re: Two phases to house - loss of neutral


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If as you say, neutral has to be tied to Earth at the premises, then I
can indeed see that the situation might not be so bad.

On the face of it, having significant current flowing from neutral to
Earth would have to indicate a fault. It would be nice for the breakers
to trip in such a situation, but I can't see that they will.

Sylvia.

Re: Two phases to house - loss of neutral


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Having the breakers trip because a faulty neutral in an neighbours
house (which would cause his neutral current to flow through his earth and
your earth (etc) to the common neutral) could be an annoyance.

bye.

Re: Two phases to house - loss of neutral


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I recently had such an event happen at a theater. It was county fair
season and the vendor was scrapping the bottom of the barrel for gear.
It was an old distro, with a Yamaha 3500 FOH, with an effects racks.

About 2 hours before house, everything was working when i heard the
system crackling. I looked over at FOH and the lights were flashing.
Then smoke started coming out of the effects rack. I ran to the stage
to shut off power amps and shut everything down.

It was split phase going to FOH. Measured 220 between the hots, and
nothing to neutral. Somewhere, the neutral was lost. Checked the
connector on stage and changed the plugged slot on the distro.

At that point, power came back the way it should. BUT, the effects rack
was still smoking due to fried MOV's in the Furman plug strip.
And all the Drawmer compressors were DOA. luckily, the Yamaha console
was ok. Did the show with a few less toys.

bob

Re: Two phases to house - loss of neutral


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The scenario you mention with live entertainment is particularly bad,
as when you think about it, unlike a fixed installation you can't bond
the earth and neutral in these portable 3 phase to 240v outlet mains
boxes.  If you did, it would trip any earth leakage breakers upstream
- if these are fitted.


To make matters worse, in live entertainment, the major loads are
almost all amplifiers, and lighting. None of these are stable loads
(amplifiers are changing their load current draw constantly with the
music and lights are being dimmed, flashed often in large groups. )


Therefore if the neutral lead fails, there is no bonding to earth to
back it up, unlike a residence, and with the inherent instabilty of
the load balance , massive damage is guaranteed.


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Re: Two phases to house - loss of neutral



"kreed"
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** Current good practice for professional lighting dimmers is to use 800
volt rated triacs  (so over-voltage will not trigger them) and 400 volt
rated primary transformers on each phase with a circuit on the secondaries
that detects any voltage imbalance.

Soon as a significant voltage imbalance is detected, all drive to the triacs
is instantly cancelled and it locks on.

Power amplifiers with iron transformers pretty much protect themselves by
blowing the AC fuse.

SMPSs of all sizes are vulnerable to instant damage or even exploding
electros.


.....  Phil











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Actually, the Neutral conductors are *NOT* connected to the plumbing
system. The Neutral conductor is connected by the MEN link to the main
earthing conductor, which is connected to the appropriate earth
electrode. AS/NZS 3000 then requires that conductive pipe work be
equipotentially bonded to the main earth conductor or earth terminal or
bar (NOT to the neutral).

David

Re: Two phases to house - loss of neutral




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It's a huge issue.

It has happened twice, at my house.  In both cases, a squirrel had
chewed through the neutral wire (they apparently like to sharpen /
exercise their teeth on the rubbery insulation).

The symptoms were much as you suggest.  Voltage goes wonky, with the
voltage being fed to each phase being dependent on the various loads
connected to both phases.  Voltages at the high-amperage appliances
can drop (and this can result in burned-out motors) while voltages on
the opposite phase can spike upwards (burning out incandescent lights,
damaging other appliances through over-voltage).  Worst case would be
a heavy load on one phase, and only very light loads on the other
phase... the lightly loaded side could jump up to almost double
voltage.

The oddest symptom we noticed was that when a heavy load switched on
(e.g. refrigerator or microwave), room lights would suddenly
brighten... just the opposite of what would normally occur.

Both times this happened, we noticed the weirdness immediately, called
the electric company (PG&E), and they dispatched a service team on a
priority basis.

The dispatcher told me that they consider *any* sort of "high voltage,
low voltage" trouble report to justify an urgent response.  She had
understood my use of the phrase "open neutral", and I infer that this
is a not-terribly-unusual situation for them.

After the second time this happened, the PG&E guys decided that our
existing pole-drop was not repairable (it had already been spliced at
least one).  They installed a newer, higher-amperage-rated drop (which
might even have had some armoring).  The problem hasn't recurred...
I'm keeping my fingers crossed.

--
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
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Re: Two phases to house - loss of neutral



"Dave Platt"
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** Sylvia lives in Sydney, Australia and is a  MASSIVE  TROLL.

Nuts of her the post such a COUNTRY  SPECIFIC  message to a US newsgroup.




.....   Phil






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Last time I checked, multiphase power supply was pretty universal, and
the principles to be applied in understanding the consequences of faults
would be the same. Or perhaps you want to argue that the different
voltage in Australia is the deciding factor.

Sylvia.

Re: Two phases to house - loss of neutral



"Stupider than Anyone Else Alive "

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** 100 % WRONG  !!!

The question this insane bitch posted  IS country SPECIFIC !!!

Cos it was about her house in * Sydney, Australia *.

See Trevor Wilson's reply.



....  Phil




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From what I can gather, pretty much every home in the US has a 120v -
0 - 120v system where the 2x 120v's are 180 degrees out of phase.
Between the 2 phases 240v is available for stoves, air conditioners
etc.

In Australia, most separate homes have a straight single phase 240v
supply, its not common for there to be 2,3 separate phases to the
average house.  Therefore the scenario Sylvia mentions is less likely
to occur, as not many people have this multi phase system.

The exception here could be blocks of flats, if the main neutral to
the building fails, and there isn't a good earth, the imbalance could
be a risk.


Re: Two phases to house - loss of neutral



"kreed"

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** There is always a good "earth"  available  - courtesy of the water mains
in the street being linked to neutral at every premises.


.....  Phil





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All new construction [last 30 years] around here uses iron mains with
rubber sealed joints and plastic service pipe into all homes, so there
is NO bond from the house to the main. The Gas main is plastic, as are
the service drops. So, the only earth connection is the rather 'iffy'
pair of 8' ground rods usually in the rain shadow of the roof
overhang.

Neil S.

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That is true in a lot of cases, however I have noticed with a lot of
newer houses, (in QLD anyway) they are using some kind of either black
or grey pvc or poly pipe for home water mains.  To make it worse, it
looks to me to be "crimped" together at joins, sometimes with what
look like soft metal rings but not always.

Copper or the old cast iron pipes would provide an excellent Earth,
and excellent soil electrical contact due to the enormous surface area
of the water main in contact with the soil over distance



Mine is all copper, all the way back to the water main, so no problems
here.

Re: Two phases to house - loss of neutral


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**It's going to be a lot more popular too. The stuff is around half the
price of copper pipes, is very easy to handle and use.


--
Trevor Wilson
www.rageaudio.com.au



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