Supply voltage to overhead 240V mains wiring transformer

[charles]

have they really made 11111316037 of them?

[Huge]

Chacun á son goût.

[Dave Liquorice]

If you ever wander out in the thr countryside and look at the 11 kV power distribution it's all three wire, just the three phases. With a signle phase transformer one side will be connected to a real earth connection at the pole, in our case it looks like a bit of 25 mm^2 before it disappears behind the protective capping and under ground. This earth may or may not be carried on a seperate wire into the house, around here the pole to house connection is mainly just two wire, phase and neutral earth combined.

The only glaring error is the use of LV to differentiate 240 from 11 kV. To the DNO's 11 kV is "LV", I think 33 kV might be be lowest "HV" but it could be 125 kV.

[Huge]

[22 lines snipped]

33kV is HV, according to the man from the 'leccy. The farm next door is being demolished and when the bloke from the 'leccy turned up to disconnect its electricity (& put in a 240V drop for the builders) he refused to go anywhere near the 33kV "because it was HV". Fortunately, in the end he managed to disconnect the 33kV from the transformer, using a long pole with a hook on the end (which he was allowed to do) and make all the wiring changes on the 240V side. He explained all this because they had to disconnect our power for ~30 minutes while he did it.

[Johnny B Good]

I had an idea that the "LV" upper limit was a little bit more inclusive than the 240v house supply voltage level. I just wasn't too sure whether "LV" included 33KV as well as 11KV.

I mentioned "LV" in the context of 240v to emphasise the fact that despite its potential for electrocution, 240v wasn't considered to be a high voltage by the Public Supply Utilities (PSUs).

Still, if that's the only 'glaring error' in my 'pre-google to confirm the facts' posting, I don't think I made too big a fool of myself. :-)

BTW, the sight of only 3 wires on rural 11/33KV transmission lines was also my recollection too but I've even seen 11/33KV lines strung with just two wires on rare occasions which left me a little bit puzzled to say the least.

Googling suggests these 2 wire lines might possibly be examples of a bi- phase SWER transmission line (or, more likely here in the UK, just a simple 2 wire single phase spur to a remote load not deemed worthy of the expense of a 3 phase supply).

[charles]

[Snip]

He was correct BS 7671:2008 aka The Wiring regs states:

Extra-low. Not exceeding 50V ac or 120V ripple free DC

Low. Exceeding extra-low but not exceeding 1000 v ac or 1500 V dc between conductors or 600V ac or 900V dc between conductors and earth

High Exceeding low voltage.

[charles]

there are quite a few of the in what used to be NSHEB territory.

[The Natural Philosopher]

Round here there are a few short single phase 11KV links - typically to a single property or a couple.

[Johnny B Good]

Having had some feedback and googled to verify 'my facts', I've decided to revisit your questions.

Your mention of glass insulators and pole top transformers (mini sub- stations) rather implies an 11KV distribution[1] which doesn't normally include the neutral wire at this Medium Voltage level. The transformers employed are normally delta/wye types where the 11KV primary is delta connected, obviating the need for a neutral connection, with a wye connected 240v secondary to provide a locally grounded neutral return for each customer connection.

Your concern over the rating of the neutral wire only applies when the sub-station transformer is at the remote end of a 240v LV 3 phase distribution cabling network (either underground in urban areas or else on poles in more rustic areas where you may well see all four wires strung from pole to pole).

If your property is fed from a pole transformer (possibly also serving two neighbouring properties), it seems unlikely that the 11KV line will include a neutral wire. Otoh, if your incoming supply is via a drop line from an overhead LV 3 phase line from a remote sub-station transformer, your connection will simply be a live and neutral 2 wire drop connecting to the common neutral and one of the three phase wires.

If you care to walk the route of such an LV line, close observation will reveal that every property connects to the neutral (usually the topmost or bottommost of the four wires) and one phase wire, usually in a cyclic sequence so that each phase wire only connects to every third property.

If you're observant enough, you may also spot that the neutral is no thicker than any of the other 3 phase wires (indeed, if the phase wires are insulated, you might gain the impression that the neutral is thinner than the phase wires).

Your surmise that under balanced loading conditions on all 3 phases that the neutral return current is cancelled out is quite correct. What's important in this case is that even under the most extreme conditions of unbalanced loading conditions, the neutral has to carry no more load current than any of the phase wires, hence it being sized exactly the same as the phase wires.

[1] 11KV is the usual voltage level in this part of the distribution network but lower medium voltages might also be used such as 3.3 and 6.6 KV. Not as high as the more usual 11KV but still high enough to warrant 'glass insulators' and elimination of a neutral wire, courtesy of the pole transformer's delta primary windings connection topology.

[damduck-egg]

It's not all 3 wire ,we had a two wire 11kV line over our fields which served us and two other properties, in fact they were served first when it arrived in 1960 ,as Dad had bought a new generator a few years before and wanted to see some use out of it so we didn't connect till

1964. This branched off a 3 wire about 3/4 of a mile away, a similar 2 wire went in another direction but the two only shared one of the 3 wires in common so that was probably an attempt to keep the load reasonably balanced. They had to run another four poles to carry our 250V supply, each had an earth connection to the neutral with a red metal plate on each pole with PME in white letters on it, as a curious youngster I asked what that meant so became aware of what a protective multiple earth was when quite young.

G.Harman

[JimK]

/Having had some feedback and googled to verify 'my facts', I/q

FFS?

Jim K

[Dave Liquorice]

Yeah OK wrong word "all", we are on a two wire spur...

But the two wire feeds are two of the phases from the main three phase (wire) distribution not a phase and neutral/earth.

[The Natural Philosopher]

Yes. Of course. But that is in and of itself a 'single phase'...

[Huge]

Well, I wasn't about to argue with him about it. And there's no way I'm going anywhere near 33kV no matter what the regs say!

[Huge]

S'what we're on. If you follow our 2 wire overhead a couple of miles (it runs alongside a favourite bridle way) it goes through some switchgear (operable from the ground, although padlocked) and then connects to two of the three phases of a three phase line.

[damduck-egg]

My memory might have got it wrong but I think I was told by an electricity board employee that type of switch wasn't designed to be operated under load but was operated when the line was dead and was for isolation of that spur after which the power on the feed was restored.This came up in a conversation about 45 years ago when some friends and I were caught by said employee who just happened to be passing at the time when my friends and I were lexamining the padlock for the isolator that controlled the feed to our school. Of course it was possible he deliberately misinformed us to discourage us doing it later.

G.Harman

[Andy Burns]

Way more information than those reading this thread are likely to need, but interesting reading all the same

The document seems to contradict itself whether 6.6kV/11kV circuits are HV or LV.

[Brian Howie]

Not a video, It was a bit windy, maybe camera shake.

Brian

[NY]

I'd tend to regard "low voltage" as "definitely capable of being felt but not lethal", and "high voltage" as a voltage that you have some chance of surviving an electric shock from. 240V mains gives you a very nasty belt (I still have the scars on the first knuckle of my right index finger from when I brushed it against the terminals of an on-off switch in a tape recorder in my teens, after I forgot to unplug it, and I had to surreptitiously clean up my puddle of wee from the bedroom carpet). And then apply various levels of "extra high", "super high", "extremely high" adjectives beyond that! 650 V from a third rail would give you very nasty burns and probably kill you. And

25 kV from overhead line train electrification, or 11, 33, 275 or 400 kV from pylon wires, would probably set you alight as well.

I lodged with a couple after I started my first job, and Mick worked as an emergency call-out engineer for one of the electricity companies. I remember he came back late one night, very shocked (pardon the unintentional pun) after having to clear up from an incident in which a JCB operator had plunged his bucket into a 400 kV underground cable. The JCB had melted and twisted so they had to saw it up to get the body of the guy out - and only then could they start to repair the cable. I take my hat off to the guys who work on pylon wires, firstly at great height and secondly while the lines are still live, in specially insulated cherry-pickers.

[Cursitor Doom]

In the CAT system for grading the safety of DVMs, I often see the term (for CAT IV) "The source of the low voltage installation" and (CAT II) "The circuits directly connected to the low voltage installation." What is meant by "low voltage installation" in this context exactly? Sounds a very ambiguous.