Resistance, resistivity and grounding

9 August 2016

Two very similar words in ground testing, meaning two very different things – resistance and resistivity.  What’s the difference? And how do we find ground using them.

The efficiency of a buried electrode like a rod, grid, plate or parallel ground field is evaluated in terms of resistance.  This is a measurement of how well the electrode can disperse current into the surrounding soil.  If you are taking a resistance measurement, you are testing a particular installed ground.

Resistivity is the independent electrical properties of the soil. If you are taking a resistivity measurement you are testing the soil itself.  Resistivity gives us a way to quantify the differences in materials and their ability to conduct electricity.  By doing this, we get a better idea of how to design a ground system that meets our needs.

Testing a soil’s resistivity also teaches us how conductive the soil is and whether or not a generic ground specification design will work.  It helps reduce unexpected findings at the end of installation as well as indicates the amount of corrosion that can be expected over time on components of the ground system.

Measuring resistivity is the first step in finding a good site for a ground.  And also helps you determine what its design should be. 

Resistivity can change depending on how deep you are going into the soil. Its changes can be unpredictable. Sometimes for the better, sometimes for the worse.  So it is important to know resistivity at varying depths in order to determine the amount and length of ground rods needed for an application. Measuring resistivity is like insurance.  By knowing the soil resistivity upfront, a plan can be put into place to achieve success in grounding.

Measuring the earth’s resistivity is useful for finding the best location and depth for ground rods.  When taking this measurement you are looking for a measurement of less than 25,000 Ohm-cm. 

Soil resistivity testing needs to be checked periodically because climates change throughout the year and environments can change.  It is not a once and done measurement.

A good ground electrode system is made from things like ground electrodes, ground conductors, ground bars and bonding connectors.  Each of these can be made from copper clad steel, solid copper, galvanized or stainless steel or copper plates. 

Remember, that as current flows into a grounding system, it creates electrolysis and corrosion.  As the ground rod corrodes, its resistance rises and it loses its effectiveness. 

Megger offers several ground resistance testers to fit your applications needs.  Click on Ground/earth systems testing to find the tester that is right for your application.