TN-C-S Pitfalls

[ADS]

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

I can't be bothered having the 'outbuilding' argument with you again.

As for the 'electrocutions' - I await the links.

I did 'google' Loss of supply neutral', by the way, ...........and all you get is a wiki entry and a load of forums full of scaremongering...........not a news story in sight - and certainly no electrocutions.

 

[steptoe]

try googling something along the lines of

'electrocutions outdoors'

do you really think the DNOs are going to publish the fact they have a substandard system is the cause of peoples deaths?

 

[ADS]

try googling something along the lines of'electrocutions outdoors'do you really think the DNOs are going to publish the fact they have a substandard system is the cause of peoples deaths?

Tried that - still no news stories about 'lost neutrals'

I don't think it would be up to the DNOs as to what got published if someone died - Health and Safety and the Press would publish the findings regardless.

In all the years of internet and over 60,000,000 people in the country, I can't find one report of someone dying due to a 'lost neutral' on a TN-C-S system.........and yet, apparently, it happens all the time.

 

[apprentice87]

Hi all,

First off'

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

Yes definitely, i have such a cable feeding my own supply.

Secondly, as to LV mains cable failures, I was talking to a DNO cable jointer a while ago about this. He told me a few things.

Firstly, my own supply to the shed i am building is housed in a freestanding metal "thingy" i made.View attachment 3166

The cable jointer told me that he was surprised that they connected a "PME" supply to this, as "they will now refuse to connect "PME" to anything that is made of metal"

Now, i cannot see this, but i was at a large shopping centre and noticed that there were earth rod boxes everywhere outside surrounding the buildings, so whether this was indeed a TT'ed huge install with multiple rods, or a "proper" PME one, I have no idea. [Obviously i cannot go and ask to inspect Tescos supply at the intake position to find out for sure]

Next, i said that i was amazed that the 80 year old PILC cable in the street was in such good condition. The jointer told me "Yes, they are all like that. We have LOADS of trouble with the newer stuff though. The concentric part is only aluminium in lots of it, and the water gets in all the time and rots it all away!!!!"

Oh dear......!!!!!!!!!!

Now then, here is a little theory of mine. Please have a read and tell me what you think. I have thought about this for a long time.

1, "Exporting earth" under FAULT FREE conditions.

Cannot see the difference here between a DNO earth supplied as either TNS or TNCS, Surely the only PD difference between earth, and "true" earth out in the garden BETWEEN THE TWO DIFFERENT SYSTEMS will be caused [in the case of TNCS] by the flow of current down your OWN service cable. In other words, practically none existant.

2, Say there is a neutral fault in YOUR OWN service cable on TNCS.

Then yes, you are stuffed!!!

3, One on TNCS ALL on TNCS.

Say there is a neutral fault in the DNO mains cable out in the street. So what, what difference can that make??? The neutral is in parallel with the cable sheath earth that they HAVE to maintain for TNS customers. [in fact, at every point where they connect a new service cable, they bond the neutral to the earth sheath of the "old" cable AND add an earth rod too.] Anyway, in the event of a "neutral fault" the neutral current will just return back to the star point down the existing cable sheath and so precisely nothing will happen.

Now to my point.... Now, leaving aside the fact that the "old" PILC mains cable itself will act as a giant earth rod. Say i am on TNCS. Right, there is a neutral fault "out in the street" With me up to now? Now, lets say that apart from the break in the neutral [in which case as i have explained above, i do not think much exciting will happen anyway, due to the cable sheath being in parallel] there is a break or a high impedance joint in the existing cable sheath, THEN even if YOU are on "safe" TNS, YOUR earthed metal work will all be live too!!!! It has to be, the DNO have gone and connected YOUR nice safe earthed TNS cable sheath, to MY mega dangerous neutral conductor at the point where my service cable joins the main, so if my exposed parts are "live" then so are yours!!!!!!

IT GETS WORSE!!!!!

The cable jointer also told me that "do not think we even have any SNE cable in the yard, it is ALL concentric now"

In other words, you might THINK you are on TNS but YOU ARE NOT!!!!!!!!!!!

Say there is a LV mains fault. They cut out the faulty bit and replace it with modern concentric stuff, EVERYONE is now on TNCS....... Now, when this modern stuff rots through, [as the cable jointer told me happens all time] apart from the fact that due to the loss of neutral, individual phase voltages will go all over the place, if it were not for the fact that there are these earth rods all over the place, and everyones MET is hopefully connected to bits of metal sticking out ot the ground, ALL our exposed parts will become live....

Constructive comments please!!!!

john...

 

[ProDave]

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.

Oh yes you CAN as already explained in post #39

Without picking on Dave, 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.

So, based on some other comments in this thread - and so that the OP doesn't go back to his lecturer with a totally biased view

- anyone care to give us some links to some news stories of:

......

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

4) Any report on loss of supply neutral.

Yes I have been to one such fault. The customer complained of "no power" Well there was a tiny bit of power. If you turned on just one light, then it would glow very dim, but turn on anything else and nothing worked.

I quickly determined it was a broken neutral and the only reason anything worked at all was a high impedance return path via the water pipe bonding.

I told the customer there was nothing I could do and he needed to call Scottish Hydro. The feedback I got afterwards was that they had to dig up his driveway and half way across the road outside his house to lay in a new supply cable.

---------- Post Auto-Merged at 08:39 ---------- Previous post was made at 08:29 ----------

Say there is a neutral fault in the DNO mains cable out in the street. So what, what difference can that make??? The neutral is in parallel with the cable sheath earth that they HAVE to maintain for TNS customers. [in fact, at every point where they connect a new service cable, they bond the neutral to the earth sheath of the "old" cable AND add an earth rod too.] Anyway, in the event of a "neutral fault" the neutral current will just return back to the star point down the existing cable sheath and so precisely nothing will happen.

That's the bit where you are wrong.

The Neutral and Earth are ONE and the same with a concentric cable. There IS NO EARTH CABLE SHEATH

So when the outer core of a concentric cable breaks, you lose your neutral AND your earth connection. That's what makes it so dangerous under a fault condition.

 

[apprentice87]

Hi ProDave,

You say "That's the bit where you are wrong. The Neutral and Earth are ONE and the same with a concentric cable. There IS NO EARTH CABLE SHEATH"

Yes, i agree 100%, I was referring to the sheath in the PILC LV main that MOST of our "concentric" service cables are connected to.

The point i was trying to make was, if anywhere along the line [not line in the electrical sense!] concentric IS used, and the concentric part does rot through, then that is when all the problems start, as with either TNCS or TNS if the neutral DOES develop a fault, and the sheath DOES have a break or a high impedance too, the sheath, and everyones exposed parts TNS OR TNCS will all be live too. [i think!!]

That is what i meant by "one on TNCS ALL on TNCS" because break in the sheath and your nice "safe" sheath [now un-earthed at the supply end] is connected to my dodgy neutral at the point where my service cable joins the main!!!! Sooooooooo, a break in the sheath and EVERYONE is on TNCS too!!!!

Obviously this all leaves aside the fact that the PILC itself will act as a giant earth rod as will everyones extraneous parts connected to their MET, plus the rod where the service joins the main. [but you all know what i mean!!!]

Anyway, the fact that the DNO's do not want to connect up to "metal things" anymore, PROVES they know that have an unsafe system......

john....

 

[ProDave]

Just an asside can you refuse a PME connection? i.e can you ask for TNS instead?

My guess is they would just say you don't have to use the PME earth, and if you don't like it, TT it.

 

[apprentice87]

Hi ProDave,

My understanding is that ALL new supplies are now PME ones, especially as the DNO people told me that apparently the only sort of cable they have in their yard is the concentric stuff!!

I think what they do is to supply everyone with PME, but if it is an installation where they choose not to, then they make, in their own words, "the earthing terminal unavailable to you"

So you are 100% right, you will get PME if they deem it suitable, if not, you will still get it, but they will not drill the little hole for you to poke your earth conductor into, and tell you to make your own arrangements!! If you CHOOSE not to use PME and TT out of choice, then that is all the better from their point of view!!!

john....

 

[sb_89]

Apprentice 87

Now, i cannot see this, but i was at a large shopping centre and noticed that there were earth rod boxes everywhere outside surrounding the buildings, so whether this was indeed a TT'ed huge install with multiple rods, or a"proper"PME one, I have no idea. [Obviously i cannot go and ask to inspect Tescos supply at the intake position to find out for sure

I'd imagine it would be for the lighting protection, if I was all the way around the building

 

[kelvfoozinda]

When installing new supplies to flats etc we normally would supply a distribution unit with wavecon (like concentric but in massive sizes and a wavey outer copper strands rather than wrapped around). Then split concentric is used from this point to the flats. Split concentric has a separate neutral and earth determined by the neutral strands being further insulated. Therefore your tns supply in the flat is only tns as far as the cupboard down the hall. The same wavecon cable is what is stuck in the road in most new situations, so everything ends up tnc-s.

Faults i've seen include three bungalows on the same phase and the neutral drops out in the road, the earth became live and melted in most of the ccts within these three properties, causing an old unsheathed earth stapled to the rafters in one property to char the timber it was clipped to. The lead twin for the lighting ccts to melt and all other manner of stuff to need rewiring!

Neutral to drop out intermittently in a substation causing for a tnc-s supply to one half of a block to find a neutral path down an aerial cable through one blokes tv to the other supply to the block, we clamp metered 80A on the co ax before pulling the plug on the TV!

 

[Doc Hudson]

When installing new supplies to flats etc we normally would supply a distribution unit with wavecon (like concentric but in massive sizes and a wavey outer copper strands rather than wrapped around). Then split concentric is used from this point to the flats. Split concentric has a separate neutral and earth determined by the neutral strands being further insulated. Therefore your tns supply in the flat is only tns as far as the cupboard down the hall. The same wavecon cable is what is stuck in the road in most new situations, so everything ends up tnc-s.Faults i've seen include three bungalows on the same phase and the neutral drops out in the road, the earth became live and melted in most of the ccts within these three properties, causing an old unsheathed earth stapled to the rafters in one property to char the timber it was clipped to. The lead twin for the lighting ccts to melt and all other manner of stuff to need rewiring!

Neutral to drop out intermittently in a substation causing for a tnc-s supply to one half of a block to find a neutral path down an aerial cable through one blokes tv to the other supply to the block, we clamp metered 80A on the co ax before pulling the plug on the TV!

Very interesting post, thanks for that input. Some of our members appear to be a bit oblivious of the potential dangers of TN-C-S with supply neutral faults.

Doc H.

 

[kerching]

Apprentice 87Now, i cannot see this, but i was at a large shopping centre and noticed that there were earth rod boxes everywhere outside surrounding the buildings, so whether this was indeed a TT'ed huge install with multiple rods, or a"proper"PME one, I have no idea. [Obviously i cannot go and ask to inspect Tescos supply at the intake position to find out for sure

I'd imagine it would be for the lighting protection, if I was all the way around the building

I will wager a slap up tea at Mrs Miggins Pie Shop that those 'pits' are for the Lightning Protection system......... :coat

 

[sb_89]

Put me down for a pie too .... Lighting protection , specially with tesco having a tin roof

 

[apprentice87]

Hi Kelvfoozinda,

That is exactly what i meant by "one on TNCS ALL on TNCS", in my original post. The DNO cable jointers told me that they do not even have any of the "old" type cable in their yard anyway. So if they repair a cable out in the street and replace a section of the old stuff with wavecon, everyone fed by that cable is now on TNCS anyway whether they realise it or not....

Now, i am only a newbie, so i am not sure about this, but that is the way it seems to me. What difference does it make if the neutral and earth are only combined at the end of your service cable, or a mile up the road, they are still combined, so your nice safe TNS that you THINK you have, is no more...

I think all you really have now, would be TNCS with your "old" TNS cable sheath relegated to acting as a "concentric" earth rod of sorts.

What you all think......

john...

 

[kelvfoozinda]

All the outer of the old pilc cable does is end up acting as an earth path in a massive fault, like a digger going through it! Our jointers are not even allowed to re-terminate a pilc if the head is damaged, now a joint must be done outside with a new service head and wavecon/concentric supply into the building. The faults they seem to have are on consac cables more than pilc's, with stories of repeated visits for repair etc and flames coming out from the joins in between paving slabs!

Oh, and pilc would cost a bloody fortune nowadays, copper cores and tonnes of lead.

 

[apprentice87]

Hi again Kelvfoozinda,

That is what the jointers told me, that they have many more problems with the modern stuff than they ever do with PILC.

Would you agree though with my little theory, that all this stuff about you cannot safely "export" earths outside of the equipotential zone [say to a shed in the garden or to exterior metalwork] with TNCS [but it is ok with TNS] [in case the DNO lose a neutral and all your earthed metal work becomes "live"] is a bit silly really, because even if they do lose the neutral the PILC sheath is in parallel with it anyway, and in the case that there is a break in the continuity of the sheath, THEN lose the neutral and your "nice safe" TNS earth is going to be live too!!, because, as you have explained, somewhere along the line, the sheath and neutral will be linked anyway, either at the point where the old PILC has been repaired with a section of wavecon, or where the service cable to a "PME" supply joins the main. So, effectively then, if i am on PME, then, in the event of a fault, so are the houses around me!!!

Just as well that the remnants of the old sheath will act as an earth rod....

What you think??

john..

 

[steptoe]

APP87

the BIG issue I have with TNCS/PME is that for it to be proper PME is that IMHO it should be rodded as close to the point of entry to your property as possible, ie, at your entry point,

therefore there is virtually no possibility of the cable being damaged unless on your premises, then its your problem.

nothing stopping you driving your own rods in either, is there?

---------- Post Auto-Merged at 20:48 ---------- Previous post was made at 20:44 ----------

the other issue with TNCS being exported is this,

if it is only TNCS and NOT proper PME, the point of EARTH reference may only be at star point, this could in actual fact be miles away from your point of export, therefore due to variations in ground resistance and VD your DNO earth could in fact be at a different potential from true earth, this can be measured, in much the same way you can verify your TNCS supply to be true PME or not.

 

[ProDave]

Good points steps.

The more I read about PME, the more I realise it's a flawed system. NOT because in principle it's flawed, but because the DNO's persist in using concentric cable which is seriously flawed.

As for DNO's splicing in sections of concentric into a TNS supply, show me where in the regs it allows for a supply type that starts it's journey as TNS, has a mid section of TNCS spliced in, then it's back to TNS for delivery to the customer. What gives the DNO authority to ignore 7671 and devise such a non standard, non recognised earthing system?

The other thing that bugs me, up here in the sticks, I've seen several installs where a single property is fed from an overhead transformer on a pole. It's usual to dig a trench in two directions. Away from the property they bury an HV earth, then towards the property (usually the same trench carrying the customers concentric supply cable) they bury an LV earth.

So that's a TNCS supply courtesy of the combined earth and neutral conductor from the transformer to the customer.

But HOW does it qualify as PME. Please tell me where the MULTIPLE earth's are (I can only count one)? But they still stick a PME label on the supply head.

 

[kerching]

Maybe, just maybe, the P in PME stands for Possible and not the more widely accepted Protective !................... :coat

 

[apprentice87]

Hi Steps,

Yes i agree with everything you say. My own supply to my shed counts as "proper" PME [i hope!!!] There is an earth rod at one end of the service cable [courtesy of the DNO] and at the other end i have earthed it myself.

Ze using my rod and not their earth terminal is about 47 ohms, so that will do me!!!

Hi Prodave,

You say about the DNO and their dodgy cable. All i can tell you is this; The cable jointer told me that all they have got is the concentric stuff so all over the place they will have this strange TNCS/TNS hybrid you mention. Even worse, they said that they have loads of trouble with some of this concentric stuff. Some of it, but not all i gather, had the concentric part made of aluminium wire. The jointer told me that the wet gets in all the time and they lose the neutral as a result. He said that this was very common.

Presumably it is only because most houses MET is bonded to metallic services and the sheath of the old PILC acts as a neutral return to the transformer too, coupled with the establishment of ann EQ zone by proper bonding in the house that i presume that the DNO notice something amiss before [most] consumers.

I would think that the thing that most people would be affected by, would be funny voltages caused by the dodgy neutral and out of balance loads on the network.

My install, they forgot to do up the screws in the cutout...... On L2, Ze was .52 or so on TNCS, tighten the screws, .28 like the others....

john..