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3 good ways to improve 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 n. Using the 40 percent
reduction rule, 88 x 0.4 = 35 Ω reduction.

By this calculation, a 4-ft deep rod would have a resistance of 88- 35 or 53 Ω一comparing closely with the curve values.

Maybe you 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 0 to 5.6Ω.

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

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. for three rods are used,the reduction is 60 percent. for 4 rods. 66 percent ( see Figure 3).

When you use multiple rods, they must be spaced apart further than the length of their immersion.

3. Treatment of the Soil

Chemical treatment of soil is a good way to improve earth electrode resistance when you can't drive deeper ground rods for hard underlying rock, eg: it is beyond the scope of this manual to recommend the best treatment chemicals for all situation, you have to consider the possible effect on the electrode.

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.

But Chemical treatment is not a permanent way to improve the earth electrode resistance. it is gradually washed away by rainfall and natural drainage through the soil, Depending on 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.

the advantage of Chemical treatment can reduce the seasonable variation on resistance that results from periodical wetting and drying out of the soil.

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

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