Application Note Earth resistance measurement and the 62% rule Everybody, who is familiar with the earth resistance measurements knows the 62 % rule: The rule (recommendation) is defined in IEEE 81-2012 in Annex C. It is based on simplified model of rods and some additional preconditions: - Current trough both rods (E and H) is the same: IE = IH - Uniform soil - Hemispherical electrodes If the distance between E and H rode is big enough, then the rods can be treated as hemispherical electrodes. If the soil can be treated as uniform soil then the following equations are valid: The distance between the E and S is calculated out of the following equation (C.9 in IEEE 81): 1 1 (D-x) D When performing the earth resistance measurement with the Fall of Potential (FoP) method you have to put the measurement S probe on a distance of 61,8% of the distance between the centre E of the tested earthing system and the auxiliary test probe H. The positive root of equation is the location of the S probe x0 = dE_S=0.618 D The S probe must be located on the straight line between E and H probe.

During the measurement both probes E and H conduct a current therefore the potential around the electrodes raises (GPR – ground potential raise). The graphical presentation of this potential raise around the probe has a form of a funnel (Figure 2). is influenced by the GPR of the other probe. When performing the measurements of the potential raise around the probe only the summary of both GRP can be measured. Therefore simple measurement of the voltage between E probe and zero ground results in an error and one of the possible solution is to use the 62% method. Within this method and the above...

Therefore the formula Eq.1 must be corrected with the coefficient ki: ki = This results in shorter distances between E and S probe: Figure 4: 4 flex clamp method Some typical values of ki and corresponding distances dE_S between E and S are listed in the table below: ki METREL D.D. Measuring and Regulation Equipment Manufacturer Ljubljanska 77, SI-1354 Horjul, Slovenia T +386 (0)1 75 58 200, F +386 (0)1 75 49 226 [email protected], www.metrel.si 46% 62% For example, if ki = 0,4 then the S probe must be positioned on 46% of the distance between E and H probe. The funnel around the auxiliary H probe...