We Wrote The Book on Electrical Earthing

Hi Gajanan,

Thank you for your question regarding how to get an earthing rod below 1ohm earth resistance.  It is our pleasure to help.

To reduce the resistance to earth, you have two choices:

• A) drive your earth rod deeper or

Since you have not provided soil resistivity data, we cannot calculate scenario A.  However, we can estimate the resistance to earth using scenario B.  Multiple earth rods yield a lower resistance to earth than a single rod.

If your earth rods are placed at least one rod length apart, you can use a formula to estimate the earth resistance of multiple earthing rods.  So, if you are using 10-foot earth rods, you must place them at least 10 feet apart.  Actually, twice the length (20 feet) is ideal if you have the space.

You can divide the resistance of a single earth rod by the total number of rods and then multiplied by a factor F supplied in IEEE Standard 142—Recommended Practice for Grounding of Industrial and Commercial Power Systems (i.e. “The Green Book”).

Table 4-6—Multiplying factors for multiple rods (excerpt)

• F (2 rods) = 1.16
• F (3 rods) = 1.29
• F (4 rods) = 1.36

Thus if your first ground rod measures 2.5 ohms, we would expect the resistance of multiple rods to be the following:

• Resistance (2 rods) = 2.5 ohms / 2 rods * 1.16 = 1.45 ohms
• Resistance (3 rods) = 2.5 ohms / 3 rods * 1.29 = 1.075 ohms
• Resistance (4 rods) = 2.5 ohms / 4 rods * 1.36 = 0.85 ohms

To reach your goal of 1-ohm earth resistance, you will need three or four rods total.  These can be placed in a row or at the corners of a 10-foot by 10-foot square.  A final measurement will let you know if you have reached your 1-ohm goal.

We hope this helps!  If you should have any further questions, please do not hesitate to contact us again in the future.

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