Can earth electrode resistance values be too good/artefactual?

[pabrinex]

Hi all,

Would like some advice on the below. Context:

Rural French domestic installation
TT system
Earth rod is situated inside the property, buried in the concrete subfloor
Distribution transformer (overhead supply) is very close, on a pole approx 40 metres from the property

I did some 3 pole earth electrode tests today using my Megger 1741. The distance between the electrodes was approx 20-25metres. The two test electrodes were buried to around 25cm. The local soil is fairly light with a little clay and some gravel. It last rained yesterday but has been fairly dry past few weeks. There is no other bonding to gas or water.

I consistently measured around 6 Ohms. I haven't done much earth electrode testing but from what I've read that is very good. I suspect it could be due to multiple things:

• Short distance to the distribution transformer
• Possibility that the earth rod is tied to the metal framework of the foundation (though if encased in concrete, is that effective?)
• Recent rain

I wonder what your thoughts are. Is there a way this could be artefact? If not, which things (that I've mentioned or not) are likely to have the biggest impact on the good readings?

Many thanks in advance!

 

[davesparks]

Yes if it is connected to the metal reinforcing in the concrete this will improve Ra, potentially by quite a lot. The use of a metal grid buried in concrete as an earth electrode is known as ufer earthing.

Proximity of the distribution transformer should have no influence as you have isolated power and completely disconnected the earth electrode to carry out the 3 wire test.

 

[pabrinex]

Yes if it is connected to the metal reinforcing in the concrete this will improve Ra, potentially by quite a lot. The use of a metal grid buried in concrete as an earth electrode is known as ufer earthing.

Proximity of the distribution transformer should have no influence as you have isolated power and completely disconnected the earth electrode to carry out the 3 wire test.

Thanks for your reply.

Yes of course I'm getting confused with doing an earth fault loop test.

Any idea how common ufer earthing is? This was built in the late 80s.

 

[UKMeterman]

It only gets too good if it is a PME network and you might get more than what the cable is rated for long term through the bonding conductor during a combined earth neutral conductor break. This could be seen with a substantial radio installation where great efforts are made for a function earth with a very low resistance. Have a read of https://www.nienetworks.co.uk/docum...ning-standard/ena_er_g12_4_1-12_2015-pme.aspx

 

[pc1966]

Any idea how common ufer earthing is? This was built in the late 80s.

No idea about French practices, but in the USA it has been common to require rebar in concrete to be bonded to the supply earth (they are typically TN-C-S but the N-E link is usually in the consumer's first (or only) DB and not like the UK where TN-C is only for DNO style use).

 

[Rockingit]

No idea about French practices, but in the USA it has been common to require rebar in concrete to be bonded to the supply earth (they are typically TN-C-S but the N-E link is usually in the consumer's first (or only) DB and not like the UK where TN-C is only for DNO style use).

Is it not now a requirement of new-builds to have structural (aka Ufer) foundation earthing installed or did that go away in the end? I remember there was much talk of it a couple of years back but then I got bored of keeping up with it...

My personal best for a TT is 0.17Ω - obtained via 2 x 8' 5/8ths rods driven below and cross-bonded to about 18sq m of reinforced concrete slab, on 35mm conductors. Admittedly, that was central London so there was probably a multitude of DNO tapes within a few metres, but I'll still take the win!

 

[pc1966]

Is it not now a requirement of new-builds to have structural (aka Ufer) foundation earthing installed or did that go away in the end? I remember there was much talk of it a couple of years back but then I got bored of keeping up with it...

It was dropped, which is dissapointing as probably one of the best safety improvements for TN-C-S going forward.

My personal best for a TT is 0.17Ω - obtained via 2 x 8' 5/8ths rods driven below and cross-bonded to about 18sq m of reinforced concrete slab, on 35mm conductors. Admittedly, that was central London so there was probably a multitude of DNO tapes within a few metres, but I'll still take the win!

Impressive!

As you say, probably quite close to DNO earths and/or service pipes to get anything that low.

Our site has damp clay soil and I am seeing a decent 10-15 ohms per 2m rod, we have 6 in total (more as lightning than supply use) so should be seeing something in the 2-3 ohm region.