The latest UKPN earthing guidance doc states that all LV supplies even ones originally constructed using SEN should be regarded as being PME, if the neutral in the street is lost you may see a rise in voltage between anything earthed/bonded in your installation and earth even if your cut out is TN-S.

Regards

so is there no longer any benefit/disadvantage between tns, and tncs?

Does it say anything about plastic pipes though !!

It noticed thier implemenation of G89 treats terraced houses with metallic services as "Common Buildings" and therefore wouldn't be offered PME - they'll get a SNE service.

Fine - but then bonding would need to be for PME conditions anyway

Regards

OMS

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Failure is always an option

TN-S is superior in many ways - the problem is finding out if the supply is TN-S, or "mis-sold" TN-C-S.
- Andy.

I wonder what earthing system these beauties have?

http://www.flickr.com/photos/allmyownwork/4721667361/

Regards

ok, well mine is tn-s and always has been. im sure though that the new houses connected to the same substation will be tn-c-s though

the cable installed to our 1950s houses would not have earth rods earthing the neutral throughout its length though would it, just at the substation end?

The main reason i didnt want tn-c-s at the time was because I was thinking about adding outbuildings, and the thought of a lost neutral on the supply didnt appeal,

I'm going to stop thinking about it now, i have forgotten half of it now, ill re-remember it when i build my garage

The problem is that if the DNO have had to work on it at any time (e.g. to repair damage or even to connect a new service (even to replace a lamp-post)), they have a nasty habit of joining N & sheath (PE). Any replacement section of cable is likely to be CNE. They might drop a rod into the bottom of the trench before filling it in - but even then the benefit of such rods is at best "limited".
- Andy.

ah yes, i have seen them do that before.

it doesnt really seem ideal that we have mixtues of systems installed without any real knowledge of whats happening


on a totally separate note, what would an installation be called if it was like this:

11kV - 400v customer transformer, star point connected to about 12 rods and tape, this then connected to the main earth terminal, and then just before the main switch, a solid link from neutral connects to the earth bar in the main panel, plus a connection to the main earth terminal.

its been called numerous things in the past, well, in fact, everything! TT, TN-C TN-C-S and PNB

TNC-S (PNB)

Regards

are tncs and pnb one in the same thing or is TNC-S (PNB) a distinct system?

PNB normally serves only 1 customer, but I seem to remember reading somewhere that it can serve up to 4. I guess it depends who owns the TX.

Regards

it just serves the one place, the transformer is customer owned

I always call it PNB but all the test certs vary as i said above

Some say there are two kinds of TN-C-S - PME and PNB.

Others say that PNB is actually a kind of TN-S.

(for me it depends on the relative positions of the earth connection and the N-PE link)

- Andy.

funnily enough, that's the same conclusion I came to!
- Andy.

when the confict of opinion came about, i did a spot of research, and decided it was PNB, think part of the reason was because there werent multiple earths, just the star point , and the MET.

the N-PE link is directly below the 1600a ACB