Two phases to house - loss of neutral
in [Repair]
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From: Phil Allison on 23 Nov 2009 19:58 "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. ..... Phil
From: Sylvia Else on 23 Nov 2009 20:24 Phil Allison 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. 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.
From: TonyS on 23 Nov 2009 20:28 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? > > Sylvia. I had exactly this happening two years ago. My supply has 3 phases and I lost part of the neutral connectivity through corrosion on the terminal coming in from the power pole. The result was a brown out on some and over voltage on other phases. In top of this I got a tingling zapping in the shower. I guess, depending on the quality of the earth rod, this can be a pretty bad situation. Where will the returning current go if there is no neutral? Through a different phase I guess. Neutral will have a more or less floating potential. Tony
From: D Yuniskis on 23 Nov 2009 20:38 Sylvia Else wrote: > Phil Allison wrote: >> 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. > > 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. Grounding the mains to "plumbing" *was* common in the US many years ago. Now, I believe, you *must* ground to a metal rod driven into the earth (8 ft?). Note that if the water meter was removed, your earth-through-plumbing would fail. > 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. This is how GFCI breakers work -- they watch for current "leaking" off to ground someplace other than in the "return" conductor.
From: Sylvia Else on 23 Nov 2009 20:43
D Yuniskis wrote: > Sylvia Else wrote: >> Phil Allison wrote: >>> 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. >> >> 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. > > Grounding the mains to "plumbing" *was* common in the US many > years ago. Now, I believe, you *must* ground to a metal > rod driven into the earth (8 ft?). Note that if the water > meter was removed, your earth-through-plumbing would fail. > >> 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. > > This is how GFCI breakers work -- they watch for current "leaking" > off to ground someplace other than in the "return" conductor. In the scenario I outlined, the currents in the live and neutral conductors passing through the breakers would remain equal, so they wouldn't trip. What I need is something to detect current through the link between neutral and ground, which would then disconnect both phases. Of course, it could trip as the result of a fault with the neigbour's neutral wire, but I could live with that. Sylvia. |