Discover the Importance of Soil Resistivity & How to Accurately Measure It

By Steven Harrisson, September 13, 2018

The soil is the most abundantly available resource on this planet. We usually think of it in terms of gardening or archaeological excavations, but interestingly enough, the soil is also a major component of properly functioning electrical systems. But why is that so? While soil can poorly conduct electricity under normal circumstances, if the area of a path for current is large enough and resistance is quite low, it can become a good conductor. And since it’s so readily available everywhere, the soil is the best conducting connection between an electrical circuit or equipment and the earth.

As a rule, the lower the ground resistance – the safer the electrical system will be, which is important if you want to prevent electrical shocks and equipment damage. For this reason, as part of the maintenance plan for electrical systems, it’s important to regularly perform soil resistivity testing. Many factors can influence the resistivity of soil, including its composition, moisture, salt content, and temperature. Since these factors can change with time, it’s recommended that soil resistivity testing is performed at least annually.

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To measure soil resistivity, it’s recommended to use a highly sensitive ground resistance tester. While it can be tempting to purchase a cheap tester, it’s more advisable to invest in a quality model from a reliable brand, like for instance AEMC or Sonel. This is because quality instruments have superior current outputs, which is an especially important requirement to look for. And considering how the soil in most parts of Australia is dry and rocky, you really need a tool that can generate a test current of 200mA and more for an accurate reading.

What’s more, you also need to pick a model that’s suitable for your exact needs. For instance, if you need to test an existing earth installation, a two or three pole ground resistance tester is the best tool to use. However, if you plan to measure the ground’s resistance before new installations, it’s best to employ a four pole tester. While most ground resistance testers require you to manually calculate the resistivity, there are some models that can perform automatic calculations which are more accurate and, not to mention, convenient.

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But even the best instruments can produce wrong figures if you perform the testing under conditions that are less than ideal. For instance, if you do a ground resistance test in the aftermath of a major rainfall. In that case, the results can be overly favorable and may even cause you to miss an important problem, such as the corrosion of ground rods. With that being said, it’s best to test the ground resistance on a day when the soil is especially dry.