PME vs TN-C-S

I’ll go for:

2 or more DNO earths = PME protective multiple earthing.

1 DNO earth = PNB protective neutral bonding.

Both are TN-C-S combined in the DNO network and separate in the consumers installations. 

Not in the slightest.


PME is only PME in a public distribution network.


PNB is also (most of the time) PME in a public distribution network,


PNB is almost the same as TN-S in a private installation, where:

(a) The N-E bond is only made at one point (usually the main switchboard); AND

(b) The cabling to the transformers / generators have no connection between star point and earth save within the transformer/generator themselves (i.e. the PE between the main switchboard and the transformer/generator is "gapped" at the transformer/generator, making the only items that are TN-C-S the transformers/generators themselves).


So .. clear as mud, but at the same time, not all that difficult.

Yes, indeed!


A couple of years back on the day before Christmas Eve I think it was, the 5-core PILC cable (it had a lighting core in as well) failed. We live between the sub-station and the failure and we were back on power fairly quickly, whereas our next door neighbour was off a bit longer as they joined the other side of the failure to the new cable Wavecon(?) first. (The failure appeared to be due to the PILC being slightly damaged (at the time) when a water meter was fitted for next door.)


Being my usual nosey self I inquired about the street light and was told that they only use one type of cable for repairs and the street light was left isolated for about twelve months until a supply was found for it on the other side of the road.

I didn't see any earth rods being driven in near the joints. Similarly when local link boxes were being renewed, they joined on tails where the earth and neutral were combined. Here though, earth rods laid horizontally in the trench were used.

Clive

PME systems do indeed have extra requirements to be considered PME,  such as electrodes just before the end of the PME run and part way along at defined intervals if the run is more than a certain length, PEN conductors must have double crimped joints or an equivalent measure to make sure neutral is less likely to break then live...

Equally nothing stops TNS or PNB having extra electrodes or extra NE bonds, and still not being PME.

Much of the DNO paper work avoids the saying "PME" altogether, and refers instead to CNE  (Combined Neutral and Earth) and SNE (Separated Neutral and Earth).

IF only there were labels that could be fitted at the intake position that told you what was going on, like this but ideally with the addition of a small white square to the right hand side for writing in a Zs result and a PSSC in crayon.


.

There is absolutely nothing to prevent such labels ... although I would question whether they need to be safety signs?


Just an information/instruction notice is good enough I think.

Can it ever be PME if there is only one earth rod?

As I understand it, PME describes a specific method of providing a consumers earth. TNCS is an earthing arrangement that includes the PME terminal but extends in to the installation to additionally describe the relationship between neutral and earth therein.