suppliers neutral break?

[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.