TN-C-S or TN-S?

[westie101]

Missed this one as I was on hol.

As far as the electrical installation is concerned they are both TN-S.

Both cables look to be PILC which cannot be used to supply a PME supply to a customer. As has been commented cable work in the street may have converted it to PME, possible but very very rare within the constraints of PME and the network design has to be taken into account. (I've never known it to occur as we have access to 4c cables to use in PILC networks)
We work to very strictly laid down scenarios to cover these situations!


Oh and
1/

Looks like they've took a few strands of the neutral part of the concentric cable for the earth. To my mind that would make the neutral undersize at the cut-out.

Nope, not legal and would earn whoever did that loss of authorisation and a serious interview with senior operations staff.

2/
We can only supply a TN-S earth from a non-PME system
We can only supply a TNC-S earth from a PME system
There is no interchangeability

 

[ricicle]

We can only supply a TNC-S earth from a PME system
There is no interchangeability

What about PNB.....

 

[holmslaw]

..

 

[ban-all-sheds]

holmslaw - I doubt that westie has much knowledge of the rules, regulations, DNO practices etc in the Wallis and Futuna Islands...

 

[westie101]

What about PNB

PNB is interesting, I found this in relation to agricultural buildings on a PNB supply,

Where isolated premises are connected to a local transformer which provides connection to no other customers, for example, it is possible to use Protective Neutral Bonding (PNB). PNB is not PME, and is not subject to the PME provisions of the ESQCR excepting that Regulation 8(3)(b) makes specific provision for this method of earthing.
In this method of earthing, the LV neutral is not earthed at the transformer, but instead is earthed at the cut-out.
No bonding is required.
In this way a broken neutral anywhere on the system does not result
in elevated potentials on metalwork.
PNB is the preferred method of earthing in these cases, and is particularly effective where livestock is involved.
All PNB installations shall be recorded on the mains records. The resistance to earth of the earth electrode shall be such that twice the current necessary to operate the HV protection will flow following an inter-winding fault on the transformer. A value of 40Ω or less shall be achieved.

 

[westie101]

My place is fed via a pilc installed no later than 1957, and possibly much earlier. And I'm certain there is no earthing of the cable within at least 500mts.

Its all labelled up as pme, do the dno's differentiate between pme and tncs?

If other DNOs do it it that form it must be within their COPs

There is no mention of TNS, or TNC-S within our documents as they probably pre date the descriptions within the Wiring Regs (TT is though)

Services with a SNE cable shall not be converted to PME by the insertion of the neutral/earth link in the cut-out without the written agreement of the Protection Policy Manager, ***. If PME is required then the service
cable shall be replaced with a CNE cable at the cost of the customer.
Alternatively where the existing SNE service cable is long and the costs of the additional joints can be justified, the SNE cable may be reconnected. In suchcase:
at the service termination, a short length of CNE cable terminated in the
cut-out shall be installed;
and at the main, the prescribed neutral-earth bond shall be applied in the joint
and, if necessary, a short length of CNE cable and straight joint installed.

 

[holmslaw]

..

 

[westie101]

so perhaps theres an n-e link in the joint.

I can't find the reference but a N-E link in a service cable to convert a supply to PME is not permitted. It must be done on the main! (well here at least)

 

[RF Lighting]

Not sure how relevant this is, but I had a TN-S supply on a PILC a few years ago.

Tested Ze and it was about 6 ohms, so called the DNO (YEDL/CE)

They came out and swapped the head for a PME head and bonded the sheath of the PILC.

Is that really allowed?

 

[westie101]

They came out and swapped the head for a PME head and bonded the sheath of the PILC.

Is that really allowed?

Here.... No

In YEDL, I don't know.

Each DNO is free to come up with it's own Codes of Practice to comply with the guidelines, so there is not really a national yes/no answer.

I have known it be done as a temporary measure but not permanent

It's not unknown to lose the earth continuity on SNE PILC systems, common causes can be broken lead sheaths (often just below the cut-out) or loss of continuity in certain older joint designs