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.