Earth Electrode Systems (BS 7430)

<...why...not taken into consideration the resistance of the cable that connects the rods or pipes to the Main earth terminal ?...>

Because that is taken into account in other testing that needs to be carried out in accordance with BS7671.

thank you

We need to know its value before the test, to verify that the residual current device chosen works well or not.

All the data that is needed for the calculation is independent of the cable.

A <..residual current device...> is mandatory for TT-earthed systems in many countries' standards. The resistance of the earth cable in TT is tiny in comparison with the electrode resistance and in practice can be disregarded when selecting the device.

You must enter some realistic values into the formula, e.g. 100 Ω-m for resistivity of soil, 3 metre pipe of 0.05m diameter. That 100 ohm-m is e.g. a good moist soil suitable for growing crops, so many locations are much worse.

I think you will find a value of about 30 ohms. Compare resistance of 6 sq.mm copper ~ 2.9 milliohm/metre - so 100 m is 0.3 ohm.

Hence the connecting conductor resistance is negligible [except where there is a really big earth farm, substantial power stations specify 0.5 ohm] - in any case it would be included in resistance when earth resistance is measured properly. Usually the connection to earth system is sized by current carrying capacity & physical robustness rather than resistance. Examine a lightning conductor installation on a building.

67model

in any case it would be included in resistance when earth resistance is measured properly

Yes, you are right. But in a project, the choice of residual current circuit breaker is made before the measurement of the resistance; we must calculate an approximate value of grounding to choose the right residual current circuit breaker.

In many cases the <...right residual current circuit breaker...> will be determined by local practices. 100mA is typical for a domestic TT here to protect the entire installation, such a value protecting against a "poor earth" with 30mA breakers downstream protecting individual circuits for personal protection under BS7671.

• A residual current in excess of 100mA might suggest a serious safety risk with the installation, for this value, if it were passed through a human, can cause fibrillation, etc..
• Individual circuits protected at 30mA are considered safe under the standard.

Information to the forum about <...a project...> might yield additional guidance.

See post #6. You may deduce that ten times the 3m rod example of 30 ohm - a 300 ohm earth x 0.1 amp is only 30 V and a small part of a typical 230V supply - BS7671 gives a maximum value of 500 ohm earth loop impedance for a 100 mA non-delayed RCD, but notes that earth resistances over 200 ohms are unstable. A 100 mA RCD would be used on main incomer, but 30 mA on its distribution circuits for personnel protection.

Following has reference to helpful documents.

http://www.esgroundingsolutions.com/how-do-you-meet-a-grounding-system-requirement-for-0-5-ohm-earth-resistance-in-thailand/

Any approximate value could be a very wrong guess compared to a specific site. Values measured for known rods at existing sites nearby are better than speculation. Note that seasonal changes & weather year to year changes mean that measurements have to be made regularly.

...and a disturbingly large value may be reduced by tipping a large quantity of seawater over the earth electrode area.

If the water is pure, it has very high resistivity - it is the dissolved minerals which make it conduct.

Given the typical minerals, about 12% water gets 100 ohm-metre. Below 10% water, resistivity increases rapidly.

see https://www.scribd.com/presentation/112775002/Earthing-system-design-Myths-Facts#download

So I guess if you get 1 kg of soil, heat it to drive off all moisture and it then weighs 0.9 kg, it has about 10% water content. But surface soil can be quite different to that at depth.

Banging in a test rod or few is probably easier than digging a hole [also easier than digging holes to bury earth plates]. Maybe you could test someone's existing rod to your own rod at several distances separation.

https://en.wikipedia.org/wiki/Soil_resistivity

At points outside the earth grid, the potential rise decreases. The simplest case of the potential at a distance is the analysis of a driven rod electrode in homogeneous earth carrying I amps. The voltage profile is is inversely proportional to radial distance r rom rod given by the equation in Wikipedia.... V = ρ x I/(2 x 3.142 x r)

https://en.wikipedia.org/wiki/Earth_potential_rise

If the current I comes from another rod at a distance, so there is a length of uniform potential between them, this can be taken as the earth potential & voltage at various distances r_x from rod examined for conformity with inverse law.

Thank you PWSlack

This may be useful.....

https://en.wikipedia.org/wiki/Earth_potential_rise

thank you very much 67 model

This is how not to do it.

See-for instance:

ERITECH Grounding Products and Systems

https://stevenengineering.com/Tech_Support/PDFs/ERICO_GROUNDING-BONDING.pdf

in order to see the earthing [grounding] products.

Since the earth resistance is very high with respect to copper [or aluminum or steel] the connection and the electrode itself resistance may be neglected.

Thank you 7anoter4