ze 12 ohms

[westie101]

sparkticus

Generally we use a "one size fits all" cutout. So it would be suitable for any of the earthing methods.
Makes stock control easier and keeps costs down!

rotted away old earth sheath

Quite possibly, as you say an attempt at a solution without involving the DNO. (as discussed elsewhere there is a lot of urban myth about what we do and don't charge for) If a property has an existing earth we are obliged to maintain it.

Oddly enough we had a query today where an electrician (we think) was suggesting he just connect the earth to the neutral as there was no earth at a property!
We told him it would be TT and an independent earth rod should be fitted!

 

[sparkticus]

Sure - but that's totally moving the goalposts in comparison with what you wrote (above)

As if I would


Yes (and this is where my knowledge of the DNO world is lacking) I suspect that they might convert a rotted TN-S to a TN-C-S by adding a link?



I am not 100% sure about that though it is just something I have partly assumed happens sometimes.

 

[sparkticus]

I can sometimes get a Ze reading off a damp wall

You will wake up the sleeping "cockburn"

 

[JohnW2]

As if I would
Yes (and this is where my knowledge of the DNO world is lacking) I suspect that they might convert a rotted TN-S to a TN-C-S by adding a link? I am not 100% sure about that though it is just something I have partly assumed happens sometimes.

If they did that, they would presumably have to add PME, otherwise it would be very naughty?

Kind Regards, John.

 

[EFLImpudence]

Is that how the wall got wet?

 

[sparkticus]

As if I would
Yes (and this is where my knowledge of the DNO world is lacking) I suspect that they might convert a rotted TN-S to a TN-C-S by adding a link? I am not 100% sure about that though it is just something I have partly assumed happens sometimes.

If they did that, they would presumably have to add PME, otherwise it would be very naughty?

Absolutely but even with adding PME (which might be multiple neutral-earth spikes along the rout) if the link is missing (in other words if it was supposed to be converted but the conversion did not happen) you would have a cut-out that might look like TN-S or TN-C-S but is really a TT connection by virtue of the PME being implemented (and the remnants of the old earth cable of course)

This is where I really think thee DNO should indelibly mark a true TN-C-S/PME installation so there is no doubt.

 

[sparkticus]

sparkticus

Generally we use a "one size fits all" cutout. So it would be suitable for any of the earthing methods.
Makes stock control easier and keeps costs down!

rotted away old earth sheath

Quite possibly, as you say an attempt at a solution without involving the DNO. (as discussed elsewhere there is a lot of urban myth about what we do and don't charge for) If a property has an existing earth we are obliged to maintain it.

Oddly enough we had a query today where an electrician (we think) was suggesting he just connect the earth to the neutral as there was no earth at a property!
We told him it would be TT and an independent earth rod should be fitted!

Thanks Westie, one more question: do the DNO convert rotted out TN-S to TN-C-S?

 

[westie101]

I suspect that they might convert a rotted TN-S to a TN-C-S by adding a link?

From posts on here I suspect some might. Certainly in this part of the country we are prohibited from doing this. The level of checks on an underground system to convert it to PME are considerable.
Whilst it may appear simple in the worst case (fault) scenario there are serious risks of losing the neutral with the attendant effects of that.

 

[sparkticus]

Oddly enough we had a query today where an electrician (we think) was suggesting he just connect the earth to the neutral as there was no earth at a property!
We told him it would be TT and an independent earth rod should be fitted!

Yes, I know that you have to "PME" the rout first. (at least I think I know that - my knowledge of the DNO world is very limited)

 

[westie101]

Yes, I know that you have to "PME" the rout first

Correct, for example if we have underground cables feeding up exterior walls to overhead mural wiring, we have to bond the N to E in each one.
Of course all the houses will need bringing up to an acceptable bonding standard

 

[JohnW2]

If they did that, they would presumably have to add PME, otherwise it would be very naughty?

Absolutely but even with adding PME (which might be multiple neutral-earth spikes along the rout) if the link is missing (in other words if it was supposed to be converted but the conversion did not happen) you would have a cut-out that might look like TN-S or TN-C-S but is really a TT connection by virtue of the PME being implemented (and the remnants of the old earth cable of course)

I'm not sure I get that emboldened bit. If there is no N-E link, then all the cutout's earth terminal is going to provide is either 'nothing' or a high resistance return path, depending upon whether the TN-S earth is completely or partially rotted - I don't see how it could benefit from the PME earthing.

This is where I really think thee DNO should indelibly mark a true TN-C-S/PME installation so there is no doubt.

I think that most (maybe all) DNOs do provide the 'PME Warning' labels/stickers - but what is probably needed for situations such as we're discussing (and the DNOs don't provide) is a label saying that it is NOT a TN-C-S/PME system.

Kind Regards, John.

 

[sparkticus]

I suspect that they might convert a rotted TN-S to a TN-C-S by adding a link?

From posts on here I suspect some might.

I just talked to my son (he does mostly domestic) he just confirmed the following about a house near the centre of Bristol:

- Until 2006 TN-S
- He quoted for rewire in 2006 but could not obtain a reasonable Ze
- DNO called - investigated and said house will need to be converted to TT
- In 2009 property became available for TN-C-S at a price of £78+VAT
- My son and property owner reminded DNO that the service was once TN-S
- They converted to TN-C-S within 6 week free of charge.

Now this general street became "available for PME" so I guess that means that the DNO had completed some planned investigations and "PME'd" the general area?

 

[sparkticus]

duplicate sorry

 

[sparkticus]

If there is no N-E link, then all the cutout's earth terminal is going to provide is either 'nothing' or a high resistance return path, depending upon whether the TN-S earth is completely or partially rotted - I don't see how it could benefit from the PME earthing.

In the case of a fully open circuit earth sheath (rotted through) where the earth current (test current for Ze) would flow across the rotted gap and via parallel paths, would the PME earthing (along the rout) not lower (slightly or greatly) the earth return resistance?

Now I now the picture is complex because of what is going on elsewhere in the street/vicinity but intuitively (I do not know for sure) I would say the PME would generally lower the measured Ze in such circumstances.

 

[westie101]

In the case of a fully open circuit earth sheath (rotted through)

Don't forget that the lead sheath is usually the earth, not the steel tape armouring (though the two are bonded together). It is not that common as obviously if the lead is damaged dampness can get into the paper insulation.