As per the BS standard BS-7430 Page no:38 the formula can be used for finding number of parallel rods to arrive 1 Ohm value.

Mr. Venugopal,

Could you please post page #38 containing the formula from BS-7430 for everyone.

Regards,

M.T. Ansari

First you must identify the soil type and then achieve the reading of 1 rod in the soil onsite. this must not be assumed.

RGDS

In that formula we have to back work to arrive the desired resistance value. it is possible you can fix the soil resistivity then you vary the length of the rod you can bring desired value of resistance 1 ohm.

"it is possible you can fix the soil resistivity" ?????

Not sure what you are saying here, please clarify.

Mr. Venugopal,

If I may ask, what is the significance of 1 ohm? Where is it coming from? What assumptions were made in order to derive this value?

From my experience, achieving such an arbitrary value does not relief you as a designer from your duty of care or safety obligations. These seem to be indefensible in a court of law.

Just achieving such a value does not say a lot about the location of the asset. Location is important as it defines the ultimate risk exposed by the asset. What is suitable / applicable in one area is not necessarily applicable in another.

Be careful when chasing such values.

Hi All,

Agreed with Jan, the required resistance to earth is depended on various aspects and we must consider them while designing the earthing system of the building according to its purpose.

Only in case of Protection Against Lightning, IEC or EN calls for resistance to earth value of max 10 ohm.

For Mr. M T Ansari : Sending you the following by message as I' unable to paste any formula here.
Calculation of Resistance to earth – parallel electrode & horizontal conductors
(eq. 9.5.5 : BS7430 : 2011)