UK TT undetectable PEN fault

Hakuhou · 2025-01-19T13:27:55+0000

Tom256 said:
The requirement was for a TT system but is that now what you have? What earthing system do you now have? Is the intended installation earthing now compliant ?

Ah right it’s now inadvertently a TN-C-S and where there is no predictable path for current to take.
Current can travel down into the source earth, the installation earth, back somewhere in the house, somewhere completely different than the house if that thing is somehow connected to something conductive in the house, or some combination of all the above.

mainline · 2025-01-19T13:45:48+0000

Hakuhou said:
Ah right it’s now inadvertently a TN-C-S.
I would say no, since a fault can either travel down into the source earth, the installation earth, back somewhere in the house, somewhere completely different than the house if that thing is somehow connected to something conductive in the house, or some combination of all the above.
Given the goal of an earthing system is to predictably direct a fault this installation would not be compliant.

As already said the Earth rod is connected before the Consumer unit so all the rod is doing is adding to the PME earthing.

If the Rod was connected to the earth bar in the Consumer unit and penetrated the Neutral, it would no longer be a TNC-S as there would be no Separation from the earth bar.

DefyG · 2025-01-19T13:56:42+0000

Hakuhou said:
where there is no predictable path for current to take.
Current

Unless the CNE conductors are completed severed, current will always take the path of least resistance which is most unlikely to be your new earth rod.

mainline · 2025-01-19T14:01:11+0000

DefyG said:
Unless the CNE conductors are completed severed, current will always take the path of least resistance which is most unlikely to be your new earth rod.

Current flows through all paths in an amount inversely proportional to their resistance.

Tom256 · 2025-01-19T14:02:59+0000

mainline said:
Current flows through all paths in an amount inversely proportional to their resistance.

Is this the point now reached where we move on to touch voltages? Because if we do I need a coffee first... with some bacon to dunk into it

Hakuhou · 2025-01-19T14:05:55+0000

mainline said:
As already said the Earth rod is connected before the Consumer unit so all the rod is doing is adding to the PME earthing.

If the Rod was connected to the earth bar in the Consumer unit and penetrated the Neutral, it would no longer be a TNC-S as there would be no Separation from the earth bar.

Thank you, getting there.
So this is a TN-S system but where the installation’s earth has been connected to the “first” earth in the PME chain.

DefyG · 2025-01-19T14:07:51+0000

DefyG said:
current will always take the path of least resistance which is most unlikely to be your new earth rod.

Probably should have said 'most' as the earth rod having a significantly higher resistance, the current would be insignificant (probably) but this is all hyperthetical!

Tom256 · 2025-01-19T14:07:52+0000

Hakuhou said:
Thank you, getting there.
So this is a TN-S system but where the installation’s earth has been connected to the “first” earth in the PME chain.

View attachment 119470

That is not TN-S. Do you understand why?

Hakuhou · 2025-01-19T14:08:50+0000

mainline said:
Current flows through all paths in an amount inversely proportional to their resistance.

Ohm’s law right? V=I/R

DefyG · 2025-01-19T14:14:33+0000

Hakuhou said:
Ohm’s law right? V=I/R

Wrong

Hakuhou · 2025-01-19T14:18:24+0000

Tom256 said:
That is not TN-S. Do you understand why?

Hmm, for TN-S the installation earth is directly connected to the source earth (usually via the DNO’s supply cable’s metal sheath, this is not the case so not TN-S.
For TN-C-S the separate bit is missing
So yes not TN-S, but not TN-C-S since the S is missing.
By process of elimination it’s a TN-C, in the book I read.
“TN-C is not generally permitted in the UK.”
Ok time to read up on TN-C.

Hakuhou · 2025-01-19T14:19:13+0000

DefyG said:
Wrong

Typo V=IR -> I=V/R

Tom256 · 2025-01-19T14:27:58+0000

Hakuhou said:
Hmm, for TN-S the installation earth is directly connected to the source earth (usually via the DNO’s supply cable’s metal sheath, this is not the case so not TN-S.
For TN-C-S the separate bit is missing
So yes not TN-S, but not TN-C-S since the S is missing.
By process of elimination it’s a TN-C, in the book I read.
“TN-C is not generally permitted in the UK.”
Ok time to read up on TN-C.

Consider the 'S' in TN-C-S. What part of the electrical system does this aspect apply to? Where do the neutral and the protective earth become separated never again to come together?

Tom256 · 2025-01-19T14:35:12+0000

Tom256 said:
Consider the 'S' in TN-C-S. What part of the electrical system does this aspect apply to? Where do the neutral and the protective earth become separated never again to come together?

To help with your consideration of this. Using your diagram in post #21.
The earth point on that 3rd electrode (closest to the installation) was moved along by some form of pixie a tad and was now at the origin of the customers installation.

mainline · 2025-01-19T16:50:47+0000

Hakuhou said:
Hmm, for TN-S the installation earth is directly connected to the source earth (usually via the DNO’s supply cable’s metal sheath, this is not the case so not TN-S.
For TN-C-S the separate bit is missing
So yes not TN-S, but not TN-C-S since the S is missing.
By process of elimination it’s a TN-C, in the book I read.
“TN-C is not generally permitted in the UK.”
Ok time to read up on TN-C.

For it to be TT earthing, you would have to have the ROD connected to the earth bar in the consumer unit.

The PEN would be isolated from that consumer unit.