It gets very complicated and expensive when this assumption does not pan out. Because once the ground wire is installed on the compound, it becomes challenging to get any accurate data readings from a 4-point soil test with copper already in the ground. The 4-point soil test may take a half-day to perform, start to finish, but it can alleviate many headaches before breaking ground. And, a soil test is relatively inexpensive to perform even if you utilize an outside contractor to perform this test. IEEE Std 81 7.2.1 notes that is difficult, or impossible, to obtain a useful approximation of soil resistivity from sample measurement and recommends the Wenner 4-Point Soil test method.
A Wenner 4-Point Soil Resistivity test is performed using a specific meter. Meters are available from several companies, such as AEMC, MEGGER, FLUKE, and others. The test consists of hammering four metal probes into the ground, spaced according to Motorola R56 shown in table B-2.
A 4 Point soil test meter is tied to each of these probes, as instructed by the meter manufacturer. Current is injected into the soil first at 40-foot pin spacing, and progressively smaller pin placements down to 5-foot spacing. The meter takes voltage readings on the other 2 probes and results are displayed on the LED. This test is performed optimally, 5 times: on each of the 4 compound borders and once diagonally across the compound. The data, as taken directly on the readout of the meter, in ohms, is recorded in a table–see table B-3
Once all the data is recorded from each run, you may convert these ohm values to the label, ohm-cm. The formula is ρ = 191.5 x A x R, where A is the distance between rods, and R is the resistance, in ohms, from the meter. If the data is not converted, ALLTEC engineers will convert these readings to ohm-cm when they receive the soil resistivity test before starting on an enhanced grounding system design. The table B-3 will give us the results shown in table B-4 for all depths recorded.