TN-C-S Pitfalls

[r0baj0b]

A conversation came up between myself and my lecturer (I am nearing the end of my apprenticeship) about the reasons for not taking the earth outside of a building on a TN-C-S system to, say, a shed. He tried to explain it but couldn't quite figure out how to.

With reference to this thread I found (http://www.talk.electricianforum.co.uk/question-answer-board/16596-tnc-s-earth-external-building.html) I don't understand how, if the supply neutral was lost, everything earthed in the house would become live, and how the exported earth would be at the same potential as the star point of the transformer.

At one job I was at, the DNO came to move the intake, and installed a TT system, as opposed to a TN-C-S as we were expecting. When asked, the engineer said something about it being dangerous for them to install TN-C-S (I can't remember exactly what he said as it was over a year ago)

What other pitfalls are associated with this system?

 

[Andy™]

What other pitfalls are associated with this system?

if the neutral fails and the earth becomes live, and someone comes into contact with it, no RCD will trip

 

[steptoe]

grab a seat young man,

get yourself a box (or 2) or cold ones and sit down while Uncle Albert tells you a story,,,,,,,,

or,,,,

you could do a forum search (search box in top right of screen somewhere) or two then come back and ask what it is you still dont understand.

depends how much of a sadist you are if you really want to listen to me go one,

anyway,

during the war...........................................................................................

 

[r0baj0b]

I like stories! haha.

 

[steptoe]

as a famous general once said,

during the war,

'I shall return'

just grabbing some grub and a beer and I'll post in a bit.

 

[JPA]

Can I grab a seat, I like to learn new things...

 

[ProDave]

The "problem" with TNC-S is neutral and earth are one and the same thing at source.

Now in a normal install, say TNS, if the neutral conductor broke, then not much would happen, apart from everything in the house would stop working, and you would call someone to fix it. your earth, would remain at earth and all would be well. But Neutral would float up to or close to L potential. But nobody would notice this.

With TNC-S the neutral and earth come into the property sharing the same conductor. At the supply head, neutral and earth come out of the same block of metal.

So if the "neutral" cable ruptures, the neutral would float up to somewhere close to L potential. Now this time, because E is joined to N at the head, then E will float up towards L potential as well.

Now that's not such a huge issue as you may think within a building as everything will float up together.

Now take the example of the TNC-S earth exported to the garden shed. Someone working in the garden, say using a power tool from an extension lead. He will be standing on the ground, on real local earth, so if the exported E is floating up towards L, then clearly a potential difference exists and a shock hazard exists.

Hey I've just done a good job of convincing myself that running an extension cable from the house out into the garden is not a good idea.

Anyway the final part of the jigsaw is TNC-S installations are very often fed by concentric cable, and it's more common than you might think for the outer core (which is the combined E and N) to break. This cable is prone to getting punctured if not laid well, and water ingress can corrode the copper outer core, undetected, until it eventually breaks.

EDIT: Steps will be back in a bit to add more (I hope he agrees with the gist of what I said)

 

[sprocketflup]

**takes a seat**

and waits patiently......

---------- Post Auto-Merged at 19:39 ---------- Previous post was made at 19:37 ----------

The "problem" with TNC-S is neutral and earth are one and the same thing at source.Now in a normal install, say TNS, if the neutral conductor broke, then not much would happen, apart from everything in the house would stop working, and you would call someone to fix it. your earth, would remain at earth and all would be well. But Neutral would float up to or close to L potential. But nobody would notice this.

With TNC-S the neutral and earth come into the property sharing the same conductor. At the supply head, neutral and earth come out of the same block of metal.

So if the "neutral" cable ruptures, the neutral would float up to somewhere close to L potential. Now this time, because E is joined to N at the head, then E will float up towards L potential as well.

Now that's not such a huge issue as you may think within a building as everything will float up together.

Now take the example of the TNC-S earth exported to the garden shed. Someone working in the garden, say using a power tool from an extension lead. He will be standing on the ground, on real local earth, so if the exported E is floating up towards L, then clearly a potential difference exists and a shock hazard exists.

Hey I've just done a good job of convincing myself that running an extension cable from the house out into the garden is not a good idea.

Anyway the final part of the jigsaw is TNC-S installations are very often fed by concentric cable, and it's more common than you might think for the outer core (which is the combined E and N) to break. This cable is prone to getting punctured if not laid well, and water ingress can corrode the copper outer core, undetected, until it eventually breaks.

EDIT: Steps will be back in a bit to add more (I hope he agrees with the gist of what I said)

so if the same thing happened in a TT system, wed have live and neutral at 230v and earth at 0v *( or near) surely thats as much of a hazard as the TNCS scenario above? Or do i need to think about this some more?

---------- Post Auto-Merged at 19:40 ---------- Previous post was made at 19:39 ----------

Im forgetting the 100mA RCd arent i?

 

[ProDave]

so if the same thing happened in a TT system, wed have live and neutral at 230v and earth at 0v *( or near) surely thats as much of a hazard as the TNCS scenario above? Or do i need to think about this some more?

With say TT, if the N ruptures, yes it will float up to close to 230V. But N is an insulated conductor. You would not normally be able to touch a N conductor, so what difference does it make what potential it is.

With the TNC-S example, it's the fact E can rise as well. Now that's a different ball game. LOTS of things are connected to E and easy to touch. So take that E outside the building and put it in a situation where you could touch exported E and local E together, you have a problem.

 

[r0baj0b]

Probably a stupid question, but how would the Neutral float up to the same potential as Line? Wouldn't there have to be a short between the two for that to happen?

 

[Andy™]

here a diagram i posted a while back

 

[r0baj0b]

Ok, but isn't that the same as touching the neutral bar in a live mains board? You don't get a shock from that

 

[ProDave]

Ok, but isn't that the same as touching the neutral bar in a live mains board? You don't get a shock from that

But you would or could get a shock from the N bar IF the N conductor had broken.

Remember this discussion is all about the dangers of a fault condition caused by the N conductor breaking, which as I have said, because of the concentric cable in common use, this is not exactly a rare occurrence.

 

[sparkytim]

Effectively your N becomes LIVE back to the point where the break has occured.

 

[r0baj0b]

Hang on, I've thought about it a bit more....You get a shock because the current isn't returning through the N, but rather through you to earth, and then through earth back to the transformer. Have I got that right? :S

 

[ProDave]

Hang on, I've thought about it a bit more....You get a shock because the current isn't returning through the N, but rather through you to earth, and then through earth back to the transformer. Have I got that right? :S

Yes, as shown by Andy's diagram. (note the grim reaper in the picture next to the stick man)

 

[sprocketflup]

Im amazed Andy drew the reaper so well, and then must have ran out of time for the poor sod getting toasted :^O

 

[Andy™]

Im amazed Andy drew the reaper so well, and then must have ran out of time for the poor sod getting toasted :^O

best keep the stealing image and adding it in a secret then

 

[steptoe]

The "problem" with TNC-S is neutral and earth are one and the same thing at source.Now in a normal install, say TNS, if the neutral conductor broke, then not much would happen, apart from everything in the house would stop working, and you would call someone to fix it. your earth, would remain at earth and all would be well. But Neutral would float up to or close to L potential. But nobody would notice this.

With TNC-S the neutral and earth come into the property sharing the same conductor. At the supply head, neutral and earth come out of the same block of metal.

So if the "neutral" cable ruptures, the neutral would float up to somewhere close to L potential. Now this time, because E is joined to N at the head, then E will float up towards L potential as well.

Now that's not such a huge issue as you may think within a building as everything will float up together.

Now take the example of the TNC-S earth exported to the garden shed. Someone working in the garden, say using a power tool from an extension lead. He will be standing on the ground, on real local earth, so if the exported E is floating up towards L, then clearly a potential difference exists and a shock hazard exists.

Hey I've just done a good job of convincing myself that running an extension cable from the house out into the garden is not a good idea.

Anyway the final part of the jigsaw is TNC-S installations are very often fed by concentric cable, and it's more common than you might think for the outer core (which is the combined E and N) to break. This cable is prone to getting punctured if not laid well, and water ingress can corrode the copper outer core, undetected, until it eventually breaks.

EDIT: Steps will be back in a bit to add more (I hope he agrees with the gist of what I said)

Dave, thats a real good explanation,

plain, simple, and succint,



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

 

[sparkytim]

Thats it, just think of the current serching for a path back to "home", its lost its normal route via the N conductor and along you come and provide a path thru your body!