TNC-S earth to external building?

[JONOXNAM]

Hi new bloke here so excuse what might seem like a stupid questions....

Customer has a TNC-S supply feeding a house CU, in which is an RCD protected way feeding a garage with 4.0mm SWA. The SWA has the armour terminated as the earth at both dwelling CU and garage CU.

1 Am I correct in thinking this is NOT satisfactory to use the dwelling CU earth in the external building? (rational being if the Neutral supply is lost then the external building could rise in potential with respect to anyone standing on real earth potential in the garage).

2 Can I simply add an earth rod to the garage CU and leave the SWA in place? or do I need to turn the garage into a TT and disconnect the armour from the garage earth?

3 Do I then simply treat the garage CU as having a TT supply (i.e. is the Ze value now taken as max 200 ohms and the need to fit an RCD in the garage etc)

If I am correct in this then do we need to add earth rods to all TNC-S installations that have copper water taps on their exterior as you are standing on real earth when you touch them.

Thanks, Jon.

 

[steptoe]

Im here, just thought Id let someone else prob explain it in a better way than I could, :|

I suppose I should do an

'Earthing your Garage if your house is TNCS' thread sometime, ?

Customer has a TNC-S supply feeding a house CU, in which is an RCD protected way feeding a garage with 4.0mm SWA. The SWA has the armour terminated as the earth at both dwelling CU and garage CU.1 Am I correct in thinking this is NOT satisfactory to use the dwelling CU earth in the external building? (rational being if the Neutral supply is lost then the external building could rise in potential with respect to anyone standing on real earth potential in the garage). Correct, but the customer wouldnt have to lose the neutral for this to happen, in fact if they did lose the Neutral then everything that has been earthed could potentially rise to 230v, including inside the house.! its about the exported earth being possibly at the potential of star point which could in fact be many miles away.

2 Can I simply add an earth rod to the garage CU and leave the SWA in place? or do I need to turn the garage into a TT and disconnect the armour from the garage earth? you make the garage TT, and disconnect the armour and any other earth from the garage CU, and ensure it is not possible to come into contact with the earth on the armour

3 Do I then simply treat the garage CU as having a TT supply (i.e. is the Ze value now taken as max 200 and the need to fit an RCD in the garage etc) Not exactly, the SWA going to the garage will still need to be within the max Zs required by the protective device in the house, then the garage cu will have its own Ra from the rod.

If I am correct in this then do we need to add earth rods to all TNC-S installations that have copper water taps on their exterior as you are standing on real earth when you touch them. outside lights come to mind here, in short NO, the pipework should be bonded back to the MET and within the fabric of the building, the proper way to sort this out would be to simply upgrade your earthing system to TT throughtout,

this issue would be solved though if the DNO made its supply 'proper' PME , as it stands, its NOT, and TBH, I dont think it will be proper PME for a long long time, no matter what they tell you, bunging a sticker on the cut-out doesnt make it proper PME

Thanks, Jon.

in RED

HTH

 

[apprentice87]

Hi steps, Correct me if i am wrong here, but i often read about DNO's turning up and "converting" a TNS system to "PME" by means of connecting an earth terminal to the neutral at the cutout.

As far as i know though, to convert from one system to another, they would in fact have to dig the road up, and add an earth rod at the point where the service cable joins the main..

Am i right in thinking this??

Also, can anyone explain exactly the difference between "true" PME, and PNB.

Is PNB "PME" without the earth rods???

john..

 

[steptoe]

JOHN, in order for the DNO to comply with ESQCR all current TNCS must be PME by next year, so in order to meet this they are simply sticking PME labels on TNCS systems,

IMO this simply smacks of fraudulently describing something.

TNS is slightly different though as by its inherent nature the sheath 'can' be in regular if not almost constant contact with the general mass of earth.

is PNB PME without the rods,- NO, PME without the rods is simply TNCS

 

[JONOXNAM]

Great thanks steptoe, why don't they just make it compulsary to add a rod to the eathing conductor in all TNC-S - bets and braces - cost is the obvious reason but is there any reason not to as if you can't get Zs in its one way right.

Jon.

 

[steptoe]

jono, search the forum for ' simplified earthing systems ' Ive done a basic description of them,

myself personally I dislike TNCS with a passion, there are a few heated(shall we say) threads on here about PME etc,

there is a way around your issue, but in all truth, if you need to ask then these methods are not the way forward for you, they involve many other factors which simply could never be established via this medium, and would require site visits etc,

the simplest, and usually cheapest option for most people is to simply isolate the earth to the garage and install a TT system, unless of course you wish to upgrade your complete system and install TT for the dwelling as well,.

---------- Post Auto-Merged at 00:07 ---------- Previous post was made at 00:06 ----------

EDIT,_ -- BTW Jono, for TNCS to be true PME its is, IMHO, compulsory for a rod to be added, but, the DNO make their own rules up. :|

---------- Post Auto-Merged at 00:09 ---------- Previous post was made at 00:07 ----------

http://www.talk.electricianforum.co.uk/student-learning-zone-c-g-2330-levels-2-3-c-g-2381-c-g-2391-other-co/15480-simplified-earthing-arrangements.html

 

[apprentice87]

Hi again Steps,

You say that;

"Correct, but the customer wouldnt have to lose the neutral for this to happen, in fact if they did lose the Neutral then everything that has been earthed could potentially rise to 230v, including inside the house.! its about the exported earth being possibly at the potential of star point which could in fact be many miles away"

Ahhh, never thought of that, that is not good!!!

Are you saying then, that if the DNO did the job properly and fitted an earth rod next to where the service joins the main, that leaving aside the problem of the lost neutral, that the problems with TNCS and "exporting" would more or less disappear??

What would happen if you fitted an earth rod at "your" end of the service cable?? Would this "tie the neutral down" so at least the difference between "true" earth [your garage floor say] and the earth derived from the supply maybe ten yards away in the house would no longer matter??

Having said that though, i suppose you generally already have a "rod" at "your" end, as their neutral is connected to your MET, which in turn is bonded to any metallic service pipes poking up out of the ground.

But anyway, say you added an "extra" rod, I know that in theory, if there was a break in the "mains" neutral that your rod would end up as the return path for half the street and cook your earthing conductor for you, but surely the impedance of "your" rod would be such that provided your earthing conductor was the right size to start with [16mm] then nothing much would happen??

john..

 

[steptoe]

I like your thinking John, will do you a proper reply later when on the pc, on phone at work just now.

 

[Badger]

tut tut steps does your boss know your logging on :^O in works time aswell :^O

 

[steptoe]

My boss has got her own job to do without worrying about me! :slap

 

[steptoe]

Hi again Steps,You say that;

"Correct, but the customer wouldnt have to lose the neutral for this to happen, in fact if they did lose the Neutral then everything that has been earthed could potentially rise to 230v, including inside the house.! its about the exported earth being possibly at the potential of star point which could in fact be many miles away"

Ahhh, never thought of that, that is not good!!!

Are you saying then, that if the DNO did the job properly and fitted an earth rod next to where the service joins the main, that leaving aside the problem of the lost neutral, that the problems with TNCS and "exporting" would more or less disappear?? NOT perhaps entirely, but it would make it a more feasible option, IMO

What would happen if you fitted an earth rod at "your" end of the service cable?? Would this "tie the neutral down" so at least the difference between "true" earth [your garage floor say] and the earth derived from the supply maybe ten yards away in the house would no longer matter?? localised rodding,! this is a subject that I wholeheartedly embrace, but it is not as simple as just banging a rod in, in fact, to do so may well actually make your installation less safe than it previously was, one of the reasons I have excluded this type of system from the earthing arrangements sticky,

it actually starts to become quite complex (and possibly extremely dangerous) to do this sort of thing without a proper understanding and grasp of the principles

Having said that though, i suppose you generally already have a "rod" at "your" end, as their neutral is connected to your MET, which in turn is bonded to any metallic service pipes poking up out of the ground. no, unfortunately not, most services now are in plastic, and more and more are becoming such, this is why quite some time ago the regs were changed to disallow the use of services for earthing purposes,

but in practice we do know they will usually drag the Zs down to a lower level than expected from Ze

But anyway, say you added an "extra" rod, I know that in theory, if there was a break in the "mains" neutral that your rod would end up as the return path for half the street and cook your earthing conductor for you, but surely the impedance of "your" rod would be such that provided your earthing conductor was the right size to start with [16mm] then nothing much would happen?? I like the fact you have thought again about simply banging a local rod in, so, as you are realising its not so simple as just adding a local rod, you have now possible multiple fault paths and currents to assess and take into account.

you have kinda answered your own question on putting a local rod in with this statement.

john..

Im liking your outlook and input on this 87,

have I made anything more clear to you?

Im really not very good at explaining some things.

 

[apprentice87]

Hi Steps,

No, you explain things very well and are very knowledgeable too, which is why your opinion is always so very well respected on this forum...

Meanwhile..I learn, little by little... I do try!!

john..

 

[SteveT]

Steps, your theory as always stands up to scrutiny, however you mention 'upgrading' to TT, in Bristol, Western Power do not allow TT, and will not provide a supply unless it comes with PME, in the city at all, only in rural/semi-rural locations. Don't ask me why, I cant get an answer either!!

 

[steptoe]

Probably because they DONT have a proper PME infrastructure and therefore your TT could cause them a lot of headaches with fault currents flowing up other installs bonds etc,

Although I cant think of any justification for them binding you to using their earth terminal, simply DONT use it.