TN-C-S Pitfalls

[ProDave]

Dave, thats a real good explanation,plain, simple, and succint,

Yes I don't mind being described as Plain, Simple and Succint :innocent .

 

[steptoe]

:slap

well, I was feeling lonely here all on my own, :|

 

[r0baj0b]

also goes a good way to explaining what happens under conditions of NO fault and exporting the earth.

Sorry, I don't think I got that bit :S

Feeling a bit thick tonight!

 

[steptoe]

taking a TNCS out of the EZ and to the general mass of earth can actually produce a potential difference between the artificial (DNO supplied) earth and true earth, which you will now be standing on.

 

[r0baj0b]

Would that be due to the imbalanced currents travelling down the neutral throughout the whole 3 phase local system?

 

[Andy™]

[quote name='Andy

 

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[quote name='Andy

 

[steptoe]

you could possibly have 400v wandering across phases trying to get back to star point,

in theory, Im not sure how this would actually pan out in the real world.

 

[Andy™]

To earth?

yes. depends on many factors. 400 would be worst case, but in reality it would probably be less.

 

[ProDave]

Do they use concentric for 3 phase? (i.e 3 inner phase cores and one outer neutral core)

If so I've not seen it.

With conventional cable, I don't think a ruptured neutral is particularly common, from my understanding it's a design fault with concentric used to feed a single phase install.

Not saying a neutral fault further downstream can't happen, just not so common.

 

[Andy™]

Do they use concentric for 3 phase? (i.e 3 inner phase cores and one outer neutral core)If so I've not seen it.

yes

 

[steptoe]

they do indeed Dave, dunno how common it was, but Ive seen it a few times, and seems to be getting very common nowadays with the new ali cable they are using

 

[r0baj0b]

taking a TNCS out of the EZ and to the general mass of earth can actually produce a potential difference between the artificial (DNO supplied) earth and true earth, which you will now be standing on.

Would that be due to any imbalanced currents in the local 3 phase system travelling down the shared earth/neutral?

 

[Andy™]

you can also get armoured split con

found a bit on something i had lying around during xmas clearout

 

[steptoe]

Would that be due to any imbalanced currents in the local 3 phase system travelling down the shared earth/neutral?

no,

even on single phase the DNO earth on TNCS may be at a signifigantly different potential to the true mass of earth,

remember the horse that got electrocuted at the race course a while back?

similar issue but a bit more complicated due to it being a quadraped with a bigger stride than a normal human and the stride taking the current across its heart,

you can prove it to yourself, but im not going to explain it to you as it could end up getting stupidly dangerous,

let ANDY do the stoopid experiments!!!

 

[Andy™]

no,even on single phase the DNO earth on TNCS may be at a signifigantly different potential to the true mass of earth,

remember the horse that got electrocuted at the race course a while back?

similar issue but a bit more complicated due to it being a quadraped with a bigger stride than a normal human and the stride taking the current across its heart,

you can prove it to yourself, but im not going to explain it to you as it could end up getting stupidly dangerous,

let ANDY do the stoopid experiments!!!

tell me what experiment you want done and if i can, i will

 

[r0baj0b]

you can prove it to yourself, but im not going to explain it to you as it could end up getting stupidly dangerous,let ANDY do the stoopid experiments!!!

Haha, I don't intend to electrocute myself anytime soon!

 

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[quote name='Andy

 

[Andy™]

You're not going to get 400 volts neutral to earth by losing the supply neutral on a three phase supply.400 volts occurs between lines because they are out of phase with each other.

yes, you can

see drawing

supose the cable from the left is the transformer and it only feeds 3 properties. 2 of them loose a neutral

house 1 is OK, since it still has both L&N

but house 2 and 3 are not. there is now 400v between the incoming phases, with neutral common to only those 2

ignore external resistance (keeps maths easier)

house 2 has a resistance of 99 ohm. house 3 has a resistance of 1 ohms. so each ohm is 4v

house 2 will now be at 396 volt L-N, house 3 will be 4v

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3) Someones electric not working, due to losing the supply neutral.

4) Any report on loss of supply neutral.

whilst there is often reported power failures (especially larger ones), i dont think any of the DNO publish what the actual fault was, wether broken L or N

and there are plenty news stories about peoples stuff being fried by upto 400 after some scumbag as stolen a neutral link from a substation

 

[steptoe]

this seems to be a common statement by a lot of the 'you can't run a TN-C-S earth to an outbuilding' brigade.

err, well lets ask the DNO if they permit it then shall we.

the standard answer is what?

or maybe you never ask for permission to alter the supply parameters of your installations.?

it is a requirement to ask the DNO for permission to take their supplied earth to a building other than the one they have supplied it to,

statutory law BTW, so its a criminal offence if you export the supplied earth outside the original building without their permission,

now, would you care to mention or link the actual amount of cowboy installs that ignore this fact?

to your question, I cant be bothered to drag up the amount of times people have been electrocuted in their gardens due to hedgecutters/lawnmowers etc cutting cables/extension leads etc.

try a google search, Im sure you will fill your evening with joy at peoples mis-information and dodgy workmanship by 'sparks' that have absolutely no understanding of the consequences of their actions simply because they didnt understand how earthing worked in the UK