DEEP EARTH GROUNDING VERSUS SHALLOW EARTH GROUNDING
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DISCUSSION
The field data includes earth resistance values for every 5 foot depth of ground rod installation. Ground rod depth was determined by achieving the desired resistance or hitting an obstruction. The resistance of the rod was plotted on a depth vs. resistance graph as shown in Figure 1.
Figure 1. Sample ground rod resistance: ohms versus depth.
The resistance data from over 140 ground rods is averaged and plotted in Figure 2. Note that the average 5-foot ground rod measured 66 Ohms and at 10 feet is 29.8 Ohms, by interpolation an 8-foot ground rod would average approximately 40 Ohms. The average 8 and 10-foot ground rod failed to meet the NEC minimum of 25 Ohms or less. Depths of 30 feet are required for 5 Ohms or less. The first 20 feet of depth represented the greatest change in earth resistance.
Figure 2. Average resistance graph.
The final depth and resistance of each rod is plotted on Figure 3. The majority of the rods ranged in resistance of 0.9 -2.0 Ohms at a depth of 40-60 feet.
Figure 3. Scattered plot chart.
A comparison of the resistance at different times is shown in Figure 4. This graph shows the average resistance for rods installed in each year of the survey period. Note how the resistance varies considerably in depths of 10 feet or less. The early part of 1993 was a very wet period and is represented by much lower resistance. At 30-foot depth the year-to-year variance diminishes to below 10 Ohms resistance. Depths below 30 feet increase the stability and decrease the resistance even further.
Figure 4. Year comparison graph.
