Firstly I would like to start with an apology to ADS, and Plumber.
Some argument have very good points raised, my only problem is the structure of these arguments. If you read through the posts there are many twists and turns, and references made to steer away from, uncomfortable areas, only to be brought up later in a different guise. I have allowed myself, due in part to personal comments to become embroiled in this, without remaining objective.
There are two thoughts on this subject, both are not wrong, and one is no more right than another.
My position on this is, that whilst two methods of earthing are present, the installation can be classed as one installation, and the supply as presented should not be overlooked. The supply as presented is a TNC-S, sometimes referred to PME, the two should not be confused, the supply companies still have a little time left to address this. Now there is some confusion as to when, and if, any TNC-S earthing can be exported. At one time there was an almost blanket no, given by the supply companies, in the past I have even seen this taken to extremes, in that whilst the internal building was TNC-S, the garden socket was presented as TT, and was fixed to the outside wall of the property. There are also instances where we can find that the earthing conductor from the earth electrode is taken back to the earthing terminal at the origin of supply.
Whichever way you look at it, there will be a relationship between the type of earthing and the provided supply, this is even more important when we have one presented earthing and within the installation add another type.
If we take an installation where we have subdivided the distribution, we will have to consider all eventualities. One of the main considerations is the prevention of shock. Even in a situation where we have an earth free location, the idea being that no earth path through the body is present, we can still have the potential for a shock because of other characteristics not accounted for or designed against.
We have all heard of the fears presented with regards to a dropped or broken neutral, this is rare, and one of the reasons why PME has been pushed forward, in the hope that should this occur there will at least be along the length of supply cable various references to earth. Connected loads from neighbours, also have an effect on the shared supply as presented, one of the reasons why radio hams hate TNC-S.
I have a number of concerns when any TNC-S is taken from origin and then TT'd. If we take the instance of a broken neutral for instance, the immediate danger would be the fault current passing from all the shared houses to your earth electrode. Another problem could occur when the main earthing conductor is removed, or become ineffective, from the earth electrode, as I implied before, this is where the neutral becomes a very important part, the TT part of the installation under this type of fault would in effect become TN-C. Compounded where no extraneous conductive parts are present.
If we take Ze, which ADS and Plumber have correctly identified as a reading external to the installation, could we have two separate Ze readings within the installation? In theory yes we can, however, great care needs to be taken so as not to confuse the two. The TT part of the installation will still be part of the general, total installation. The origin being sub divided, and this relationship should not be ignored. The Ra, of the earth electrode is used as a calculation, if the Ra is not known it may be replaced by Zs. I will continue this post at a later time, as its a long long explanation, and far too much to read in one go.
