TT but TN-S earth clamp present

[rytech]

Hi I'm fairly new to the forum and have a situation I've never came across before. I visited a customers house to quote for a new consumer unit, the existing installation is a wylex pop out MCB board metal clad with a 100 mA RCD main switch with the supply being TT. The earth rod is not visible as a clever building firm built an extension over it, I got an RA measurement in the region of 46ohm. The confusion comes because the supply cable has a lead sheath with earth clamp integrated, I have measured ZE here and got 0.38 which so is providing a perfectly good earth and miles better than the current non accessible TT system. I just wondered if anyone could shed any light as to why there is an earth rod in use if there is a perfectly good earth provided by the supply cable. The owner of the property has always known it to be a TT system. Would it be wise to contact the DNO to ask permission to use the supply cable as the main earth converting it from a TT to TN-S.

 

[Jay Sparks]

I would contact the DNO. This could be a DIY tn s. What sort of earth clamp is used? If its the one used for bonding then its likely to be DIY. Ring and ask, they may have changed it due to the building of the extension. But I dont come accross that many TT round my way but I think E54 is online and he will probably shed better light on it than me.lol

Jay

 

[trev]

It could be a DIY TN-S so check with the DNO to make sure, you can't do the conversion though.

 

[Engineer54]

I'd leave it well alone!! If someone else has installed this earth clamp to the DNO cable, then it's nothing to do with you!! lol!!

Back in the 50s/60s it was a common practice on council housing estates to install an earth rod or earth plate to TN-S systems... Nothing detrimental about it at all!! You can mix any TN system with a TT, but you can't mix different TN systems.

 

[Jay Sparks]

Told ya...lol

 

[Baker1988]

The only time I have seen a tt when there was a tns connection is that the ze was way too high for a tns so they put a tt system in instead but as the ze of the tns is low enough I would just get that reconnected as that will be better then an in accessible tt rod(which im sure is not allowed by 17th edition) so I would just put it back to a tns

 

[Baker1988]

As above check with dno that it is not a diy job

 

[Jay Sparks]

Hi I'm fairly new to the forum and have a situation I've never came across before. I visited a customers house to quote for a new consumer unit, the existing installation is a wylex pop out MCB board metal clad with a 100 mA RCD main switch with the supply being TT. The earth rod is not visible as a clever building firm built an extension over it, I got an RA measurement in the region of 46ohm. The confusion comes because the supply cable has a lead sheath with earth clamp integrated, I have measured ZE here and got 0.38 which so is providing a perfectly good earth and miles better than the current non accessible TT system. I just wondered if anyone could shed any light as to why there is an earth rod in use if there is a perfectly good earth provided by the supply cable. The owner of the property has always known it to be a TT system. Would it be wise to contact the DNO to ask permission to use the supply cable as the main earth converting it from a TT to TN-S.

With an Ra that good, i would not be worried. Its below the accepted value in the osg of 200ohms.....Sorry E54, only joking, dont hurt me..lol I know how much you love the 200ohm Ra values..lol Just thought Id stir the pot!! It is Friday!!

Jay

 

[rytech]

I think I will contact the DNO to see what their view on it is which will probably that they will want a fee to come and check it and as yes use it as TNC-S. The earth terminal on the supply cable is not just a BS951 clamp it is integrated as part of the sheath, as far as I can see there is no way of releasing the earth terminal from the supply cable it just has an M6 screw with washer ready to accept a lug.

 

[rytech]

Just to add I'm going to use this consumer unit change as my elecsa assessment job and wasn't too sure on how they would view the earth rod being totally inaccessible.

 

[Engineer54]

The only time I have seen a tt when there was a tns connection is that the ze was way too high for a tns so they put a tt system in instead but as the ze of the tns is low enough I would just get that reconnected as that will be better then an in accessible tt rod(which im sure is not allowed by 17th edition) so I would just put it back to a tns

What isn't allowed??

And why would you want to involve the DNO?? The only time to involve those buggers is when you DON'T have a decent Ze value!!! You'll just be asking for trouble!!

 

[Engineer54]

I think I will contact the DNO to see what their view on it is which will probably that they will want a fee to come and check it and as yes use it as TNC-S. The earth terminal on the supply cable is not just a BS951 clamp it is integrated as part of the sheath, as far as I can see there is no way of releasing the earth terminal from the supply cable it just has an M6 screw with washer ready to accept a lug.

Then it's almost certainly a DNO provision!! I was assuming it was a 951 clamp you were on about!! So why not connect your main earthing cable to this sheath earth connection then?? I really can't see what your problem is now??

 

[rytech]

Then it's almost certainly a DNO provision!! I was assuming it was a 951 clamp you were on about!! So why not connect your main earthing cable to this sheath earth connection then?? I really can't see what your problem is now??

Easy!! It was merely an inquisitive question to see if anyone had came across a similar situation before and because it will form part of my elecsa assessment I just wanted a bit of forum input!!!!

 

[Engineer54]

Oh, and leave the TT rod connected to the MET, there is no need to remove it!!