these pictures are describing a TN-S or TN-C-S system?
TN-C-S or TN-S?
- Thread starter dsuley
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these pictures are describing a TN-S or TN-C-S system?
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Even though I imagine (hope!) that you wouldn't find a TN-C-S supply that didn't also have PME, they certaintly aren't the same thing, and theoretically don't have to come together.
TN-C-S means literally what it says - the Neutral and Earth start off (at DNO end) combined ("-C" - i.e. there is just a 'neutral' conductor) and then at some point, usually on entry to a building, separate ("-S") into separate neutral and earth conductors. Anything that fulfills that definition is TN-C-S, and still would even if it didn't have PME.
'PME'is a totally different issue and, again,simply means what it says - multiple earth connections (to the 'combined' neutral conductor.
As far as I am aware, the only 'advantage' of TN-C-S is that it saves the DNOs some money, but to make it a bit less dangerous in the event that a break occurs in the neutral supply cable, they install PME, such that there are frequent connections between their neutral and earth.
As has also been observed, I presume there is no theoretical reason why PME cannot be applied to TN-S systems, but I don't know if this is ever actually done.
Of course, thanks to Main Protective Bonding and metal service supply pipes, both TN-C-S and TN-S would tend to aquire 'PME' via customers' premises, even if the DNO didn't provide it!
Kind Regards, John.
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Where is the earth connection on the second grey fuse unit? - normally it comes out of the right hand side of the fuse unit in the case of a TNCS or PME type system. This "right hand side earth connection" convention has been found on my former 1968 built house, and also on my new Twenty-teens new build house.
I took photo 20 min after DNO did PME!
pictures are describing a TN-S or TN-C-S???
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What concentric cable?
The one coming out of the ground. It "looks" like split con and (as ricicle says) they have borrowed a few neutral strands and connected them to the earth block - making it a TNC-S installation.
Need a good macro close up photo of that to be sure..
Looks like an older cloth covered cable to me, with an earth braid sweated on...
It's a TNS head on a PME system
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It is this "at some point" that is the problem is identifying TN-C-S from TN-S. There are installations where one can see it is TN-C-S but there is only one way to find if it is TN-S and that is enquiry to DNO.
The combined bit may not be visible to the user. If the earth loop impedance is over 0.35 and under 0.8 then this may point to a TN-S supply but it still does not prove it.
TN-C-S system. A system in which neutral and protective functions are combined in a single conductor in part of the system.
Protective multiple earthing (PME). An earthing arrangement. found in TN-C-S systems, in which the supply neutral conductor is used to connect the earthing conductor of an installation with Earth, in accordance with the
Electricity Safety, Quality and Continuity Regulations 2002.
Reading those then PME is TN-C-S by definition. It is possible that TN-C-S is not PME but it is very unlikely. I can't think of any installation where we don't have multiple earths. May be a block of flats?
But the real point is why are we worried about which TN type of supply we have. The ELI is of course important but in the real world only Petrol stations, Caravan Parks and Marinas really have to worry as to if the supply is TN-C-S or TN-S for most installations it does not matter.
Some paperwork asks for supply type. If you don't know it is wrong to guess as if you have it wrong it may get copied again and again on all documents issued after. So the answer is simple. Just enter it as TN. If you want enter in comments supply type not verified.
It's the same where the paperwork wants you to enter the supply fuse size and type. If you don't know then enter <100A or leave blank but to enter a value when you have not a clue is wrong.
The outer earth could be derived from the neutral further back along the cable at a joint. This can cause concerns as a lot of the time it is done so TNS systems become PME without any notification or labelling
Agreed.
I obvioulsy cannot disagree that ... per that definition of PME, 'PME is TN-C-S by definition'. However, as I and others have said, I see no theoretical reason why PME earthing could not be applied to a TN-S system. Indeed, maybe it sometimes is?
Is not the reason that although still very rare, the risk of losing both neutral and earth (but not L) simultaneously, with the hazards that presents, is much more likely with TN-C-S? The bureacratic reasons (forms etc.) you mention for wanting to know the TN type are simply bureacratic!
Kind Regards, John.
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yes but you're missing the point, PME and TNCS are different things. This is an example of a TNS head on a PME system
The outer earth will be derived from the neutral as thats the case in both TNS and TNCS systems....
And me...
I'm not missing the point at all. I.m fully aware TNCS is not always PME but PME is always TNCS.
Yes but the TNS earth is formed at the Tx.......
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