Two phases to house - loss of neutral
in [Repair]
|
From: Sylvia Else on 23 Nov 2009 19:10 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.
From: D Yuniskis on 23 Nov 2009 19:31 Sylvia Else wrote: > I have to phases of power supply to my house - so three power lines, two > phases plus neutral. I suspect it is really "single phase" (think: center tapped transformer) -- though in Oz they probably do things differently! :> > 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. "Large" is a relative term. Homes are typically wired to try to distribute the loads roughly equally on the two legs. And, REALLY BIG loads (e.g., air conditioner compressor) tend to straddle *both*. > My electrician says it's not an issue, but I can't see why. "Luck"? But, in theory, he is wrong. The more significant issue is *how* you would lose neutral. Here (US), your house is tied to *earth* at the main panel. And, neutral is similarly tied to earth. So, you would have to lose the neutral feed from the utility *and* the earth at your house. This has happened to two neighbors in the past 10 years, though. I think in both cases it was caused by a cable actually failing (corrosion). In one case, the folks sharing his distribution transformer (typically four homes to a transformer) lost power when the incoming power was effectively *shorted* to ground, (I don't know what the distribution transformers are fused at but it was enough to set some adjacent utilities -- below grade -- on fire)
From: Sylvia Else on 23 Nov 2009 19:38 D Yuniskis wrote: > Sylvia Else wrote: >> I have to phases of power supply to my house - so three power lines, >> two phases plus neutral. > > I suspect it is really "single phase" (think: center tapped > transformer) -- though in Oz they probably do things differently! :> > >> 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. > > "Large" is a relative term. Homes are typically wired to > try to distribute the loads roughly equally on the two > legs. And, REALLY BIG loads (e.g., air conditioner compressor) > tend to straddle *both*. I imagine that's the case in terms of large potential loads, but if I think about what's actually running in my house at the moment, apart from a couple of lights, it'd be the refrigerator and my computers. I don't know whether they're on difference phases, but if they are they'd represent significantly different loads. Sylvia.
From: D Yuniskis on 23 Nov 2009 20:03 Sylvia Else wrote: > D Yuniskis wrote: >> Sylvia Else wrote: >>> I have to phases of power supply to my house - so three power lines, >>> two phases plus neutral. >> >> I suspect it is really "single phase" (think: center tapped >> transformer) -- though in Oz they probably do things differently! :> >> >>> 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. >> >> "Large" is a relative term. Homes are typically wired to >> try to distribute the loads roughly equally on the two >> legs. And, REALLY BIG loads (e.g., air conditioner compressor) >> tend to straddle *both*. > > I imagine that's the case in terms of large potential loads, but if I > think about what's actually running in my house at the moment, apart > from a couple of lights, it'd be the refrigerator and my computers. I > don't know whether they're on difference phases, but if they are they'd > represent significantly different loads. Exactly. And, those loads *change*. So, when the refrigerator's compressor kicks off, *that* load is gone (here, refrigerator sits between one leg and neutral; yours may straddle both legs?)
From: kreed on 23 Nov 2009 19:57
On Nov 24, 10:10 am, Sylvia Else <syl...(a)not.at.this.address> 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. 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. |