suppliers neutral break?

[liddley]

suppliers neutral break?
can someone explain the reason why the voltage in a domestic dwelling rises when the neutral is broken out in the street? 3 phase is not something I deal with and not too clear on the theory?! probly a daft question just me being thick?

 

[hasel5]

Tony one for you lol i know the theory but to many pinots

 

[D Skelton]

In a way it turns the star system you're on to a delta. Vl (line voltage) on a star is 230V and Vp (phase voltage) is 400. On a delta however Vl and Vp are the same.

A star winding has a neutral at star point. The point of the neutral is to provide earth potential (0V) to eliminate an unbalancing between phases, this gives a potential difference between line and neutral of 230V and a potential difference between line - line of 400V. A delta system has no neutral hence why line - line is the only potential difference you can measure (discounting of course line - earth).

Not enough time to go into a huge amount of detail but if you understand the fundamental difference between star and delta windings this should make sense.

Edit: Just realised you don't know much 3P theory. Go on wikipedia and look at the difference between star and delta windings. You don't really have to know the maths to understand it a little, all you need to know is that without a neutral connection on the suppliers side you only have the potential difference between phases, which of course we all know is 400V. All loads on any other phase, after the break but before you essentially turn their neutral into a phase. As do you for them. One thing to bear in mind is that the voltage will only rise according to the unbalancing of each phase.

In short, if this sitution occurs, it is a disaster! It is rare but extremely dangerous.

Edit 2: Just re-read that and realised I'd make a crap teacher Sorry to the OP if I've only confused you more!

 

[Guest55]

a load of confusing stuff.........

;-)

please explain how this would increase voltage in a domestic thats almost certainly only single phase lol.

 

[D Skelton]

Ok, I'm on my 5th beer but I'll try lol.3 houses on 3 phases fes by star tranny. Break in suppliers neutral before any of the three houses. Voltage on L1 into first house, returning on neutral which is no longer at 0V, this voltage on the neutral then picked up by next house along and then returning via L2. Voltagfe on L2 into econd house, returning on neutral then picked up by third house returning via L3, third house on l3... Returning via neutral to first house then to L1.I can visualise it quite clearly, explaining it in english is another thing Biff! Ya get my drift tho?

 

[Guest55]

i'm sure i can see slurring in that post ^^^ lol.
i think you need a whisky chaser to help clear your thoughts.
;-)

 

[Sintra]

A couple of diagrams might help

 

[D Skelton]

Indeesd Lol,

I get the feeling however that I'm sitting here and quite drunkenly teaching you how to suck eggs

You know exactly how it works, you just wanna make fun of my inebriated explanation haha.

 

[Brightspark2]

A couple of diagrams might help

Good diagrams...


but there not harmonized colours aaarrrggghhhh!!!!!...

 

[Silly Sausage]

If you lose the Neutral on an unbalanced star, you get a 'floating neutral'.
The voltage on the neutral will rise wrt Earth (0V) depending on the loads on the 3 legs of the star.
The phase angle between the 3 phases will change as well.

edit...

I could give a link, but it's prohibited!

 

[Sintra]

A picture tells a thousand words mate

 

[Sintra]

Good diagrams...


but there not harmonized colours aaarrrggghhhh!!!!!...

Luckily the street we are talking about was built in 1990 before those Europeans changed our cable identification.

 

[Knobhead]

With a floating neutral the voltage on each phase shifts in relation to the load. In other words the heavily loaded phase the voltage drops, whereas a lightly loaded phase the voltage rises. So if the only load in you’re house is the television and other bits and pieces on stand by and you neighbour turns on the oven. Down goes the voltage in his supply and yours shoots up. In bad cases up to the full Ph to Ph voltage.
So long as the neutral is contiguous the voltage Ph to N and Ph to Ph will remain constant.
Neutral doesn’t necessarily have to be tied to earth, it is done purely to provide a fault path back to the source. Without it, one leg of the supply to an appliance could short to the chassis and you would be non the wiser. Get two faults like that and you’ll soon know about it if the faults are on two different legs of the supply. Turn the kettle on while you’re popping bread in the toaster and bye bye cruel word.
(The morel here is to leave multi tasking in the kitchen to the wife).

I’ve worked on large furnaces where the supply is deliberately free of earth. They are a pain in the backside to fault find on. To detect a fault to earth we injected 48V DC in to each leg of the 660V AC supply. It was nice to know considering the AC was 2.6MW single phase.

 

[D Skelton]

Thank god someone found pictures! Thanks Sintra, much easier to understand than my beer soaked explanation

 

[Darkwood]

It was noted earlier that this is a rare situe' but with substation cable theft and the ill educated thieves nicking the centre tapped earth/neutral strapping thinking its just an earth link then this situe is very common now..... local village had half million pounds of damage to all customers electricals because a druggy pinched this strap.... needless to say he pleaded guilty but in his defense he said he knew about electrics and thought it was an earth cable he was pinching and didnt realise the damage it would cause...... Errrrmmm so i assume if it was just a n earth then this would have been fine in his head :32:..... just a shame he didnt bridge the gap with his body.

 

[DNS1]

needless to say he pleaded guilty but in his defense he said he knew about electrics and thought it was an earth cable .

So he thought that removing an earth, therefore potentially killing people, was a sensible course of action?! He was bloody lucky the only damage was consumer goods!

 

[PEG]

He's a lucky lad but he faces a stern ticking off at his next scheme assessment...:13:

 

[liddley]

cheers for the info I grasp what your saying it's a tad clearer now.
just experienced the fault the first time yesterday afternoon . the suppliers aluminium neutral broke in the street and the voltage rose to well over 400v in a small culdesac causing no end of damage. RCBO's knacked through out the street 12 combi boilers with damaged pcb's and no end of tv's ,sky boxes, washing machines and various other appliances damaged. the insurance assessor from NEDL had his work cut out.

 

[Knobhead]

Damn! My 947" television has just blown up. Where are you?

 

[Sintra]

I have the engineer fixing mine

 

[SYKRAPS]

I have the engineer fixing mine

Thats not a TV its a Cinema Screen

 

[Brightspark2]

cheers for the info I grasp what your saying it's a tad clearer now.
just experienced the fault the first time yesterday afternoon . the suppliers aluminium neutral broke in the street and the voltage rose to well over 400v in a small culdesac causing no end of damage. RCBO's knacked through out the street 12 combi boilers with damaged pcb's and no end of tv's ,sky boxes, washing machines and various other appliances damaged. the insurance assessor from NEDL had his work cut out.

So you can claim for the damage can you?...

 

[Sintra]

Thats not a TV its a Cinema Screen

Its only a little 156" Panasonic LCD 3D

 

[SYKRAPS]

Its only a little 156" Panasonic LCD 3D

only............get you

 

[Sintra]

only............get you

Well it's little compared to Tony's 947" tv

 

[Knobhead]

We have some big TV's in Nottingham, some must be about 19 stone.

 

[Stroppy]

Tony's explanation makes a lot of sense (thank you). I think it also explains a fault I was called to in a hotel kitchen, where they had a 3-phase distribution board which, for want of a better phrase, went mental - all the fluorescent lights were flashing and wouldn't strike, appliances didn't work, RCBO's were clicking away like a load of drugged-up click-beetles at a rave.

Testing the voltages on all the phases gave very odd readings: Red: ~180v, Yellow: ~230, Blue: ~300v! Odder still (at the time), switching an industrial toaster ON, made the flickering lights settle down and come on, and some other appliances started to work as normal. As an apprentice with no practical experience of 3-phase I admitted defeat and got someone else in to look at it, and they ended up changing the grey Wylex 3ph trip for a new one.

My theory was that the insulation in the 3ph trip was somehow breaking down letting voltage creep across the phases.

Now I read this about floating neutrals causing the voltage to vary with load I think a broken/bad neutral may have been our problem - explaining why switching the toaster ON brought the voltage up closer to 240v on the red phase making the lights and stuff work again. Sadly most of the things on the blue phase didn't pull through, God rest their souls...

 

[PRR]

> the reason why the voltage in a domestic dwelling rises when the neutral is broken out in the street? 3 phase is not something I deal with...

This is NOT particularly a "3-phase" problem.

Some places use single-phase center-tapped, and we have the same problem.

One circuit needs two wires.

But you can do two circuits on three wires, three circuits on four wires. This can save a lot of costly copper.

US, Canada, and others use 120V lamps but usually delivered as 240V 1-phase, center-tapped to give two 120V circuits on three wires (and also a 240V connection for cooker).

When the Neutral comes loose, all heck can break loose.

I set up an audio mixer in a college lounge for a concert. There was some problem but then it worked. Worked really well. About 20 minutes later BOOM! and a lovely 1m wide mushroom cloud of smoke from the mixer. Meanwhile a coffee-maker in the adjacent office wasn't getting hot.

Immediately after the event I measured 170V at my "120V" outlet, but as we will see this will vary.

College electrician was called and found a bad Neutral joint in the fusebox. (Just as in your Pet Peeves thread, low-bidders don't always get ALL the screws TIGHT.)

The drawing shows the approximate conditions.

Office and lounge were fed separate circuits. 120V circuits are staggered over both sides of center-tap for balancing and reduced Neutral current. Office and lounge turned out to be opposite sides of the split.

There was a 1,000+ Watt coffee-maker in the office, my ~~100 Watt audio mixer in the lounge.

With all three wires, each load got nearly 120V. Small drop in line loss.

With-OUT the center-tap Neutral, the big pot and small mixer were in SERIES across the full 240V.

You all know how to derive voltages in a series circuit with known resistances. But I put some numbers on anyway. The mixer is no-way a pure resistance, but I've assumed it is to get a quick answer valid-enough to show the general trend.

In this case the mixer got about 1.8 times the nominal voltage, because its 'resistance' was about 10 times the resistance of the coffee-pot and they were splitting double-voltage. Back in the office the pot was scarcely warm having about 0.2 times what it wanted.

As we see, the lost-center can give more or less than nominal depending what loads are on each side.

OK, you have 3-phase. The main difference is that your loads fight, not one, but two other loads. And in domestic service these will be other houses or even other streets. (I could observe an adjacent load running cold; you didn't know Ms Smith's tea wasn't even tepid.) Through diversity there is a better chance you won't have an eXtreme unbalance like my 10:1. But it is sure to be way off the nominal. Could be low, could be high.

(There's also a lot of root-3 in calculating 3-phase splits. But since you don't know the connected loads at other services this is minor.)

 

[Darkwood]

Your set-up is very different to ours PRR and the majority of our new builds have a PME system where the N/E incomming are the one and the same and are grounded multiple times on route back to the transformer center tap so in the event of a broken N/E it can still return vi a few grounding points, we have other systems too that do have similarities to yours but a reminder that our single phase voltage is 230v and 3ph is 400v hence our voltage references will differ from your experiences.

 

[PRR]

> Your set-up is very different to ours

You state the obvious. Please generalize the example. It shows the problem.

The electrons don't know what country they are in.

And US is generally wired TN-C-S PME just like UK.

Lamp-voltage 120V or 230V is not significant except when comparing nominal to fault voltage.

(The same problem can be shown with two 9V batteries and two different loads. Wire two 9V in series with centre-tap. Load one side with 10K and the other with 1K, both to CT. Both show 9V. Now break the CT to the load. The 10K will show 16V, the 1K will show 1.6V.)

As the desktop battery experiment shows, earth is not needed.

Power currents could travel through earth bonding conductors; but a healthy coffee-pot or audio-mixer has NO connection from Line to Earth. Line current goes only to Neutral. Earth is brought in only to drain leakage or internal shorts (a fault, but different from the broken-Neutral fault).

Anyway power currents don't travel well through Earth, the resistance is usually too high. Here I have 0.4 ohms to transformer Neutral and 40 ohms to dirt. A 200W lamp returned via dirt hardly lights.

The only difference is that in the US we may see high/low in the same building. In the UK the high/low would be over three circuits generally not in the same (small/domestic) building. However Stroppy's story shows it happening on different phases in one large kitchen. (AND the high/low relations varying with toaster use.)

And an exact computation of bad-Neutral 3-Phase requires a factor 1.73 between phases. But we are rarely interested in exact computation of a disaster.

 

[Darkwood]

No disrepect meant but you sound more like a tutor than a onsite electrician, i didn't challange your post nor did i say Earth is required or not just explaining our set-up to a non UK poster....
May i enquire what your field is in electrics (theory or practical) as your posting IMHO give you a theorist angle.

Im also aware of ground resistances too but its all down to local conditions and even 40ohms will give a return circuit rather than a floating neutral which is alot safer and less destructive to any connected load.
Thanks for the lessons in factors etc but after 25yrs in the job i hope im fluent in all the supplied info you've expressed.
(Again a humorous comment no dig intended)