When you find that your earth electrode resistance is not low enough, there are several ways you can improve it:
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.
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 this reason, you normally only consider increasing the rod diameter if you have to drive it into hard terrain.
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).
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.
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).
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.
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.
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.
Ground rod resistance (VIDEO)
Reference // A practical guide to earth resistance testing by Megger