3 phase service head and no earth

[Benny_Boy]

Evening chaps,

I've been out to a property to do some work (CU and shower circuits) and there's a 3 phase supply but no earth or main bonding. (only one phase in use, single phase meter)

The perplexing thing is that I got a Ze of 1.17 Ohms at the board, as I removed every means of earthing from the MET it transpired that the reading I was getting was through the immersion circuit. with the CPC from that circuit removed, there was no connection to earth.

I was thinking there must be a dead short between neutral and earth (or some cheeky monkey had joined them somewhere) but insulation resistance tests came back hunky dory.

I can't see any sup bonding but testing has confirmed its presence. Surely there's no way that the service pipes could act as electrodes to the extent required to get a Ze of 1.17? I'm well confused.

I'm going back tomorrow (today when you lot read this) to bang in a spike or two, as I didn't have 25+m of 16mm or any spikes on the van, I sorted the main bonding, installed dual rcd CU and stuck some 16mm in with the neutral of the supply till I get back. I realise this is not good but I figured better to leave them with rcds and some means of earthing than what they had. PME is on order.

Anyone have any ideas as to how I could have got a 1.17Ohms Ze through an immersion heater circuit CPC which hasn't been connected to its neutral. I've never heard of a TT with such a low Ze.......

Bear in mind that all circuit are now 30mA RCD protected and nothing trips!

I eagerly await your opinions.

*edit* I should point out that I called out central networks and the bloke wasn't interested. The fact there was "some" connection to earth even though it wasn't clear how that was happening was good enough for him to say "you have to apply for a PME and it's £154"

 

[Des 56]

If an adjascent property was for instance a Tnc-s system with adequate bonding
The reading you are getting with the immersion connected may be via shared metalic services
You may be picking up the reading via your immersion cpc then via these services through the adjascent properties bonding to its met, which is earthed to the pme neutral,hence the low reading

 

[Deleted member 9648]

If plumbing to the immersion is electrically continuous to the water main a good earth reading would be quite likely via the water main.

 

[malcolmsanford]

Des 56 sounds spot on. If they are offering you a TNC-S system then the chances are that all units from that sub are PME. So you will have rods at various intervals going back to that sub and chances are your picking that up via your pipework if it's metalic through mass of earth.

Are you considering PME for the price. I don't mind TT systems as bung RCD on everything is the norm nowadays, but how many of us have been to TT systems and found that the earth as either eroded off the joint or been remove for wahtever reason. At least with PME it's the DNO responsibility to keep that earth constant.

 

[Benny_Boy]

I Hadn't thought of an adjacent TNCS picking up through the pipes.
Thanks very much for stopping my brain aching chaps!