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Earth electrode resistance

When you find that your earth electrode resistance is not low enough, there are several ways you can improve it:

3 good ways to improve earth electrode resistance
3 good ways to improve earth electrode resistance (photo credit: cubis-systems.com)
  1. Lengthen the earth electrode in the earth
  2. Use multiple rods
  3. Treat the soil

1. Effect of Rod Size

As you might suspect, driving a longer rod deeper into the earth, materially decreases its resistance. In general, doubling the rod length reduces resistance by about 40 percent. The curve of Figure 1 shows this effect. For example, note that a rod driven 2 ft down has a resistance of 88 Ω.

The same rod driven 4 ft down has a resistance of about 50 Ω. Using the 40 percent reduction rule, 88 × 0.4 = 35 Ω reduction.

By this calculation, a 4-ft deep rod would have a resistance of 88 – 35 or 53 Ω — comparing closely with the curve values.

Earth resistance decreases with depth of electrode in earth
Figure 1 – Earth resistance decreases with depth of electrode in earth

You might also think that increasing the electrode diameter would lower the resistance. It does, but only a little. For the same depth, doubling the rod’s diameter reduces the resistance only about 10 percent. Figure 2 shows this relationship.

For example, a 10-ft deep rod, 5/8 in. in diameter, has a resistance of 6.33 Ω. Increasing its diameter to 1-1/4 in. lowers the resistance o to 5.6 Ω.

For this reason, you normally only consider increasing the rod diameter if you have to drive it into hard terrain.

Diameter of a rod has little effect on its earth resistance Curve A, from Ref. 19 Curve B, average of Underwriters Laboratories tests at Chicago Curve C, average of Underwriters Laboratories tests at Pittsburgh
Figure 2 – Diameter of a rod has little effect on its earth resistance Curve A, from Ref. 19 Curve B, average of Underwriters Laboratories tests at Chicago Curve C, average of Underwriters Laboratories tests at Pittsburgh

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2. Use of Multiple Rods

Two well-spaced rods driven into the earth provide parallel paths. They are, in effect, two resistances in parallel. The rule for two resistances in parallel does not apply exactly. That is, the resultant resistance is not one-half the individual rod resistances (assuming they are of the same size and depth).

Actually, the reduction for two equal resistance rods is about 40 percent. If three rods are used, the reduction is 60 percent, if four, 66 percent (see Figure 3).

Average results obtained from multiple-rod earth electrodes
Figure 3 – Average results obtained from multiple-rod earth electrodes

Comparative resistance of multiple-rod earth electrodes. Single rod equals 100%
Figure 4 – Comparative resistance of multiple-rod earth electrodes. Single rod equals 100%

When you use multiple rods, they must be spaced apart further than the length of their immersion. There are theoretical reasons for this, but you need only refer to curves such as Figure 4 above.

For example, if you have two rods in parallel and 10-ft spacing, resistance is lowered about 40 percent. If the spacing is increased to 20 percent, reduction is about 50 percent.

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3. Treatment of the Soil

Chemical treatment of soil is a good way to improve earth electrode resistance when you cannot drive deeper ground rods because of hard underlying rock, for example. It is beyond the scope of this manual to recommend the best treatment chemicals for all situations. You have to consider the possible corrosive effect on the electrode as well as EPA and local environmental regulations.

Magnesium sulfate, copper sulfate, and ordinary rock salt are suitable non-corrosive materials. Magnesium sulfate is the least corrosive, but rock salt is cheaper and does the job if applied in a trench dug around the electrode (see Figure 5).

Trench method of soil treatment
Figure 5 – Trench method of soil treatment

It should be noted that soluble sulfates attack concrete, and should be kept away from building foundations. Another popular approach is to backfill around the electrode with a specialized conductive concrete. A number of these products, like bentonite, are available on the market.

NOTE! Chemical treatment is not a permanent way to improve your earth electrode resistance.

The chemicals are gradually washed away by rainfall and natural drainage through the soil. Depending upon the porosity of the soil and the amount of rainfall, the period for replacement varies. It may be several years before another treatment is required.

Earth electrode
Earth electrode

Chemical treatment also has the advantage of reducing the seasonable variation on resistance that results from periodical wetting and drying out of the soil. (See curves of Figure 6 below)

However, you should only consider this method when deep or multiple electrodes are not practical.

Chemical treatment of soil lessens seasonal variation of electrodes’ earth resistance
Figure 6 – Chemical treatment of soil lessens seasonal variation of electrodes’ earth resistance

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Ground rod resistance (VIDEO)

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Reference // A practical guide to earth resistance testing by Megger

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

16 Comments


  1. JACKSON SAMUEL
    May 23, 2021

    Kindly send me the PDF of the earthing systems… Thank you
    West Africa, Ghana

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