A fall of potential test--also called an earth resistance or three-point test--determines the capacity of an earth ground system to dissipate electrical energy. While all (safe) electrical circuits feature an earth ground system to discharge voltage spikes, proper selection of grounding location is of particular importance on oil production sites, where electrical arcs can be catastrophic.
Earth ground systems are able to discharge enormous voltage because the number of paths available for current grows rapidly with distance from the electrode. This can be practically illustrated with tree roots: in rocky soil, a poor conductor, they develop singular, long-running paths. Earth resistance grows with the distance that fault currents, such as lightning strikes, must travel within soil to be effectively dissipated.
Earth resistance is measured by establishing a circuit within the soil and introducing a predetermined voltage. The circuit consists of a multimeter with three leads along with three probes--the electrode under test, a potential probe, and a current probe--arranged inline:
|==MULTIMETER==|
_________/ | \___________
/ | \
/ | \
ELECTRODE POTENTIAL CURRENT
UNDER TEST || ||
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GROUND==============================================================
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Once attached to all three ground probes, the multimeter passes a known current between the current probe and test electrode. The fall in voltage potential, or resistance, is measured from the potential probe. Distance between probes is an important factor, as each will exert an electrical sphere of influence that may impair accurate reading. It's sensible to locate the two outside probes as far apart as possible: this allows for multiple measurements, which are usually graphed, and accounts for the infinite variability of soil conductivity. The most stringent readings are accomplished using a fourth lead from the multimeter to the test electrode, which eliminates the multimeter's inherent resistance from the reading.
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