TT or PME?

[OnOff]

Some help please - think my "thinking" is correct. Posting here as no doubt a newbie question so here goes:

My property is an old smallholding so "rural" and dating back to about 1930. Traditionally poles along the road with overhead cables to each property. Originally each property was a 5 acre plot but a lot of these have been subdivided over many years. So through time a number of new build have gone up so there have been many visits from EDF etc with what I assume is upgrading of the supply/poles to offer TNC-S. I'm TT and have always been accepting of that. I've known for a while that some newer properties are TNC-S, next door for instance - some again have run armoured underground from their properties to the road where EDF have connected it (you don't want unsightly overhead cables on a

 

[Manator]

Only the DNO could assess if you can have PME, the problem with rural areas is theft and distance from the Tx, often the only earthing suggested is TT.

Personally I would stick to TT.

 

[Steve3948]

I was once told that if you walk either way from your pole and you see earth down pole at least on the 3rd pole (so every 6) it's likely.

However he was only the linesman, but I have found that its not always the case, as last year I was told there were already too many properties connected.

I was advised to take it further using the Quality of Service act, however decided to stick a rod in and get the job done.

 

[NozSpark]

Personally I'd stick with TT as well,

if it's looked after and tested properly then IMHO it's the safest and best earthing system you could have.

 

[batty]

As Manator says rural areas the distance of cables may mean the loop is not low enough to give a PME earth it needs to be below .35 ohm for this. Personally if its your own property I would stick with TT and that certainly looks like some old cabling on that pole which would suggest a dropped neutral is more likely. The modern concentric cables they use these days its less likely but even still anything is possible with local pikeys.

 

[OnOff]

I'd stick with TT and fit a custom board with Double Pole RCBO's, fit an earth plate and get the Ra to a decent level

When I checked the Ze I got a reading of about 4.2ohm. I figured I could get this down lower based purely on the (visual) state of the connections each end of the 16mm sq cable. At the rod the clamp looked all corroded and at the MET in the CU the copper strands were just "shoved" into the little double tabbed washer/ M8 stud arrangement. I re-made both ends fitting proper insulated ring crimps, a new 5/8", 4' rod & clamp (only about a foot from the existing one). Re-tested and got 4ohm!

To go DP RCBOs would be great - I was thinking one of those double height CU's. Appreciate this is in some respects an expensive option in one hit but the plan is to install the new CU (relocated inside) and keep the old one going, swapping circuits over as I go. Pretty much every room is being re-done so it would work and spread the cost. Must post a drawing up on this as there's some extending tails, enclosing in trunking & switch fuse questions to address!

Now to CU make. Hager don't appear to do anything bigger than a 20-way with a 100A main switch. Anyone use GARO? They appear to offer 2 and 3 row "stacked" CU's. Also they do DP RCBO's for only

 

[batty]

So what are the water pipes and gas pipes made of?

 

[Sidewinder]

As Canoe says, stick a plate down as well as your rod, mind, if you have an Ra of 4 ohms with 1 4' rod, if you can get another down that could give you 2 ohms, getting 4 down @ 4 ohms will give you 1 ohms after that you are on a hiding to nothing tbh

You'd need 8 rods @ 4 ohms per rod to get 0.5 ohms, ignoring the resistance of the interconnecting cables, it could be done say 2 rods off each corner of the house, allowing for you getting 4 ohms max per rod, all interconnected by say 16mm or 25mm sq wiring, to keep the interconnect resistance as low as possible.

However, a plate or mat may be better.

 

[batty]

But he probably is getting his 4 ohms of the water or gas pipe.

 

[Sidewinder]

But, batty, he has stated this as Ze, not Zs(db).

In which case this "should" be the "measurement of Ra at the DB by Ze measurement".

Please note the use of the inverted commas for emphasis!

 

[batty]

Sorry missed that I personally would be over the moon with a resistance of 4 ohm on an earth rod.

 

[Sidewinder]

Well Canoe, we can only work with the information we have, which is Ze of 4 ohms from a single rod.

 

[OnOff]

I'm here to learn hence posting in the Student & Learning Zone. So ref my Ze test, this is what I did:

- Using a calibrated (by RS) AVO Megger LCB2500/2 Loop/RCD Tester

- Main Equipotential Bonding, (water steel/copper)and in my case oil (copper) DISCONNECTED to account for the possibility of parallel paths

- Have an upfront RCD between meter and CU so RCD OFF

The manual refers to a High Current Loop Test done before the RCD on the supply side. Makes reference to 'Zd' "Where Zd is the impedance of the distribution circuit supplying the point of test". It also notes that "In many systems this (Zd) will be the measurement of Ze, the external fault loop impedance". As I figured mine fitted this I referred to it as Ze.

So meter set to L-PE, black, red & green test leads connected the meter side of the RCD and to the MET - SORRY, MEANT TO SAY END OF DISCONNECTED EARTH LEAD and hit the TEST button. Just found my notes and apologies - I was mistaken in the OP saying it was about 4ohm! I've noted it was 3.24ohm then after fitting a new rod and making off both ends with a 16/6 and a 16/8 ring crimp it went down to 3.22ohm. What can I say, house is built at the bottom of a valley adjacent to an old water course so ground is never really "dry". It's also only about 20' from that pole in the above picture with it's PME earth cable - is that maybe "helping"?

IF I'm doing something wrong then please tell me. If I'm right then I can deliver the Dooo Daaah.......just say if you want it warmed up!

 

[Sidewinder]

You have tested at the earth bar in the CU with all final circuits still connected and the bonding disconnected, with the supply L & N disconnected by the single up front RCD being turned off?

 

[steptoe]

If I'm right then I can deliver the Dooo Daaah.

its still not Ra,

it is Ze

its still very good.

 

[OnOff]

You have tested at the earth bar in the CU with all final circuits still connected and the bonding disconnected, with the supply L & N disconnected by the single up front RCD being turned off?

Apologies, just realised what I've written. Green croc clip was on the end of the disconnected 16mm sq lead and NOT the MET. Sorry again!

 

[Sidewinder]

OK, so you tested between the L & N of the incoming supply, disconnected from the installation by the RCD being switched off, & the disconnected earthing conductor leading only to the means of earthing of the installation, that is the earth rod?

 

[OnOff]

OK, so you tested between the L & N of the incoming supply, disconnected from the installation by the RCD being switched off, & the disconnected earthing conductor leading only to the means of earthing of the installation, that is the earth rod?

YES, putting into a few words what I tried with an essay! That is exactly what I did. Sorry for the confusion.