Earthing on TNCS

[Slimmct]

I realise this overlaps with the Earth Rod install post a little, but I am attempting to plan part one of an upgrade. Currently I am mostly confident I have TNCS supply (there is a query with the DNO to confirm) but I am a little confused by the additional strap to the lead sheaf in a TNS kind of way.

Currently, there is a gas meter about an inch away from the old fuse box, that is supplied by a plastic main, and a lead water main. Neither of these are bonded. There is a 6mm earth cable from the existing fuse box to the DNO earth point. The fuse board has a 40A cooker circuit, a single 32A breaker for the entire 4 bed house and a single 5A breaker for all the lighting.

The meter was recently replaced, and the chap put an isolator switch in for me.

The long term plan is to replace it all, but I am splitting into 2 building regs applications.

The first will put in a consumer unit with a 63a 30ma RCD protecting a supply to some outside sockets 32a B, outside lighting on a 6a B, a hot tub supply on a 32a C, and a 32a B radial circuit to supply a laundry room. There will also be a 80a fused switch off a Henley block supplying a 37m run to a garden office. This will have a smaller consumer unit in with a 80a 30ma RCD and 40a B for a 10kw instantaneous water heater, a 32a B for sockets in the office and workshop, a 6a B lighting circuit, and 20a D for a 16a 2PE commando for a welder.

Part of the work will also replace the lead water main with MDPE. My question relates to the earthing and the possibility of PEN failure. I had first considered bonding the new system to the gas and water, but reading around it, there seems to be the suggestion that with plastic supplies on both, it is not necessary, and may introduce risk. Also, the braid from the earth point to the lead sheaf. It looks like ancient aluminium braid, or could be tinned copper, either way, I am genuinely concerned that it may burst into flames if asked to carry any level of current.

With the talk of PEN fault taking an alternative path to earth, I am left considering whether it should have a 16mm bond added, or whether the braid should be removed.

I dare not ask the question about adding an earth rod to the TNCS system. I cannot find a copy of the apocryphal document by Paul Cook of the IEE on the benefits of adding a stake to TNCS. I have seen some discussion that adding a stake to TNCS is effectively the same as PME, but also the counters of it being the neutral for all your neighbours and the stake not having a good enough earth to realistically reduce touch voltages below 50v in a PEN failure. It appears that most installs of a hot tub within reach of a house use the available earth rather than TT, because it is better.

The rest of the ring mains would be replaced over winter. The plan is to use the garden office and hot tub during summer, then power off and isolate the new install, submit another application for the house ring mains, and do those over winter by installing new sockets and switches, in parallel, to the old ring mains and lighting, then once inspected and if passed, rip out the old and plaster up the holes.

I am understanding that the old stuff doesn’t have to be brought up to regs if not touched. Would the addition of a Henley block in its supply and upgrading the earth to 16mm change that, or can it remain as is?

 

[Slimmct]

Funnily, I've just had a call from the DNO. They have no record for the type of earthing at the property so an engineer will be coming out to look at it. I have no intention of touching anything south of the isolator, but accept naivety on the simplification of demarcation between the DNO system earthing terminal and consumer side with regards of the possibility that they are both part of the DNO system.

 

[Slimmct]

and I totally take on board the advice about finding a sparks. I will contact a few local companies, see if I can contract some of their time to cast an eye over my plans before submitting, and then again to test the earth rods before inspection. I am not after cutting any corners, or skimping. I just like projects and learning opportunities, the maths element has been quite eye opening.

 

[westward10]

I suspect the supply was TN-S with the braid originally separately terminated. For some reason or another, maybe this TN-S broke down and your earth is now derived from the supply neutral and the braid also connected to the neutral to essentially earth the supply cable. This is not uncommon.

 

[Slimmct]

I also wanted to say thank you for the feedback, it is really appreciated, positive and negative.

I ought to put across, that when I said my plan was to do such and such, I am not about to set off with a set off with snips and just do it. I am just putting across my current intention and welcoming challenge. If research or advice says it is not best practice, or just plain dangerous, I will of course update my intention. I didn't want to drop into a forum like this and just ask how do I do a rewire. I am trying to do the homework. The earthing type was quite a foundation question for me and my installation didn't match up to any of the books.

 

[Slimmct]

Just been a little bamboozled on this. A lady from western power phoned up and said they didn't want to send an engineer as they are not responsible for the earth in the property. I tried explaining that the earth termimal had both connections to the meter, like tncs, and to the sheath, like tns, and whilst she said I can't have that, she seemed to think it is not their problem and I should get electrician in to sort it. Unless I wanted to buy a pme upgrade.

Went in circles a little until she offered to send someone to look at, but they wouldn't touch it, and there's no appointments till the 10th June.

 

[Rockingit]

I repeat, again, that you need professional intervention at this point.

However..... because DNO's frequently annoy me on this, here's what the actual law of the land says on the matter:

The Electricity Safety, Quality and Continuity Regulations 2002 -
28. A distributor shall provide, in respect of any existing or proposed consumer’s
installation which is connected or is to be connected to his network, to any person who can
show a reasonable cause for requiring the information, a written statement of—
(a) the maximum prospective short circuit current at the supply terminals;*
(b) for low voltage connections, the maximum earth loop impedance of the earth fault
path outside the installation;
(c) the type and rating of the distributor’s protective device or devices nearest to the
supply terminals;
(d) the type of earthing system applicable to the connection; and
(e) the information specified in regulation 27(1),
which apply, or will apply, to that installation.

* In your situation they wont be able to

 

[marcuswareham]

I repeat, again, that you need professional intervention at this point.

However..... because DNO's frequently annoy me on this, here's what the actual law of the land says on the matter:

The Electricity Safety, Quality and Continuity Regulations 2002 -
28. A distributor shall provide, in respect of any existing or proposed consumer’s
installation which is connected or is to be connected to his network, to any person who can
show a reasonable cause for requiring the information, a written statement of—
(a) the maximum prospective short circuit current at the supply terminals;*
(b) for low voltage connections, the maximum earth loop impedance of the earth fault
path outside the installation;
(c) the type and rating of the distributor’s protective device or devices nearest to the
supply terminals;
(d) the type of earthing system applicable to the c weonnection; and
(e) the information specified in regulation 27(1),
which apply, or will apply, to that installation.

* In your situation they wont be able to

Him trying to get the DNO out to sort it is professional intervention

 

[marcuswareham]

Just been a little bamboozled on this. A lady from western power phoned up and said they didn't want to send an engineer as they are not responsible for the earth in the property. I tried explaining that the earth termimal had both connections to the meter, like tncs, and to the sheath, like tns, and whilst she said I can't have that, she seemed to think it is not their problem and I should get electrician in to sort it. Unless I wanted to buy a pme upgrade.

Went in circles a little until she offered to send someone to look at, but they wouldn't touch it, and there's no appointments till the 10th June.

I was under the impression that although the DNO dont need to provide an earth

they DO need to maintain an earth which they have provided in the past?

The DNOs do seem to be a law unto themselves although,

You said you will get an electrician involved to help/oversee the projects, I wonder if the DNO behave any differently with a request from a qualified spark.

When you say they wouldn't touch it has someone already been out then? Really the DNO are the only people allowed & trained to touch it, it is there property and they have a duty to maintain it,

 

[Slimmct]

Thank you @Rockingit , that section makes it very clear what I can expect, and covers what I wasn't sure about in my discussions on the phone with Western Power. I appreciate your concern, but I'm not sure I need intervention just yet. This is still just a plan on paper. Once I have it to where I think it is a good plan, I will engage with an electrician as to how it looks on paper, and take advice on the whether building regs, or working with the electrician is the best plan.

@marcuswareham I did try covering that with the lady on the phone about provision of earth, and whilst I had read that they are under no obligation to provide an earth, and there are several installations where they won't, that I thought that where they did, they were responsible for it. The conversation got bogged down on where the demarcation was. She was convinced that they were only responsible for the cutout, and that the earthing terminal was mine. I was trying to point out that the earth cable that goes into the cutout was obviously theirs, and that the earth terminal was where the consumer side would first connect to that, and the question about the braid being apparently provided by the DNO, or its predecessor. We went around in circles until she gave in and said I can have appointment, but they were very busy, and middle of June would be earliest. I wasn't going to say no. That is where she said that all they would do is look at it. They wouldn't disconnect or change anything, the only way I could get any work done on it was to order a PME upgrade.

I did call back and speak to someone different as she had finished shift, but all they would say is they couldn't comment on it as it already assigned to her.

 

[Slimmct]

The DNO have been out this morning and said that the connection is PME, and both the braid and wire together are part of the system. They've slapped a couple of stickers on it and gone.