Converting to a TT system... possibly

[chris day]

1.25ohm external earth fault loop impedance on TN-S supply to building

Supply authority stated our only option is a TT conversion as they will not covert to PME.

What would be my best course of action when converting to a TT as the building is quite old and i do not want a 100ma main switch for diversity reasons and nuisance tripping, so Would it be suitable to convert to TT and have all final circuits in a RCBO with a board charge?

 

[Nigel]

This would be a hell of lot more economical than making the system a TT and doing away with a stable earth in favour of an unstable earth.

hmm yes Essex i think this would be the best way to go, as long as all final circuits will meet the required disconnection time then we have no issues.
I think converting to a TT is money for old rope, would the 500ma downstream RCD just to for backup and to protect the tails?[/QUOTE]

No
It is to provide circuits that exceed the max permitted Zs fault protection.

A 500mA RCD has a max permitted Zs of 100 ohms so is good.

 

[Wilko]

...

 

[mhar]

Would anybody really just go ahead and just use this tns earth? You have already asked the DNO for a pme earth which they have refused and they have advised you to go tt, presumeably because they are not happy with the existing earth.
You need to report an earthing fault under ESQCR. If they are unable / unwilling to do anything (in my area if it is a genuine dno supplied tns earth they would likely arrange for a contractor to provide a tt) then your second question which should be directed towards a network planner is are you happy for me to continue to use this tns earth.
I am suprised at the recommendations you have been getting to continue to use this earth without any reference to the DNO, I would have thought that only the DNO could give such an option.

 

[westward10]

There maybe no fault and that is the correct Ze. ESQCR quotes no maximum values.

 

[Ian1981]

Very unlikely that this is the correct ZE and too be fair if you ever contact the supplier then they will quote the figures that everyone is familiar with ie 0.8ohms TN-S
0.35ohms TN-CS

If the earth is unstable then no chance I would ever use it as it could deteriorate further in the future

 

[Nigel]

Would anybody really just go ahead and just use this tns earth? You have already asked the DNO for a pme earth which they have refused and they have advised you to go tt, presumeably because they are not happy with the existing earth.
You need to report an earthing fault under ESQCR. If they are unable / unwilling to do anything (in my area if it is a genuine dno supplied tns earth they would likely arrange for a contractor to provide a tt) then your second question which should be directed towards a network planner is are you happy for me to continue to use this tns earth.
I am suprised at the recommendations you have been getting to continue to use this earth without any reference to the DNO, I would have thought that only the DNO could give such an option.

The DNO has no obligation to provide an earth. There is also no evidence that there is a fault. It is higher than we would like but if it is stable then there is no issue and the DNO have no obligation to install a PME. It may actually not be possible in the area.

So given all this why would anyone in their right mind change a stable 1.25 ohm earth and change it to an unstable 0.10 - 200 ohm earth?

It makes absolutely zero sense.

 

[Midwest]

Would anybody really just go ahead and just use this tns earth? You have already asked the DNO for a pme earth which they have refused and they have advised you to go tt, presumeably because they are not happy with the existing earth.
You need to report an earthing fault under ESQCR. If they are unable / unwilling to do anything (in my area if it is a genuine dno supplied tns earth they would likely arrange for a contractor to provide a tt) then your second question which should be directed towards a network planner is are you happy for me to continue to use this tns earth.

To be fair, whenever I've rung my DNO's customer service line, the computer always says NO. Ring the local depot, and you get a different kind, more improved service.

Tp the OP, have you tried try emailing them, putting in all the above quotes/advice, see what the reply is then?

 

[Midwest]

The DNO has no obligation to provide an earth. There is also no evidence that there is a fault. It is higher than we would like but if it is stable then there is no issue and the DNO have no obligation to install a PME. It may actually not be possible in the area.

So given all this why would anyone in their right mind change a stable 1.25 ohm earth and change it to an unstable 0.10 - 200 ohm earth?

It makes absolutely zero sense.

I had a CU replacement job a year or so ago. When I checked the supply, it looked like a TN-S, but it also had an earth rod. Tested each in its entirety, and both came back good to go.

Thinking the previous sparks had an issue with the supply, I rang the DNO and asked, also asked about converting to PME. DNO engineer came out to have a butchers. Said TN-S was ok, but if I wanted to convert to PME, showed me 'how to do it!'. He said, the incoming might be TN-S, but in the street, it was probably PME. Every time they repair a cable in road/street, its converted to PME.

 

[Ian1981]

The DNO has no obligation to provide an earth. There is also no evidence that there is a fault. It is higher than we would like but if it is stable then there is no issue and the DNO have no obligation to install a PME. It may actually not be possible in the area.

So given all this why would anyone in their right mind change a stable 1.25 ohm earth and change it to an unstable 0.10 - 200 ohm earth?
It makes absolutely zero sense.

Whether it be 1.25 ohms or 100 ohms it’s irrelevant as it’s most likely that an rcd will be required to provide fault protection to most if not all of the circuits.
If it’s a failing ze problem (we don’t know ) then I would not like to rely upon a failing earth.
Who would?

 

[Nigel]

Whether it be 1.25 ohms or 100 ohms it’s irrelevant as it’s most likely that an rcd will be required to provide fault protection to most if not all of the circuits.
If it’s a failing ze problem (we don’t know ) then I would not like to rely upon a failing earth.
Who would?

There is no evidence given to suggest the earth is deteriorating.

The difference between the two is a TT will need at least 100mA protection on all circuits where as the TN-S will only need 500mA on a few.

 

[Midwest]

Perhaps that's why the IET wanted to earth rod everything for the 18th, just in case

 

[mhar]

It makes absolutely zero sense.

It makes absolutely zero sense not to confirm with the DNO that they are happy for this tns earth to be used.
There is every reason to be suspicious about it, the fact that someone has been out and said to tt it being one reason.

 

[Nigel]

It makes absolutely zero sense not to confirm with the DNO that they are happy for this tns earth to be used.
There is every reason to be suspicious about it, the fact that someone has been out and said to tt it being one reason.

You don’t have to ask the DNO. You have the knowledge and the guidance (BS7671) to make that call.

 

[Ian1981]

You don’t have to ask the DNO. You have the knowledge and the guidance (BS7671) to make that call.

The suppliers Earth is nothing to do with bs7671

 

[Nigel]

The suppliers Earth is nothing to do with bs7671

I know.