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.
I suspect it is really "single phase" (think: center tapped transformer) -- though in Oz they probably do things differently! :>
"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*.
"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)
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.
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.
** 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.
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?)
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.
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.
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.
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.
In this case the GFCI (or RCD in AU) will not trip as the current through the load is the same going in and out. I have RCDs on all 3 phases and none of them tripped.
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.
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.
--
Dave Platt AE6EO
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**Not an issue, due to the MEN system used in AUSTRALIA. Cross-posting to an international group will guarantee a bunch of useless comments. Pay attention to Australian posters only.
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.
Please don't go there, there are many experts that really don't know how a GFCI is suppose to work.
They thieve on the idea that most units now sold have the arc detection in them and no longer label arc (AGFCI) detection as part of the unit,which does not require a ground path, along with it having the original ground fault system which does require grd path to fault.
Those that have installed these units in outlets that have no ground in them, there by, allowing the unaware person to become the ground for it. They think it's ok because just sliding the plug in/out, which causes those nice little arc's to form, from the appliance load, will trip it.
And it's obvious, if you don't have a working ground from the receptacle to the appliance, the GFCI isn't going to work very well when the appliance starts generating small little intermitting shorts that would normally trip the device if the ground on the appliance was present to unbalance the common mode device in the GFCI, with out taking out a human in the process.
Devices like lamp cords, that normally do not have a grd terminal, are excepted because, the arc detection will actually trip, some times. They have to be rather large ones before they work, and other times, things like high powered radio's going by, will false trip them.. That's what you get for trying to accommodate those that don't want to do it correctly. This problem is also prevalent in AF-breakers and code allows variance to accommodate for life saving equipment and the like.
I just love it when the building inspectors use their U-ground testers on GFCI's that have no working ground at the receptacle to find it not tripping when they hit that GF test. It brings a smile to my face..
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