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3 good ways to improve earth electrode resistance
3 good ways to improve earth electrode resistance (photo credit: cubis-systems.com)

Earth electrode resistance

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

  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

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

7 Comments


  1. Stephen
    Feb 08, 2017

    Hi,

    Nice photo – that was from a trial site for COnductive concrete – with no requirement for an earth rod.

    The product has an estimated life span of 25 years and in soil conditions of 250 we can get reading of 1 Ohm. Check out our web site Cubis-systems.com

    Stephen


  2. Stephen
    Jan 27, 2017

    Hi, I first notice the photo – which is me! We are testing a conductive concrete product which we use to provide an earthing solution

    Excellent Grounding Qualities
    SAN-EARTH installed in dry powder form, is both convenient and effective. It makes ideal contact with the ground possible over large effective areas and has qualities far superior to more traditional grounding techniques.

    Electrolytic Corrosion Resistance
    SAN-EARTH significantly extends the life of grounding systems. In normal conditions, an electrolytic reaction occurs when any metal buried in the ground is exposed to a positive electric current, resulting in serious corrosion of the metal.

    Covering the metal with SAN-EARTH creates conduction between the metal and SAN-EARTH reducing the electrolytic reaction and preventing the metal from corroding.

    Cost Efficient Grounding & Easy to Install
    No water is required when grounding with SAN-EARTH. Once installed, it absorbs moisture from the surrounding soil and hardens. Common obstacles such as rocks or stumps will not interfere with grounding and need not be removed.

    Because SAN-EARTH is granular and not liquid in nature, trouble free installation is possible even when the site is sloped.

    Environmentally Safe
    SAN-EARTH provides completely pollution free grounding because it is composed of very safe inert chemical matter. It will neither melt into the soil nor change into an electrolyte.

    Reduces opportunity for theft
    Traditional grounding systems are prone to theft due to the scrap value of copper. By using SAN-EARTH, the grounding system is safe from threat of theft or sabotage.

    Surge Impedance
    Surge Impedance is reduced by more than 50%
    The surge impedance of the SAN-EARTH electrode was particularly low during the first critical 0-0.1 µsec of the surge
    Fast response provides enhanced protection for sensitive equipment.
    Significant advantage over other grounding methods

    Check out the web site http://www.cubis-systems.com


  3. Anirban
    Dec 28, 2016

    What are the effects on resistance in plate earthing? How resistance varies with surface area of the plate?


  4. Evans Armah
    Dec 03, 2016

    Thanks for this post, very useful. Please can I connect main earthing of a 500KVA transformer to the neutral terminal. The main earth electrode resistance is 0.5ohms.
    Thank you


  5. Sergio
    Nov 29, 2016

    Well Done, very interesting and practical article!


  6. andrew
    Nov 28, 2016

    strange in the UK you can purchase an earth inspection and test box and the supplier also supplies a conductive mineral fill for high resistance soils but this needs the rod to be buried in a pit like a fence post hole to the same depth as the rod. One other thing missing is using higher gauge wire like 10 mm .square


  7. Aung Moe Moe
    Nov 27, 2016

    How about earth plate? Can you explain me? How about mountain side how to do earthing and can made earth lake or not?

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