TN-C-S? No, TN-S.

[HandySparks]

Thought this might interest some. Doing a CU change. Took a quick look at the supply head, saw the earth terminal on the side and assumed TN-C-S. When the chap from EDF turned up to change the meter and connect to my isolator, it turned out to be TN-S. It looks like the underground supply cable is split concentric; you can see the black insulated strands to the neutral terminal and bare copper to the earth terminal.
Ze was 0.17ohm, Z line was 0.18ohm, so testing wouldn't really have revealed what the earthing arangement was. By the way, does anyone know if there's a standard symbol for the line impedance, line to neutral?

 

[Strima]

Had some head scratching ones on the telecoms cabinets. A lot are TN-S/TN-CS with some really good readings, lowest being 0.03 ohms to date.

Several have had no obvious signs of earthing to the cut-out even with the lightning conductors in place with no connection back to the MET but still get a fairly reasonable Ze when tested, there's that much bonding in place the pikeys would have a field day.

Everyday's a school day...

 

[johnboy6083]

if the site has its own dedicated transformer, then you can almost guarantee that the site is a TNS, even if you open the feeder cabinet and see a neutral-earth link. This is simply the transformers start point being earthed. This is called PNB (protective neutral bond)

Some sites can seem to have dedicated transfromers, but share their transformers with other sites. These can be PME, and the lost neutral can be problematic. if you look at a ground mounted transformer, and it has one set of cables leaving the terminal box, and there is no lucy pillar nearby, then it is likely a dedicated transformer.
Pole mounted transformers can feed multiple sites too, but visually there is often no clue.

If there is a lucy pillar nearby, then these usuallly contain outgoing fuses to protect multiple circuits. Newer transformers have the fuses built in

 

[Strima]

I see what your saying mate and it's good for a heads up. All the streetwork sites I've been to so far have no apparent dedicated transformer or Lucy and all Ze tests are done with the earth disconnected, I was just really surprised at the really good readings as I would be expecting to hear a low hum in the background. Still a learning curve for me, I can only hazard a guess that they require a good supply with a clean earth, just don't look in some of the CU's... :lol: