Understanding the relationship between TNCS, TNS and TT

[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.