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What is the 'grounding path' and how good is it?"

For safety reasons, grounding of electrical power systems is common practice throughout the world. What links the system to the earth itself is the grounding electrode, in its simplest form a metal rod driven into the earth. The resistance of the grounding path through that rod is meant to be low enough to allow sufficient ground fault or induced surge current to operate protective devices on the system, and to keep voltage drop throughout the path low enough for personnel safety.

Many studies have been made of methods for maintaining low ground electrode resistance. What's most important is the interface between rod and earth, and the resistivity of the soil itself--particularly that immediately surrounding the rod.

Soil chemistry and moisture content, temperature, and the depth of rod penetration into the soil all influence the grounding path resistance. Some electrodes contain chemical salts, or are equipped with water supply systems, to keep resistance low. In large industrial installations, encasing electrodes in concrete has been beneficial, leading to use of building reinforcing steel as a system ground.

In recent years, considerable controversy has involved electrode materials. In corrosive soils, ordinary steel ground rods may be quickly destroyed. Stainless steel, once thought to be immune from corrosion, has sometimes proven to be otherwise.

Ground path resistance is tested by applying a measured voltage between the electrode and a separate probe in the soil some distance away. Current flowing between those two points causes a voltage drop along the soil path, which is measured between that probe and a second one. Resistance is then calculated by Ohm's Law. In evaluating soils themselves as suitable electrode locations, soil resistivity is measured in a similar way using a four-point Kelvin bridge connection.

A self-contained clamp-on tester is useful for measuring resistance of a single electrode path when the system contains a number of other low-resistance grounding electrodes as well. It injects an alternating current into the electrode under test, through transformer action, and automatically calculates the impedance of the electrode circuit from the voltage and current involved.

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