EICR. Cu "armoured" 3Ph supply cable. Is this a thing?

[1Justin]

I've come across a DNO installed (SSEN) 3 Ph 3 core PNE supply cable from a private substation to a large Lucy domestic head. Cable 3 Ph is copper "armoured" (or is there a better name for it?). Entirely underground, entirely within a domestic single owner property. No obvious practical or visible locations for any intermediate earthing, it's a PNE supply, earth breakout in the head. It dates from ~ 1989.
I was not aware before this that copper "armoured" (or might it be called concentric since it's not armour) was a thing? Cable is Pirelli. Marked 6-60 with 3 cores and about (my my careful measurement) about 60mm^2 of CU in the outer NE conductor.

 

[westward10]

It is a concentric cable.

 

[1Justin]

I was tempted to call it "concentric". I'm only previously familiar with domestic 1 Ph supply TN-S which splits N & E on the outer.

 

[westward10]

If your cable has all bare outer cores then it is concentric where these cores form a PEN conductor. If some of the cores are sleeved blue or black then it is split concentric. Concentric is commonly used for TN-C-S supplies with split concentric used for SN-E supplies or in old money TN-S.

 

[1Justin]

Definitely concentric. Not split. I'd hesitate to call it P"M"E because there is definitely no opportunity for any multiple earths this side of the substation.

 

[westward10]

It isn't PME. Don't forget TN-C-S doesn't necessarily mean PME as you say is is not protective multiple earthed so it is either TN-C-S or PNB.

 

[R-fur]

You said it comes a private substation? You have to get home office permission to have a TNC system the electricity ccompanies have blanket permissions but belive it has to be more specific for private installations. Does this instalation have permission? You said the cable was DNO installed why were they working on private network? If they were working as a contractor the installation should have been done to the current regs.

 

[westward10]

I agree I have overlooked the "private" part. Most private transformers use a separate neutral and earthing conductor often bonded together at the installation or left separate.

 

[Simon47]

Just on a matter of semantics and words ...
Cold "private" really mean "DNO owned but only serving this customer" ? At a previous employer we had this - "our own" substation feeding just our site, but it was DNO owned and things only became ours when the cables came through the trench under the wall and into our switchboard.
So I assume to many it would be a "private" substation - but in reality it's just another DNO owned substation that happens to be on the customers site, in a building the customer built, and serving only that customer.

 

[westward10]

@R-fur is more versed in these matters than me but I would say the DNOs involvement stops at the transformers primary.

 

[1Justin]

You said it comes a private substation? You have to get home office permission to have a TNC system the electricity ccompanies have blanket permissions but belive it has to be more specific for private installations. Does this instalation have permission? You said the cable was DNO installed why were they working on private network? If they were working as a contractor the installation should have been done to the current regs.

It was installed in ~ 1989. That would have been 15th brown book, but I don't think that affects anything. I'm not sure if ESQCR had anything different to say back then. Substation in a building on the domestic site. Original contractor no longer trading. No information otherwise.

 

[1Justin]

Just on a matter of semantics and words ...
Cold "private" really mean "DNO owned but only serving this customer" ? At a previous employer we had this - "our own" substation feeding just our site, but it was DNO owned and things only became ours when the cables came through the trench under the wall and into our switchboard.
So I assume to many it would be a "private" substation - but in reality it's just another DNO owned substation that happens to be on the customers site, in a building the customer built, and serving only that customer.

I believe this is the case, yes.

 

[R-fur]

If its a DNO substation and a DNO cable then I would say its a perfetly normal heavy duty TNC-S supply.
The cable is probably waveform type. The neutral/earths dont wrap around the cable but run along the cable in a wavy patern, this is so that if a jointer is connecting a service to it live, he can get enough slack in the neutral/earth so he does not have to cut it to get access to the live cores inside. The DNO I worked for used 95, 185 and 300mm cable of this type but we had alluminium neutral/earths which were a bad idea as a nick in the outersheath would let water in and the neutral would corrode, as aluminium corrodes it swells up and allows water to travel along the cable causing furthed corrosion. This could eventualy mean loss of neutral. We changed to copper neutral/earths about 25 years ago. About the same time we started using 4 core waveform as a replacement for PILC cable to get away from having to do plumbed wipes on cables, this could also be used for heavy duty TNS supplies.

 

[1Justin]

Thank you R-fur, useful info. Rgds

 

[David Prosser]

You said it comes a private substation? You have to get home office permission to have a TNC system the electricity ccompanies have blanket permissions but belive it has to be more specific for private installations. Does this instalation have permission? You said the cable was DNO installed why were they working on private network? If they were working as a contractor the installation should have been done to the current regs.

I'm confused by this, but it likely I'm misunderstanding you. Most of our privately owned transformers have four cables coming of the secondary side, 3 phases and neutral. The earth neutral link is introduced in the LV distribution panel down stream.

 

[R-fur]

I'm confused by this, but it likely I'm misunderstanding you. Most of our privately owned transformers have four cables coming of the secondary side, 3 phases and neutral. The earth neutral link is introduced in the LV distribution panel down stream.

DNO substations normaly have a fuse cabinate mounted on the transformer the N/E is in there. What you describe is normal for private networks and is still classed as TNS. People get confused by this as most text books show the starpoint of the transformer being earthed, this is rarly done.