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Measurement of Earth Spike Resistance


Jerac
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This query follows on from the earth resistance issues raised in the 'Consuel Being Unreasonable' thread.

I am approaching Consuel inspection and I want to get a reasonable idea of the resistance in advance.

The earthing consists of two spikes 3 metres apart, bonded together to connect back to the barette.

Would I get a realistic figure for resistance if I break the bonding between the two and using a small 230volt inverter  run a 60watt lightbulb in series with the two spikes, measure the voltage between the spikes and the current flowing between them and calculate the resistance between them  using ohms law and then halve the value on the basis that in use the spikes will be in parallel rather than series.

I will keep the neighbours dog away from the spikes while they are live!!!

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I'm far from an expert in these matters but wouldn't that only work if it were DC current?  Since it's AC you would be getting some measure of the reactance or for 50Hz are they effectively the same?  If you have an Ohm meter, why can't you just use that?  Sorry if this is a silly question, I'm sure the more knowledgable here assembled will give a proper answer soon, I was just curious......
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Thanks for your reply. I don't know why you can't use a straightforward low voltage ohm meter, but everything I have read says it will not work.

I suppose I could rectify the AC, but I didn't think that 3 metres of damp earth had much reactance at 50Hz.

 

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Stray soil currents flowing in the vicinity and electro-chemical potentials on electrodes, make earth resistance measurements impossible using the ohms range of a multirange meter. Much larger test currents must be used.

Resistances between 20 and 2000 ohms, typically 200 ohms, can be expected with standard test rods 25 mm in diameter and 1000 mm in depth.  However, in all the years I have been working in France I had never achieved below 100 ohms ( required by the Consuel) with a standard 1 metre rod . I always use a 1.5metre rod and typical resistances with one rod this length can be around 60 ohms in good soil, or a combination of rods spaced at least one rod length apart. Even better if a septic tank is being installed as earth plates and a boucle ( loop) can be installed at this time.

Power units should be capable of supplying 100 mA. To drive 10 milliamps into a high resistance soil a supply of 25 volts or more may be needed. This is why I use a dedicated earth rod tester. These work by measuring the current flowing when a known voltage is applied.

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Not quite sure what you meant by that Build Doctor - you obviously didn't read the last paragraph of my post!

Funnily enough, the yellow tester that is shown in your link is the exact one that I have ( bought in France 2003).  I had a Consuel visit on Monday of this week and tested the earth - I got  a reading off 71 ohms. Then the Consuel inspector tested it on his visit using a different make tester and guess what he got ? - 71 ohms. Nice to see it is still working properly!

 

 

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I have seen it done using a Variac and the roughly the method you describre. The math is by substitution, I found the method via Google. Technicaly the reslts are complex but I believe only the resistive element is ever required in this case.....s

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I am sure that my consuel inspector measured the resistance between the earth terminal in the tableau and the incoming neutral, explaining to me that they should be at the same potential, perhaps he was cutting a corner to save time or he had forgotten his tester but he certainly didnt put any probes into the ground.
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