Earth resistivity testing in wet ground

Paulus33 · Apr 14, 2024

Hi. The question is..... can saturated ground affect earth resistivity testing equipment?
Some days there's an unacceptable earth reading, another day it'll be perfect. The only variable seems to be the weather - ie very wet.
I've asked northern powergrid to convert to a PME system but they say they can't do that at my remote property due to local capacity... which seems unusual.
Is the issue wet ground 'tricking' resistivity test equipment (to 3 new earthing rods) as described in Grey Matter Global article? Or is this something more sinister? Thanks

Soil Resistivity Testing in the Wet | CDEGS | Greymatters Global

Soil Resistivity Testing in the wet or saturated ground can cause headaches. Ninja tricks to help with your earthing design

greymattersglobal.com

nicebutdim · Apr 14, 2024

Indeed it can, which is one of the reasons why we aim for a figure much lower than the theoretically permissible 1666Ω

pc1966 · Apr 14, 2024

Seeing 5k ohm in any situation should be ringing alarm bells!

It is not uncommon to see a 2:1 or more variation over the seasons due to moisture levels, but that high looks like it is buried in some fundamentally dry area (e.g. basement), in VERY dry soil which is uncommon in UK at this time of the year, or very rocky ground so practically little contact area from rod to anything conductive. Or maybe the connection has practically corroded through?

nicebutdim · Apr 14, 2024

pc1966 said:
Seeing 5k ohm in any situation should be ringing alarm bells!

It is not uncommon to see a 2:1 or more variation over the seasons due to moisture levels, but that high looks like it is buried in some fundamentally dry area (e.g. basement), in VERY dry soil which is uncommon in UK at this time of the year, or very rocky ground so practically little contact area from rod to anything conductive. Or maybe the connection has practically corroded through?

I'd be amazed if it's connected to anything - picturing an earthing conductor lying on top of the ground.

brianmoooore · Apr 15, 2024

nicebutdim said:
I'd be amazed if it's connected to anything - picturing an earthing conductor lying on top of the ground.

Reported a poor one on a farm with its own Tx a few years ago. When it was investigated, earth spikes, bare earth radials, the lot, were all lying flat on the surface of the ground immediately below 75mm of concrete, right by the base of the Tx pole.

timhoward · Apr 15, 2024

Paulus33 said:
Some days there's an unacceptable earth reading, another day it'll be perfect

What does "perfect" look like in terms of a reading?
i.e. what is the variation you are seeing?

Paulus33 · Apr 15, 2024

nicebutdim said:
I'd be amazed if it's connected to anything - picturing an earthing conductor lying on top of the ground.

2 brand new earthing rods.... as the previous looked loose and corroded. The same outcome.... then another day everything will be spot on

pc1966 · Apr 15, 2024

Paulus33 said:
2 brand new earthing rods.... as the previous looked loose and corroded. The same outcome.... then another day everything will be spot on

I am guessing you are testing the rods by doing a Zs on them?

If so what you are actually measuring is the overall loop from transformer earthing arrangement, via the supply cables, to your rods and back again via the Earth. The fault might be at the other end!

Do you have any option to use spikes to measure the rod Ra independently from the supply?

Do you have two rods in parallel separated by a modest distance? If so when you see high Z then connect one to the supply and test to the other, if you get a sane value of a few hundred ohm or less under such as test it proves the local rods are sane, then it looks like the supply end is going high-Z.

Paulus33 · Apr 15, 2024

pc1966 said:
I am guessing you are testing the rods by doing a Zs on them?

If so what you are actually measuring is the overall loop from transformer earthing arrangement, via the supply cables, to your rods and back again via the Earth. The fault might be at the other end!

Do you have any option to use spikes to measure the rod Ra independently from the supply?

Do you have two rods in parallel separated by a modest distance? If so when you see high Z then connect one to the supply and test to the other, if you get a sane value of a few hundred ohm or less under such as test it proves the local rods are sane, then it looks like the supply end is going high-Z.

I'll discuss this with my electrician and go from there. Thanks