Yes, you do...if you plan to test the earth resistance to a buried ground electrode. A specialized piece of test instrumentation is a must for the proper measurement of ground resistance, but this seeming statement of the obvious is often observed only in the breach. After all, it can be reasoned, ground resistance is just resistance...it can be measured with an ohmmeter, right? Wrong!

Many types of instrumentation (multimeters, insulation testers) have ohmmeter capabilities. Some are so common that they can be found in virtually any tool crib or on any bench top. It is tempting to try to use whatever is handy. And you can get a measurement...but a measurement of what?

The significant issue is that electrical measurements are normally performed on a designed circuit that has been crafted to meet some need, but this is not so with a ground measurement. The electrode under test has been installed in the available soil, and that soil (not a specifically designed circuit) is the critical part of the measurement. To deal with this situation requires a specialized instrument, as is depicted in the simplified schematic below.

The Megger^® Ground Tester establishes its own unique test current through the soil, by means of the selected placement of a current probe and the connection of it and the test ground by leads to the current terminals (C₁, C₂). This establishes a test circuit through the soil, of a unique square wave frequency that is distinct from any harmonics generated by the power grid. The tester uses only its own signal to establish the measurement. Similarly, a pair of potential terminals (P₁, P₂) are connected by leads to the test ground and a strategically placed probe, to measure the voltage drop over the soil to the point of placement of the potential probe. Voltages and current values, then, form the calculation, via Ohm's Law, of the ground resistance.

Multimeters and insulation testers, by contrast, are influenced by transient voltages in the soil. They will give a reading of series loop resistance between whatever two points are connected between their leads, but this is emphatically not a meaningful indicator of the ground electrode's resistance to earth.