Cant find where the Main Earth starts

[Norv]

Went to look at a potential job & could not find where the Main Earth started from. It was quite an old house & the DNO's tails (2 x 16mm)looked like they were coming down the wall in metal conduit. There was about 2" of conduit showing. There was no evidence of overhead lines.

I selected the biggest earth wire (6mm) in the CU (No elec shower or cooker) & used it for a Ze test which came out at 0.19 ohms. This wire goes out the back of the CU into the wall. I looked around outside for ages but could not find an inspection pit. The customer said they had the drive concreted about 10 yrs ago.

Anyone got any suggestions ?

I cant assume is a TT can I ?

 

[JohnD]

is it a terraced or semi?

Is the meter at ground floor level?

Will the DNO provide PME?

 

[Norv]

Its end terraced.

Meter at ground floor level, about a metre from the floor.

Not sure about DNO providing PME. I'll have to ring them in the morning

Thanks

 

[RF Lighting]

Can you see the cutout?

Are there any earth wires attatched to it or near it?

 

[Norv]

All I can see is 2 black wires (16mm) coming out the wall in metal conduit, the Live goes into the main fuse & then to the meter in 25mm. The N goes to a henly block & then to the meter in 25mm. Both then go to the CU

There are no earth wires attached anywhere, except for this 6mm in the CU which I used for Ze.

I measured a socket, L to E & there was 240V.

 

[RF Lighting]

It does sound like a TT.

The 6.0mm² earth may well be the main bonding, and the reading could be from a parallel path, as is far too much of a good reading to be connected to an earth stake.

You must not rely on this unless you really can verify it is actually the main earth, and if it is, it will need upgrading anyway.

 

[JohnD]

in an old terrace, the supply might come through the wall from next door.

In this case, you can't rely on their earth, as you never know if they might undo it and you never know, their main earth might just be knotted round a water pipe

 

[Norv]

There wasnt any main bonding. I'm adding a cooker circuit so I'll make sure everythings up to spec.

Do you think its worth giving the DNO a ring anyway ?

Thinking about it, If this is a TT, the CU is an old MEMERA with rewireable fuses

 

[JohnD]

IMO it is always worth asking for a Supplier's earth. the worst that can happen is they might refuse or quote a high price.

 

[SparkyMarky]

Bear in mind that if it is a TT installation then the cable from the MET to any earth spike could be as small as 2.5mm² so the largest earth cable present in the CU won't necessarily be the one you're after...

It sounds like you might have a similar arrangement to my parents house. They're on a TT supply, but the incoming cables run through the wall cavity from the front of the house and come out of the wall next to the meter and service head under the stairs. The 2.5mm² main earth also vanishes into the cavity, presumably to an earth rod. I say presumably as it's nowhere to be found (and they've never had the driveway resurfaced!). I guess the whole lot was installed while the house was being built and is now buried under the driveway or floor. The lack of a visible earth rod has always worried me slightly so I test the earth loop impedence regularly but have always got consistently good readings!

 

[Lectrician]

It does sound like a TT.

The 6.0mm² earth may well be the main bonding, and the reading could be from a parallel path, as is far too much of a good reading to be connected to an earth stake.

Or a neutral-earth fault on the installation.

When you say conduit, are you sure there is no earth taken frm this to the CU - that used to be fairly common.

Your 'conduit' could also easily be MI.

 

[Norv]

Thanks for the replies

Phoned the DNO today who said the incoming tails are on a "wall loop" from next door but they couldnt give me a definate answer about the Main Earth. They have said I should put an Earthrod in & make it a TT but I am still unsure how this 6mm wire is connected at the other end. If this was, say a TN-S at the other end (i.e I'm assuming next door) is it still ok to put the earth rod in.

I pulled up some upstairs floorboards & found the 6mm earth wire crimped to a 6mm black wire but could not trace where this went unless I pulled up loads more floorboards.

 

[Davisss]

I'm only a trainee sparky but the Ze recorded sounds like it is either TNS or TN-C-S. I thought TT systems readings were a lot higher. I've never experienced such a low reading for a TT.

 

[RF Lighting]

Thats the point we were trying to make.

If the DNO has not provided an earth, then the supply has to be a TT, as appears to be the case in this instance.

Even though there is no earth provided by the DNO, it is still possible (and quite common) that an earth (with a good Zs reading) still enters the house.

This could be a water pipe for example which is a continuous metal pipe under the street, and enters into two or more houses. If this has been earth bonded in someone elses house, it will provide an earth connection into your house. This is known as a parallel path, and is the reason why MEBs must be disconnected before you carry out a Ze test.

If the bonding was left in place, it may show a satisfactory earth reading at the consumer unit (through the parallel path) even though there is no earth supply to your property.
If the waterboard them came along and replaced their water mains with plastic pipe, then the property would be left with no earth, without anyone realising.
This is obviously then a very dangerous situation, and why a proper earth connection should be obtained, or the system is redesigned as a TT system

 

[uselessFCUK]

Wouldn't an RCD at point of supply confirm immediately whether it was TT or not?

How long have RCD/ELCB been about for TT systems?