Originally Posted by
Engineer54 
How is a PME TP&N supply into a block of flats with several sub distribution supplies that are individually metered at each flat a ''Hybrid'' system?? Each of those sub-main CU'S/DB's will still be derived from a PME supply, so PME conditions apply throughout the building. Each flats supply will have a Zdb/Zs value not a Ze value. the Ze can only be measured at the MET at the incoming TP&N supply position.
Main bonding of service pipes etc within each flat, will still need to be taken to the flats EMT
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I was going off of UKPN documentation,
Yes it is TNC-S at the head or origin, of which there is only one.
Now according to BS7671 each flat is an installation in it's own right, as you rightly say they are bonded locally inside each flat as you would do if it was a separate install like any other.
If you are classing each flat as a separate install as above then technically that flat is a TNS, there is no CNE link at that head, the earthing to that flat is separate, yes it is derived from a TNC-S supply, now either that flat is classed as a separate installation or it isn't.
This is where the complication is, that flat has technically a SNE (separate Neutral Earth) however the bonding requirements are to be taken as TNC-S, it is where the boundary is actually drawn, either at the flats own service head as per BS7671 with it's own bonding requirement as you yourself pointed out above, or the whole block of flats classed as one installation.
It is all semantics really as the definitions of where each part overlaps, I called it a hybrid in "" as it is not always clear where the demarcation point begins and ends as far as we are concerned, our remit is only to BS7671, it does not extend beyond the suppliers cut out which may (or may not) be present in that flat.
I already said if I was replacing a CU in that flat that even though it would be technically (and correctly) a Zs but would record it as that flats Ze on the EIC, because that is where that particular dwelling/installation according to BS7671 ends (or rather begins

), the chances are we would have no access to the supplier switch gear anyway, in other words treat that flat as a separate entity, which is about all we can do.
The UKPN documents go a lot further than a simple block of flats, one example given is a modern industrial estate, now previously each separate unit had it's own CNE link in each (separate) building all bonded to the various services shared between seperate buildings, and was designated as a TNC-S/PME to that building, which is fair enough so far.
The DNO (UKPN in this case) has/had decided in an effort to reduce circulating N -E currents to put just one CNE link at the supply end (sub-station/switchroom or whatever) and supply each unit with an SNE (TNS) supply, however again for the usual reason where PME is concerned (broken suppliers N etc.) that the bonding requirements for these units will be as per TNC-S.
This is similar to above but on a bigger scale
Ps. I already said in the post of mine you quoted, or in one the others related to that subject that the DNO classed the flats supply as TNC-S/PME as far as bonding was concerned and rightfully so in my opinion, so I am not sure where or if you are disagreeing with me or not lol.