question for ericmark

Lectrician · 27 Sep 2014

WPD will not mix systems on the same sub. TT is not part oft the the mix.

In saying this, there is loads of TN-S systems pulled over to TN-C-S were cables are rotting.

We have had PME Presented on sites were TN-S is preferred, and been unable to get the supply changed, even though the supply cable would permit (a four core wavecon).

One site was a caravan park, a single sub feeding the park, a harbour office and two houses. To get TN-S, we had to convert the harbour to TN-S too, the houses were ignored, as they were TT, being supplied by long runs of single cores right upto the cut out (separate fuse and N block). Had to negotiate with the harbour to allow this to be done.

JohnW2 · 27 Sep 2014

Lectrician said:
WPD will not mix systems on the same sub. TT is not part oft the the mix.

The consumer/customer surely always has option to choose TT if they so wish? AFAIAA, even if the DNO offers, or provides, a TN-S or TN-C-S earth, the customer is under no compulsion to use it.

Kind Regards, John

Lectrician · 27 Sep 2014

Exactly why I said "not part of the mix", or words / ios autocorrect to that effect!

JohnW2 · 28 Sep 2014

Lectrician said:
Exactly why I said "not part of the mix", or words / ios autocorrect to that effect!

Ah, I see - I wasn't totally sure what you were saying (or trying to persuade your device to say!)! Thanks for clarifying.

Kind Regards, John

ericmark · 28 Sep 2014

Some interesting replies.

I had been told the DNO would not mix and match earth systems but since then seen many houses moved from TT to TN-C-S without the neighbours house being also changed so had assumed I had been given duff information.

But these answers explains that I had not considered the difference with TN-S and TN-C-S.

Thanks

bernardgreen · 28 Sep 2014

My cottage and the adjacent retail unit are one stone building. Originally both were owned by the same person and a TT supply to the retail unit supplied a single meter that supplied both the cottage and the shop.

When they were separated I bought the cottage and had to arrange a new supply to the cottage. The DNO offered PME which I am using while the other end of the building still has the original TT set up.

The DNO said that I should ensure there was no possibility of the CPC or any "earthed" item in the cottage making contact with the CPC or any "earthed" item in the shop.

As far as I know both supplies are fed from the same transformer at the substation but they do come from different street cables

Lectrician · 28 Sep 2014

Ignore TT, it is TN-S and TN-C-S that they don't like mixing on the same sub. But as said, you still often see a dodgy TN-S converted to TN-C-S, although these could be historical.

Adam_151 · 28 Sep 2014

Lectrician said:
But as said, you still often see a dodgy TN-S converted to TN-C-S, although these could be historical.

We get that around here quite a bit (WPD now, but former central networks/EMEB region)

I even know of one like that on a petrol station!

JohnW2 · 28 Sep 2014

Lectrician said:
Ignore TT, it is TN-S and TN-C-S that they don't like mixing on the same sub. But as said, you still often see a dodgy TN-S converted to TN-C-S, although these could be historical.

When you say that those cases "could be historical", are you suggesting that they may no longer allow mixing to arise by that process? In other words, if, today, they had to convert one customer's TN-S to TN-C-S, would they then have to convert every other service supplied by that substation?

Kind Regards, John

JohnW2 · 28 Sep 2014

bernardgreen said:
Originally both were owned by the same person and a TT supply to the retail unit supplied a single meter that supplied both the cottage and the shop. ... When they were separated I bought the cottage and had to arrange a new supply to the cottage. The DNO offered PME which I am using while the other end of the building still has the original TT set up.

That's one thing about you which I've never fully understood. You often criticise the very existence of TN-C-S ('just to save DNO money') and frequently talk about potential hazards associated with TN-C-S/PME supplies, so I'm a little surprised you didn't stick with TT for your cottage, particularly given the potential complications associated with having a TT installation in the other half of the building. I wonder what you reasoning was.

Kind Regards, John

Lectrician · 28 Sep 2014

JohnW2 said:
Lectrician said:

Ignore TT, it is TN-S and TN-C-S that they don't like mixing on the same sub. But as said, you still often see a dodgy TN-S converted to TN-C-S, although these could be historical.

Click to expand...

When you say that those cases "could be historical", are you suggesting that they may no longer allow mixing to arise by that process? In other words, if, today, they had to convert one customer's TN-S to TN-C-S, would they then have to convert every other service supplied by that substation?

Kind Regards, John

I've not seen it done recently, so I am not sure. I did not want to say either way. I did have a high resistance TN-S last year on a housing estate, and the chap that came out to look from the DNO said if it couldn't be fixed, they would not provide an earth terminal at all. This was a split concentric cable to a 1970's build. Turned out it was fed from a feeder pillar which had been damaged and left exposed in a hedge, and was basically rotting! They jointed everything out underground and got rid of the pillar.

JohnW2 · 28 Sep 2014

Lectrician said:
I've not seen it done recently, so I am not sure. I did not want to say either way. I did have a high resistance TN-S last year on a housing estate, and the chap that came out to look from the DNO said if it couldn't be fixed, they would not provide an earth terminal at all.

I thought we were always being told that, whilst DNOs are under no obligation to provide an earth in the first place, if they do provide an earth, then they have a duty to 'maintain it'?

Kind Regards, John

ericmark · 28 Sep 2014

Lectrician said:
I've not seen it done recently, so I am not sure. I did not want to say either way. I did have a high resistance TN-S last year on a housing estate, and the chap that came out to look from the DNO said if it couldn't be fixed, they would not provide an earth terminal at all. This was a split concentric cable to a 1970's build. Turned out it was fed from a feeder pillar which had been damaged and left exposed in a hedge, and was basically rotting! They jointed everything out underground and got rid of the pillar.

I know the DNO does not need to provide an earth but once provided can't see how they could remove it?

I was aware of one property where the electrician doing the work phoned the DNO and asked what type of supply would be provided. He was told TN-C-S so did not use and earth electrode and left the tails line, neutral and earth for the DNO to connect.

However the DNO did not provide a TN-C-S supply and left the earth wire disconnected. The electrician was latter criticised for not fitting an earth electrode he in turn phoned the DNO to complain and was told he should have been on site when it was connected. However they did return and give a TN-C-S when he complained all neighbours had a TN-C-S supply.

The electrician said if he was required on site then he should have been given a date and time and if he was not there then the DNO should have not connected a dangerous installation which clearly it was with no earth connected.

Since water and gas were bonded although in theroy there was no earth in practice there was so in real terms it was not dangerous. The problem was at that time RCD protection was not required for all circuits and the ELI without the TN-C-S was not good enough.

I remember the time when the DNO would inspect the whole house and if they didn't like it would refuse to connect the supply. Seems those days are gone!

Lectrician · 28 Sep 2014

JohnW2 said:
Lectrician said:

I've not seen it done recently, so I am not sure. I did not want to say either way. I did have a high resistance TN-S last year on a housing estate, and the chap that came out to look from the DNO said if it couldn't be fixed, they would not provide an earth terminal at all.

Click to expand...

I thought we were always being told that, whilst DNOs are under no obligation to provide an earth in the first place, if they do provide an earth, then they have a duty to 'maintain it'?

Kind Regards, John

I have always read that too, but I am not sure if this really is the case. I don't work for the DNO

JohnW2 · 28 Sep 2014

Lectrician said:
JohnW2 said:

I thought we were always being told that, whilst DNOs are under no obligation to provide an earth in the first place, if they do provide an earth, then they have a duty to 'maintain it'?

Click to expand...

I have always read that too, but I am not sure if this really is the case. I don't work for the DNO

westie does (work for a DNO) and I'm pretty sure that he is one of those who is 'always saying it' - so it must be true

Mind you, from what we are told, the regs and CoPs to which DNOs operate seem to come with a good spattering of "where practicable"-type statements, so there might possibly be a loophole in cases in which they think it would require too much effort to 'maintain an earth' they had supplied!

If he's around, westie will hopefully be able to clarify!

Kind Regards, John

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