permanent moisture level and should extend well below the frost line. Periodic earth resistance measurements should be made, preferably at times when the soil would be expected to have the least moisture.
You need to test the ground rod installation to be sure it meets the requirement for minimum earth resistance. The earth resistance tester may be used to perform the test. You should make this test before you connect the ground cable to the ground rod.
When ground resistances are too high, they may be reduced by one of the following methods:
1. Using additional ground rods is one of the best means of reducing the resistance to ground; for example, the combined resistance of two rods properly spaced and connected in parallel should be 60 percent of the resistance of one rod; the combined resistance of three rods should be 40 percent of that of a single rod.
2. Longer rods are particularly effective where low-resistance soils are too far below the surface to be reached with the ordinary length rods. The amount of improvement from the additional length on the rods depends on the depth of the low-resistance soils. Usually, a rather sharp decrease in the resistance measurements is noticeable when the rod has been driven to a low-resistance level.
3. Treating the soil around ground rods is a reliable and effective method for reducing ground resistance and is particularly suitable for improving high- resistance ground. The treatment method is advantageous where long rods are impractical because of rock strata or other obstructions to deep driving. There are two practical ways of accomplishing this result, as shown in figure 3-7. Where space is limited, a length of tile pipe is sunk in the ground a few inches from the ground rod (fig. 3-7, view A) and tilled to within 1 foot or so of the ground level with the treatment chemical. The best treatment chemicals for all situations cannot be covered within the scope of this manual. You may work with your engineering office to determine the possible corrosive effect on the electrode. Examples of suitable noncorrosive materials are magnesium sulfate, copper sulfate, and ordinary rock salt. The least corrosive is magnesium sulfate, but rock salt is cheaper and does the job.
The second method is applicable where a circular or semicircular trench can be dug around the ground rod to hold the chemical (fig. 3-7, view B). The chemical must be kept several inches away from coming into direct contact with the ground rod to avoid corrosion of the rod. if you wish to start the chemical action promptly, you should go ahead and flood the treatment material. The first treatment usually contains 50 to 100 pounds of material. The chemical will retain its effectiveness for 2 to 3 years. Each replenishment of the chemical extends the effectiveness for a longer period so that the necessity for future retreating becomes less and less frequent.
Figure 3-7. - Methods of soil treatment for lowering of ground resistance.Continue Reading