Earth Loop Impedance readings

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Does not really matter but it's got me scratching my head wondering why.
A friend has a collection of cabins which is his home in a wood and the mains supply arrives at a near disused cabin where the meters are and with a Henley block it is divided into the local supply and a supply to a 100A switched fuse box and from there to next cabin down the hill.

He was going to use the cabin with the meter in it for visitors and asked me to change some of the old 15A sockets for 13A.

At which point I spotted the ELCB-v and set about some further tests.

Returning armed with RCD and Earth Loop Tester it was found there was an extremely high reading in all cabins around the 350 ohm mark but the RCD did work in the inhabited cabins.

So sent him to get earth rod, consumer unit etc. Starting at the cabin with meter I fitted new consumer unit and knocked in a new earth rod. Got a reading of 35 ohms so left it at that.

Now moved to lived in cabin and this is the strange bit. Earth loop impedance dropped to 12 ohms. How? I was going to fit a second earth rod but clearly not really needed but how do I get a lower reading at furthest point from earth rod?

As I said to start with it passes and the RCD's all trip within the time and at 30ma so all OK but can't work out why lower reading in furthest cabin from incoming mains?

As I write this I have just one thought I wonder where the boards earth rod is? The 11kV lines are closer to cabin with low reading than cabin where mains enters.

Any ideas?
 
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Is there an existing rod at the far cabin?

Are there any extraneous parts in the furthest cabin?

Any bonding present?

How does the far cabin get it's earth? Is it supplied with the L&N from the cabin with the meters in?

There's not a N>E fault at the far end or some kind of ad-hoc TN-C type setup?
 
Returning armed with RCD and Earth Loop Tester it was found there was an extremely high reading in all cabins around the 350 ohm mark ... So sent him to get earth rod, consumer unit etc. Starting at the cabin with meter I fitted new consumer unit and knocked in a new earth rod. Got a reading of 35 ohms so left it at that. ... Now moved to lived in cabin and this is the strange bit. Earth loop impedance dropped to 12 ohms. How?
Are you saying that a Ze of ~350Ω dropped to 12Ω as a result of installing an earth rod at a different cabin (and doing nothing else), with the Ze at the cabin where you'd installed the earth rod being 35Ω? If so, that obviously can't be right - either something else changed or else something is wrong with one or more of the measurements.

Kind Regards, John
 
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parralel paths via some bonding > met ? Is the cabin on metal stantions?
Yes, but none of that (or any of RF's suggestions) could possibly explain the Ze falling from ~350Ω to 12Ω, if the only change had been the installation of an earth rod at another cabin where the Ze (with rod) was 35Ω - unless one changes the laws of physics! As I said, either something else changed, or the readings are not all correct.

Kind Regards, John.
 
Unless you hammered the rod through the supply cable to the remote cabin and only shorted the armouring to neutral...
 
Unless you hammered the rod through the supply cable to the remote cabin and only shorted the armouring to neutral...
True, but if you managed that, you'd probably deserve a Nobel prize :) ... and, anyway, even that I would describe as 'something else changing' (over and above what one normally expects of 'installing an earth rod')!

Kind Regards, John
 
Returning armed with RCD and Earth Loop Tester it was found there was an extremely high reading in all cabins around the 350 ohm mark ... So sent him to get earth rod, consumer unit etc. Starting at the cabin with meter I fitted new consumer unit and knocked in a new earth rod. Got a reading of 35 ohms so left it at that. ... Now moved to lived in cabin and this is the strange bit. Earth loop impedance dropped to 12 ohms. How?
Are you saying that a Ze of ~350Ω dropped to 12Ω as a result of installing an earth rod at a different cabin (and doing nothing else), with the Ze at the cabin where you'd installed the earth rod being 35Ω? If so, that obviously can't be right - either something else changed or else something is wrong with one or more of the measurements.

Kind Regards, John
Yes as you say does not seem right. Hence question as to ideas of any further checks required.
I also could not understand the readings just does not make sense. One thought is the time delay between testing top and bottom cabin I think I must return with meters and test again. The cabins are in a wood which is very wet at the moment but as you say does not make sense.
 
Yes as you say does not seem right. Hence question as to ideas of any further checks required.
Well, the first thing I would do (after re-checking all the measurements) would be to disconnect the new earth rod and see if the Ze at the other cabin rose back (from 12Ω) to ~350Ω. If it did, I would then re-check the Ze at the 'rodded' cabin - since, unless it were a lot lower than 35, you would, as I said before, have witnessed an apparent violation of the laws of physics!

One thought. Did you disconnect any bonding (if there is any) etc. when you carried out any of the Ze measurements, or did you undertake your measurements with the full setup 'as is'? If any 'disconnecting' were involved, all sorts of possibilities jump into the picture.

Kind Regards, John
 
John D may have answer it may have rained and I want to re-check rod and it may have dropped in reading after it had settled which does seem most likely.

No disconnections were made. Just added rod. Old ELCB-v still in place as an isolator. Guy who fitted it must have been a contortionist as I can't get to it. Massive slate slab concreted in above it.

It was only after when I came to transfer notes onto installation cert that I noted readings and so started to wonder why.
 
John D may have answer it may have rained and I want to re-check rod and it may have dropped in reading after it had settled which does seem most likely.
Indeed so - the state of the earth rod could be the 'something which changed'. I had sort-of assumed that the 12Ω and 35Ω figures had been measured 'at the same time' - but, on reflection, that probably was not the case. Mind you, I would be quite impressed with 12Ω from a single earth rod, even if very wet.

So the very first thing I mentioned I would do ("after re-checking all the measurements") may be all you need to do. If both cabins had Ze ~350Ω initially, and adding an earth rod to one resulted in it falling to, say, 35Ω, then the Ze at the other cabin obvioulsy should fall to a figure no less than (in practice a little more than) than 35Ω.

Kind Regards, John
 

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