Understanding the relationship between TNCS, TNS and TT

[HappyHippyDad]

I have never actually had to make an electrical system into a TT. I have added rods in order to get an effective Ra but never had to (for example) install a TT on a new garage or shed.

It's best I give an example with regards my question..

A new garage has a supply taken to it from the house. The house is PME and this has been extended to the garage, lets say 4mm 3 core SWA. The 3rd core is perfectly adequate as an earth. Also, the installer puts a copper rod in the ground and attaches this to the earth bar in the new garage CU. Is this OK? Is this actually better? Is it now dangerous?

I've got a few ideas but I'd rather not influence the line of thought and just leave it as above.

Cheers all.

 

[pc1966]

Whatever "cost saving advantage" there is using TNC-S was always short-term, me thinks. I can understand turning to it as a "stopgap" measure, but long term? was that ever really an option?

Traditionally we have though of the supplier's Earth as trustworthy and never had to consider it going to 100V+ during a fairly basic fault when installations were being designed. It retrospect they would have been better cost-saving by pushing for TT in place of TN-S

 

[Vortigern]

Well then time for another device to sardine into our CU, an overvoltage device!

 

[Simon47]

Shhh, don't give them ideas !

 

[Cookie]

"Two rods typically do little for an open neutral". Well that certainly puts the single 4 ft rod required in most TNC-S countries in perspective. I spoke with an ex inspector recently who described the rod's we use here as "fig leafs".
Your solution is basically a form of TNS which is without doubt the best electrical system.
While the "rest of the world may not share your view", it does, nt alter the fact yours is correct

I had to smile at this! Not many are willing to fully accept the advantages put forth by TN-S without the neutral distributed.

Whatever "cost saving advantage" there is using TNC-S was always short-term, me thinks. I can understand turning to it as a "stopgap" measure, but long term? was that ever really an option?

Long terms if a disaster to the goals of the regs: protecting life and property. What many people fail to realize is that a large number of electrical practices stem from when countries where at war, recovering from war or struggling economically. Engineers, electricians and code experts were forced to improvise, often picking the lesser of several evils.

For example, during WWII the US made an exception that ranges and dryers could ground through the neutral to save on copper for the war efforts. The practice did not end until after 1996.

Today however we can look past a few extra cents for what is significantly safer and more economical in the long term.

 

[Cookie]

Well then time for another device to sardine into our CU, an overvoltage device!

Just one of the many hazards the regs do not address. Dealing with neutrals involves much, much more complexity to mitigate those risks- risks which often can not be fully mitigated.

 

[Lucien Nunes]

For example, during WWII the US made an exception that ranges and dryers could ground through the neutral to save on copper for the war efforts. The practice did not end until after 1996.

TN-C with PEN all the way to the load was popular elsewhere too and is still in widespread use in places like former Soviet bloc countries. One little screw holding the incoming PEN to the metal casing of the panel, and various load neutrals run back to it. I recall being in Germany in the late '80s and struggling to overcome the mental block on connecting the metal body of some fluorescent lights to the grey (neutral) wire on an ungrounded lighting circuit.

For me the weak link in dryer / stove thing was the 10-30 and 10-50 receptacles. The PEN is much more likely to be disconnected or high-resistance when it goes through a single prong of a plug and receptacle, than when it's a continuous wire up to the receptacle but then splits to carry N & E through two separate prongs as far as the dryer. In any case I think it's a different category of hazard than a DNO's PEN.

 

[Cookie]

TN-C with PEN all the way to the load was popular elsewhere too and is still in widespread use in places like former Soviet bloc countries. One little screw holding the incoming PEN to the metal casing of the panel, and various load neutrals run back to it. I recall being in Germany in the late '80s and struggling to overcome the mental block on connecting the metal body of some fluorescent lights to the grey (neutral) wire on an ungrounded lighting circuit.

For me the weak link in dryer / stove thing was the 10-30 and 10-50 receptacles. The PEN is much more likely to be disconnected or high-resistance when it goes through a single prong of a plug and receptacle, than when it's a continuous wire up to the receptacle but then splits to carry N & E through two separate prongs as far as the dryer.

The hazards are the same. Remember that connections/splices can and do fail- especially when they are carrying current.

In any case I think it's a different category of hazard than a DNO's PEN.

While the risk is reduced, a major hazard still exists.

 

[Simon47]

Interestingly, I was looking for something else and came across ENWL document - Code of Practice 332, LV Service Connections and Application of PME. It goes into great detail as to when different configurations may or must be used, who pays when something must be changed, etc.
There really are some interesting, and in places, eye opening, facts to be gleaned from it. Far too much to try and summarise it, but if you're interested in the practical aspects of PME from the DNO's PoV then it's a very interesting read. There's a lot of network configuration mentioned that I don't think have been considered in this thread.

Clearly PME isn't quite as haphazard as some people seem to think it is.

 

[LastManOnline]

Interestingly, I was looking for something else and came across ENWL document - Code of Practice 332, LV Service Connections and Application of PME. It goes into great detail as to when different configurations may or must be used, who pays when something must be changed, etc.
There really are some interesting, and in places, eye opening, facts to be gleaned from it. Far too much to try and summarise it, but if you're interested in the practical aspects of PME from the DNO's PoV then it's a very interesting read. There's a lot of network configuration mentioned that I don't think have been considered in this thread.

Clearly PME isn't quite as haphazard as some people seem to think it is.

Appreciate that and look forward to looking into it.

 

[Cookie]

Interestingly, I was looking for something else and came across ENWL document - Code of Practice 332, LV Service Connections and Application of PME. It goes into great detail as to when different configurations may or must be used, who pays when something must be changed, etc.
There really are some interesting, and in places, eye opening, facts to be gleaned from it. Far too much to try and summarise it, but if you're interested in the practical aspects of PME from the DNO's PoV then it's a very interesting read. There's a lot of network configuration mentioned that I don't think have been considered in this thread.

Clearly PME isn't quite as haphazard as some people seem to think it is.

Does TN-S actually exist anywhere in practice?

 

[Vortigern]

I come across TN-S quite often. More and more nowadays I find it has been hybridised into TN-S/TN-C-S. The DNO tend to take an earth braid from the outer sheath, which is soldered or spring secured, up to the neutral block. Not quite sure what that qualifies as. Sometimes the earth braid is taken to a Henley connector block which supplies the installations main earth.

 

[Simon47]

Is that a sheath of a single core cable - making it a PEN ? Or is it the sheath of a two core cable as a seperate earth ?
The former would be just TN-C-S, the latter would be TN-S if it's connected to a TN-S main in the street. But reading that ENWL document, it's clear they have a policy of converting SNE to CNE when they do works - even if customer connection remain TN-S - so it's a hybrid. Where they do that, there's a section in the doc about provision of an earth - which can be 30m of buried bare sheathed cable which is deemed to be max 40 ohm in the worst soil conditions likely to be encountered, or it can be a connection to the CNE of an adjacent circuit.
It strikes me that if bare lead sheathed cables are still common, that should be a fairly effective distributed earth.

 

[pc1966]

I remember someone mentioning that when the replace the lead sheathed cable they usually just run new XLPE alongside and keep it in the ground as an earth for the PME.

 

[LastManOnline]

Interestingly, I was looking for something else and came across ENWL document - Code of Practice 332, LV Service Connections and Application of PME. It goes into great detail as to when different configurations may or must be used, who pays when something must be changed, etc.
There really are some interesting, and in places, eye opening, facts to be gleaned from it. Far too much to try and summarise it, but if you're interested in the practical aspects of PME from the DNO's PoV then it's a very interesting read. There's a lot of network configuration mentioned that I don't think have been considered in this thread.

Clearly PME isn't quite as haphazard as some people seem to think it is.

You have struck gold there.Great Read. Have just scanned it but it will definitely be put aside for deeper analysis.

 

[Eire79]

I'm in a industrial site with 10 no traffos across site , I asked engineering firm in lately doing maintenance if site was TN C S which he said yes , but looking at it myself the neutral and earth are run separate back to each traffo and obviously linked their .
Therefore is this not TN S .
Great tread by the way.

 

[LastManOnline]

I'm in a industrial site with 10 no traffos across site , I asked engineering firm in lately doing maintenance if site was TN C S which he said yes , but looking at it myself the neutral and earth are run separate back to each traffo and obviously linked their .
Therefore is this not TN S .
Great tread by the way.

That sounds very much like TNS to me

 

[Eire79]

That sounds very much like TNS to me

So what's the implications of this in a plant does everything stay the same as TNCS with regard to bonding etc

 

[LastManOnline]

Does TN-S actually exist anywhere in practice?

So what's the implications of this in a plant does everything stay the same as TNCS with regard to bonding etc

Not so familiar with the bonding regs around TNS (have never worked with TNS). But it's without doubt a superior supply system to our TNC-S.Aside from the extra safety it provides, anyone working with electronics love to have a "clean" earth. Saves them all sorts of headaches. I think in future you will see more TNS supplies installed

 

[Eire79]

Not so familiar with the bonding regs around TNS (have never worked with TNS). But it's without doubt a superior supply system to our TNC-S.Aside from the extra safety it provides, anyone working with electronics love to have a "clean" earth. Saves them all sorts of headaches. I think in future you will see more TNS supplies installed

It's more than likely due to the fact that the traffos are located adjacent to each switch room . The 10kv is distributed via RMU'S . I must have a look through the new regs, it's a big book now .

 

[Simon47]

Aside from the extra safety it provides, anyone working with electronics love to have a "clean" earth. Saves them all sorts of headaches.

Anyone working with electronics needs to understand how to deal with issues that arise in the real world Pretty well any problem (such as hum caused by a small AC imposed on the earth connection) "caused" by having TN-C-S can be dealt with by using proper methods - though that can add to cost. TN-S doesn't magically make such problems disappear - just makes them smaller.
Don't forget that the "clean" earth provided by TN-C-S is likely to be quite dirty. You'd get the effects of faults from anyone else on the system. Some AC created by magnetic & capacitive coupling from live cores all through the network. And especially you'll get all the noise injected into the earth by all the equipment you and other users have connected.
If it's that important to you, just go TT and provide your own earth.

I think in future you will see more TNS supplies installed

I doubt it. From the ENWL document I linked to earlier, it's very clear that TN-S is being phased out, and the network converted to CNE (combined neutral & earth) when any work is done on it - and I'd be surprised if other DNOs had significantly different policies. Even if you get a TN-S connection (which is against policy), that will not go further than the connection between your service cable and the distribution cable in the street. So you would save a tiny bit of AC volt drop in your own service cable, but otherwise you'd get the same as with a TN-C-S service.

 

[UKPN]

@Megawatt as already said, in the UK only the supply authority (DNO) is allowed to bond N & E together for various safety reasons, largely to do with the consequences of an open 'neutral' making metalwork live, and to limit spurious current circulating around the earthing structures of multiple buildings, etc.

It is common in the UK for the bond point to be at the supply cut-out and that is called TN-C-S here (Common N & E to the supply point, Separate after) but within any normal installation N & E will not be bonded together. In this case the DNO is expected to have multiple low-impedance earth rods at points along the conductor (hence the PME name given for protective multiple earth). That is not always successful, and there are PME faults putting folk at risk disturbingly often across the UK.

The main advantage is to save conductor costs over the traditional TN-S system by not running separate N & E to everywhere (secondary advantage is usually a lower supply fault impedance Ze so easier to achieve fast disconnection by fuse or MCB).

The main disadvantage is the PME open-circuit fault consequences that are comparable to the TN-C risk of metalwork going live and high currents flowing in to anything bonded to true Earth (e.g. metal water pipes, etc).

There was talk of having UK properties being fitted with earth rods for TN-C-S as they are built, etc (which I think is similar to the USA arrangement) but they would go to the 'E' after the DNO point in any case (which is not isolated by incoming switch, etc) and definitely not to N. But that has not made it in to our regulations yet.

Of course with a TT setup you only have the rod(s) for earth and so in practice you need RCD protection for all circuits to have any real hope of disconnecting on a fault as it is really hard to get below ten-ish ohms for a couple of rods, and you really need one ohm or below in many cases.

"There was talk of UK properties being fitted with earth rods for TNCS as they are built. But that has not made it into our regulations yet"

It never will, it contravenes the ESQCR (real) regulations.

 

[pc1966]

"There was talk of UK properties being fitted with earth rods for TNCS as they are built. But that has not made it into our regulations yet"

It never will, it contravenes the ESQCR (real) regulations.

In what way?

After all the practical effect (a local earth supplementing the supply) is no different from bonding to metal service pipes.

 

[UKPN]

In what way?

After all the practical effect (a local earth supplementing the supply) is no different from bonding to metal service pipes.

Its a long story, and i dont get involved in long stories, but it was yet another hairbrained scheme dreamt up by the BS7671 picture guidance book to "improve" our network. Perhaps they should remind themselves of the scope of the regulations. Distribution/supply is out of scope of the regulations.

 

[DPG]

Its a long story, and i dont get involved in long stories, but it was yet another hairbrained scheme dreamt up by the BS7671 picture guidance book to "improve" our network. Perhaps they should remind themselves of the scope of the regulations. Distribution/supply is out of scope of the regulations.

But how will it contravene ESQCR?

 

[Cookie]

I want to ask now. Why is TN-S being phased out? What was the technical justification for abandoning a previous method that worked better?

 

[pc1966]

I want to ask now. Why is TN-S being phased out? What was the technical justification for abandoning a previous method that worked better?

Cost.

Remember their cost decision is about delivering power, not considering the implications for more expensive EV chargers, etc., for those using the power.

 

[Cookie]

Cost.

Remember their cost decision is about delivering power, not considering the implications for more expensive EV chargers, etc., for those using the power.

I figured. However, wasn't the UK poor as compared to the economic prosperity of today when TN-S was the norm?

 

[DPG]

I figured. However, wasn't the UK poor as compared to the economic prosperity of today when TN-S was the norm?

Yes, but I don't think TNCS had been thought of at that time.

 

[Cookie]

Yes, but I don't think TNCS had been thought of at that time.

It had been in other parts of the world. I could be wrong but East Germany and Russia was TN-C all the way up to the socket.

 

[DPG]

It had been in other parts of the world. I could be wrong but East Germany and Russia was TN-C all the way up to the socket.

Not TNCS though.

 

[Cookie]

Not TNCS though.

I'm sure those building networks in the UK would had to have known of a PEN conductor by the 40/50s? I want to think there was a technical decision behind TN-S.

 

[LastManOnline]

Yes, but I don't think TNCS had been thought of at that time.

What's not to think of? All supply systems are relatively simple to design and weigh up regarding pro, s and cons. TNC-S as, a supply system has been there as long as the others but can you imagine been the electrical engineer who suggested it....
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Cost.

Remember their cost decision is about delivering power, not considering the implications for more expensive EV chargers, etc., for those using the power.

Agreed. It was an economic decision. Very smart in the short term but long term........ (sighs!!)

 

[Simon47]

Thinking about it a little, put yourself in the shoes of those running the distribution networks "a few years back". AIUI they typically didn't provide an earth, so the distribution network was 4 core, and 2 core to the single phase customers.
Then along came plastic gas and water pipes. Customers (or their electricians, or the builders putting houses up) had become used to having a solid earth via the water supply - and this was becoming an unknown factor. At the same time I suspect, people were starting to think more about safety - and just banging a token rod in the ground was no longer acceptable for installation earthing.
So "people" start looking to the DNOs and asking why there's no earth being provided, and at some point (AIUI) the DNOs were told they had to start providing earths. Logically, it's the place to provide an earth - they have long runs of underground cable (which at the time would have been mostly lead sheathed), and for overhead supplies they have plenty of poles they can put an earth rod in close to to make a distributed earthing system.
Where they have 4 core lead sheathed in the ground, it's easy to provide an earth - but there's no guarantee that the sheaths are electrically linked at every joint. If it's not got a conductive sheath then that's a different matter. And overhead supplies would have been typically two or four uninsulated cores strung from pole to pole. So what are they supposed to do in these situations ?
Overhead they could string another wire - as long as none of the spans are marginal for height. But if there's an insulated underground section then the only way to add an earth core would be to start digging. And in urban areas with typically underground supplies, that's a lot of digging to do. Basically, adding a separate earth to sections of the network that don't already have one is a massively expensive task.

So if you look at the cost involved of providing a separate earth across the network, you can understand why combined N&E looked attractive. And even when building a new bit of network, you've eliminated a 5th core - which even allowing for it perhaps being a smaller CSA, is a big saving in copper.
And if you have (e.g.) lead sheathed cables in the ground, just linking the neutral to sheath at some joints immediately gives you a strong earth - without having to go round checking for sheath bonding across all joints.

 

[LastManOnline]

Thinking about it a little, put yourself in the shoes of those running the distribution networks "a few years back". AIUI they typically didn't provide an earth, so the distribution network was 4 core, and 2 core to the single phase customers.
Then along came plastic gas and water pipes. Customers (or their electricians, or the builders putting houses up) had become used to having a solid earth via the water supply - and this was becoming an unknown factor. At the same time I suspect, people were starting to think more about safety - and just banging a token rod in the ground was no longer acceptable for installation earthing.
So "people" start looking to the DNOs and asking why there's no earth being provided, and at some point (AIUI) the DNOs were told they had to start providing earths. Logically, it's the place to provide an earth - they have long runs of underground cable (which at the time would have been mostly lead sheathed), and for overhead supplies they have plenty of poles they can put an earth rod in close to to make a distributed earthing system.
Where they have 4 core lead sheathed in the ground, it's easy to provide an earth - but there's no guarantee that the sheaths are electrically linked at every joint. If it's not got a conductive sheath then that's a different matter. And overhead supplies would have been typically two or four uninsulated cores strung from pole to pole. So what are they supposed to do in these situations ?
Overhead they could string another wire - as long as none of the spans are marginal for height. But if there's an insulated underground section then the only way to add an earth core would be to start digging. And in urban areas with typically underground supplies, that's a lot of digging to do. Basically, adding a separate earth to sections of the network that don't already have one is a massively expensive task.

So if you look at the cost involved of providing a separate earth across the network, you can understand why combined N&E looked attractive. And even when building a new bit of network, you've eliminated a 5th core - which even allowing for it perhaps being a smaller CSA, is a big saving in copper.
And if you have (e.g.) lead sheathed cables in the ground, just linking the neutral to sheath at some joints immediately gives you a strong earth - without having to go round checking for sheath bonding across all joints.

I appreciate your points and likewise I appreciate the dilemma the various DNOs were presented with and obviously a, solution had to be arranged. I can, t comment on the situation in the UK but I do not believe it is satisfactory here (ROI) where it is becoming almost exclusively TNC-S to accept that we must now regard the risks associated with a broken neutral as the "new norm".
If the DNO is not for whatever reason in a position to provide a solution let's at least as an industry "front up" about the reality of the situation and start looking at ways to solve the issue from the homeowners standpoint.
I don't want to go into any detail about possible solutions, as its been discussed already extensively (and very adequately) on this and other threads. I can appreciate that there may be a degree of "fatigue" setting in. But I will say this. If I am to hear that the UK is going to follow our lead (and Aus, NZ) etc and implement the "four foot" rod (AKA "the fig leaf), I will despair ?. COOKIE, Are you out there? Time to launch a transatlantic campaign to fully TNC-S?
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I appreciate your points and likewise I appreciate the dilemma the various DNOs were presented with and obviously a, solution had to be arranged. I can, t comment on the situation in the UK but I do not believe it is satisfactory here (ROI) where it is becoming almost exclusively TNC-S to accept that we must now regard the risks associated with a broken neutral as the "new norm".
If the DNO is not for whatever reason in a position to provide a solution let's at least as an industry "front up" about the reality of the situation and start looking at ways to solve the issue from the homeowners standpoint.
I don't want to go into any detail about possible solutions, as its been discussed already extensively (and very adequately) on this and other threads. I can appreciate that there may be a degree of "fatigue" setting in. But I will say this. If I am to hear that the UK is going to follow our lead (and Aus, NZ) etc and implement the "four foot" rod (AKA "the fig leaf), I will despair ?. COOKIE, Are you out there? Time to launch a transatlantic campaign to fully TNC-S?

Sorry fourth sentence should read" NOT satisfactory"

 

[Cookie]

I appreciate your points and likewise I appreciate the dilemma the various DNOs were presented with and obviously a, solution had to be arranged. I can, t comment on the situation in the UK but I do not believe it is satisfactory here (ROI) where it is becoming almost exclusively TNC-S to accept that we must now regard the risks associated with a broken neutral as the "new norm".
If the DNO is not for whatever reason in a position to provide a solution let's at least as an industry "front up" about the reality of the situation and start looking at ways to solve the issue from the homeowners standpoint.
I don't want to go into any detail about possible solutions, as its been discussed already extensively (and very adequately) on this and other threads. I can appreciate that there may be a degree of "fatigue" setting in. But I will say this. If I am to hear that the UK is going to follow our lead (and Aus, NZ) etc and implement the "four foot" rod (AKA "the fig leaf), I will despair ?. COOKIE, Are you out there? Time to launch a transatlantic campaign to fully TNC-S?
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Sorry fourth sentence should read" NOT satisfactory"

I'm still here!

See my other thread, I may have a solution for full TN-S!

 

[LastManOnline]

I'm still here!

See my other thread, I may have a solution for full TN-S!

Good to hear?

 

[Cookie]

Good to hear?

Yup, and following along.

To add to this thread- few have stop to think what stray current flowing all over the earth and infrastructure is doing...

 

[LastManOnline]

I'm still here!

See my other thread, I may have a solution for full TN-S!

What's the title of your other thread..?

 

[Cookie]

What's the title of your other thread..?

"practicality of 400 volt equipment"