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.

 

[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?