Earthing system , TNCS off a TNS?

[normanton]

It is possible to have both TN-S and TN-C-S supplies from the same transformer. For example, the sheaths on some underground cables corrode and stop providing good earth connections, and so homes where "bad earths" are found get converted to TN-C-S

 

[yellowvanman]

Experienced this before. TNS supply had a Ze of 7 ohms, reported to DNO who after much faffing about made it TN-C-S!

 

[Bosi]

Possible maybe, but is it legal? Not according to 543.4.9. Depending on the routing, it takes away all the pros with a separate PE/N. I guess the cost of digging is a factor...

 

[normanton]

It's possible to get two different supplies off one transformer . But as far as getting a TNCS supply from a TNS installation , I don't think that's possible. If the cable got that badly corroded that the Neutral was touching the earth, you would still have a TNS system, just one with a fault! The consumer wouldn't know any different.

 

[Engineer54]

It is possible to have both TN-S and TN-C-S supplies from the same transformer. For example, the sheaths on some underground cables corrode and stop providing good earth connections, and so homes where "bad earths" are found get converted to TN-C-S

YES it is possible, and all too likely too!! When the DNO earth a neutral at the T off joint to a house, or any other joint come to that, they do not include the lead sheathing as part of the N-E connection. In fact they will keep the continuity of the lead sheathing in place. You need to remember also, that it may only be a section of the DNO supply cable that is affected, maybe due to a higher corrosive soil or industry chemical saturation. The rest of the cable could be just fine... It would cost the DNO's an absolute fortune to PME every installation on a cable run, just for the sake of a short section of cable turning bad!!

Your not mixing the two TN systems at the service cut-out, and you will generally comply with whatever DNO earthing arrangement is presented to you!! Where a TN-S system has been converted to TNC-S hopefully the DNO remove the earthing conductor from the lead sheath and insulate with amalgamating tape any exposed lead sheathing... Generally they remove the sheath earthing conductor but don't bother insulating the exposed lead sheathing!! lol!!

This is where the difference lies, between a PME and TNC-S, DNO supply. PME systems will have every or almost every joint PME'd, anything much less than that, will be classified as a TNC-S supply...

Not sure why anyone would think it's not legal, or quote BS7671, as this will only be affecting DNO owned equipement, and they certainly don't follow BS7671!! They tend to make there own rules as they go along!! lol!!