Domestic Zs 5ohms! pme using electrode

[Chrishands]

Hi. I have a family member who requires a new CU. She still has a rewireable fuse box and still has an ELCB. So no RCD's. Her install is a deathtrap waiting to happen from an earth faulr point of view!
I replaced a socket on wall today and measured Zs. It was 5Ohms!!!!
Shocking. I checked earthing arrangement and found its supposed to be a PME system, BUT for some reason the main earth is comming from an electrode despite having a PME incomming unit, labeled up as such. I also live 2 doirs down on the same phase and mines TNCS too.

my question is, can I get the DNO to supply main earth via PME? And get rid of earth electrode? Also how do I go about getting them to install an isolator and upgrade tails?
Regards,

 

[malcolmsanford]

If the header is set up for TNC-S then there is a good chance there is one there. The only people that can tell you for definate if there is one is the DNO themselves. That would be my first port of call.

Secondly I doubt that the DNO will supply and fit an isolator. They certainly won't upgrade the tails for you, they normal stop at the header. Again there is no harm asking and you never know.

The main worry though is that Zs, if that is a correct reading there is no chance that will trip the MCB. You do say that there is a ELCB, but there is very little chance of you testing that without specialised equipment.

I'd advise that you use your ELCB as purely your isolator. I'm not sure if you have Part P or not, but yes I would be looking at a board change. But for now if you haven't Part P at least fit a global 30mA RCD between the ELCB and the BS 3036 board, that will give her protection until things are sorted out

 

[Farmelectrics]

yes a phone call to the dno is required you can change board and tails yourself and also install isolator have you taken a ze reading at db and what was the value

 

[Chrishands]

Hi. No im not part P. But I can get a 3rd party sign off. The main concern is no RCD protection. Then secondly im corncerned about Zs. I will measure Ze and if thats the perpetrator, I can only by the looks of it ask DNO if they can put it back to PME. Cant understand why on earth they would use an electrode if the header is capable of tncs.

 

[telectrix]

you need to get the Ze reading first. a few years ago, i had a BG guy pull up an installation with a Zs on sockets of 351 ohms. it was a PME. the Ze was 0.2 ohms, but on the outgoing side of the up front 16th ed. RCD it was 349 ohms. the anomaly was caused by the RCD.

 

[davesparks]

It's more likely that the supply was TT and the DNO upgraded their distribution system to a tncs arrangement and that house hasn't yet been upgraded.
The same situation exists in the village where I live, the abc overheads were installed in the 80's but still half of the houses have old TT setups with voelcb's and 3036 boards. Most of the cutouts even have a nice PME label next to them.

 

[7029 dave]

Sounds like the DNO have provided PME, as stated ZE needs to be taken first to prove, then convert.The tails will be the suppliers responsibly as the Isolator.