Two phases to house - loss of neutral
in [Repair]
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From: kreed on 23 Nov 2009 21:33 On Nov 24, 12:16 pm, Sylvia Else <syl...(a)not.at.this.address> wrote: > Phil Allison wrote: > > "Dave Platt" > >> Sylvia Else > > >>> 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. > >> It's a huge issue. > > > ** Sylvia lives in Sydney, Australia and is a MASSIVE TROLL. > > > Nuts of her the post such a COUNTRY SPECIFIC message to a US newsgroup. > > 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. 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.
From: Phil Allison on 23 Nov 2009 21:36 "Trevor Wilson" > > **AFAIK, the US system is fundamentally different to ours. ** In the US and Canada - it is NOT permitted to link neutral and earth or plumbing within a premises. See here under the heading "Regulations". http://en.wikipedia.org/wiki/Earthing_system The MEN system used here in Australia seems to be unique. See last para on same page. ...... Phil
From: Phil Allison on 23 Nov 2009 21:40 "kreed" > 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. ** There is always a good "earth" available - courtesy of the water mains in the street being linked to neutral at every premises. ...... Phil
From: kreed on 23 Nov 2009 21:47 On Nov 24, 10:58 am, "Phil Allison" <phi...(a)tpg.com.au> wrote: > "Stupider than Anyone Else" > > >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. > > ** 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. > 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. > .... Phil
From: Allodoxaphobia on 23 Nov 2009 22:12
On Tue, 24 Nov 2009 09:28:48 +0800, TonyS wrote: > Sylvia Else wrote: >> 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? > > I had exactly this happening two years ago. > My supply has 3 phases ... <-snip-> Oh?? Really?!?! |