Meter has been moved

[dunny123]

The original supply was via lead sheath cable, TN-S. At some point the owner has had the meter moved and the supply is now via a two core concentric cable which has been jointed underground to the old lead sheath cable. I think this was done as a "foreigner" by a guy from EON. I assume that the neutral and earth have been jointed together in the UG joint. The service head is a TN-C-S type and an earth taken in 16mm from here to a terminal block.

My question is, is this now a valid TN-C-S system ? Or should the earth be connected via a rod or the original sheath cable?

The Ze measures a very low 0.1 Ohm

 

[Spark123]

TN-CS should only ever be provided if the network operator has deemed it safe to provide it. This is only normally on PME networks (Protective Multiple Earthing).

 

[dunny123]

TN-CS should only ever be provided if the network operator has deemed it safe to provide it. This is only normally on PME networks (Protective Multiple Earthing).

I have tracked down the DNO Engineer who jointed the sheathed cable to the 2 core concentric and he confirmed that he tied the earth to neutral in the joint, and that this is what they would always do for a meter move or damaged cable in this situation. Furthermore, he stated that if they find a high (not necesarilly out of spec) earth impedance on a TN-S system, say 0.5 and above they will put in a PME head and take the earth off the neutral(explains why I have seen a few PME heads on seemingly TN-S supplies!)

I know that the earth is at the same potential as neutral at the supply generator so the above seems to make sense, but from Spark123's quote I am curious to understand any potential danger from taking the earth from the neutral supply. If the neutral has gained any significant potential above earth, between the supply and the installation, then I would assume it reasonable that it should be grounded ?

 

[Spark123]

The earth on a TN-CS will normally be slightly higher than earth potential owing to the currents flowing in the CNE (combined neutral and earth) conductor.
It is up to the DNO to decide wether or not their system is suitable.
There is a risk where the CNE goes open circuit, the exposed conductive parts can become live at mains potential.
TN-CS is specifically banned in some situations such as caravan sites and marinas owing to the risk posed.

 

[Lectrician]

TN-C-S UG service runs have a bare hard drawn copper conductor (circa 70mm) connected to most of the UG joints, this connects to the CNE, and acts as an earth rod. The cable being laid in the trench with the service cable. This keeps the CNE as near to earth potential as possible.

A lead service cable in the ground also acts it self as an earth rod - the cloth and tar sheath was proven to not last long! So, the sheath is very earthy and earthy to mother earth(soil). On old dodgy runs, they do very often joint N to the sheath in the UG joints, this is almost second nature.

Now, you have a SINGLE core concentric cable feeding your property. This is now classed as a TN-C-S. The N and E are no longer seperate, so you no longer have a TN-S supply. It really is that simple! It may get a little odd if you then start to think about folk who DO have a lead sheathed supply, and have their earth sweated to the sheath.......Do they have a TN-S even if the join in the road has deliberately shorted N to sheath?

 

[dunny123]

Now, you have a SINGLE core concentric cable feeding your property. This is now classed as a TN-C-S. The N and E are no longer seperate, so you no longer have a TN-S supply. It really is that simple! It may get a little odd if you then start to think about folk who DO have a lead sheathed supply, and have their earth sweated to the sheath.......Do they have a TN-S even if the join in the road has deliberately shorted N to sheath?

...which i think is the very question I posed in the first place !

 

[Lectrician]

I have answered your question.

You now have TN-C-S.