IEEE 81:2012 pdf free download – IEEE Guide for Measuring EarthResistivity,Ground Impedance,and Earth Surface Potentials of aGrounding System
Often,the most effective way of increasing the test current is to decrease the current electroderesistance. The resistance of the test electrode can be reduced by driving the rod deeper into the soil,pouring water around the rod, or driving additional rods and interconnecting them in parallel. Theaddition of salt to the water poured around the test electrodes is of very little value; the moisture is themain requirement. If a current electrode is made up of parallel ground rods, then adequate spacing (notcloser than the below-grade length of the current electrode) between the ground rods will minimize theimpact of mutual resistances.
The maximum resistance values of the current and potential electrodes will depend on the type ofinstrument used.
lf stray currents are present when direct current (dc) tests are being performed, the interference can bemitigated by setting the test current to a value significantly above the stray dc earth currents.When testswith ac or periodically reversed dc signals are being performed, the interference can be mitigated bysetting the frequency of the test signal to a frequency not present in the stray currents or use a randomnoise signal.
A potential probe with high resistance to remote earth can also influence the voltage measured by theinstrument. if this resistance is of the same order of magnitude as the input impedance of the instrument,then the actual voltage will be divided between the measured voltage and the voltage across theresistance of the potential electrode. In cases with low soil resistivities,the error caused by thisinfluence is insignificant, but in the case of sandy or rocky soil environment, the user might considerreducing the resistance of the electrode to reduce this error. Otherwise, if this influence prevails, thenthe indicated resistance will be less than the actual resistance.
6.3 Stray direct currents
Conduction of electricity in the soil is electrolytic, and direct current results in chemical reactions andpotential differences. Direct potentials are produced between various types of soil and between soil andmetal by galvanic action. Cathodic protection systems of pipelines，dc railroad tracks，and dctransmission lines are some of the major sources of dc currents in the soil. Galvanic potentials,polarization，and stray direct currents can seriously interfere with direct-current measurements.Therefore, periodically reversed direct current or sometimes a regularly pulsed current is used in makingmeasurements. However,when using periodically reversed direct current for resistance measurements,the resulting values will be fairly close, but they might not be accurate for alternating-currentapplications. Solar-induced currents,which can be quasi-dc, might influence test results.
6.4 Stray alternating currents
Stray alternating currents in the earth,in the grounding system under test, and in the test clectrodespresent an additional complication. The effects of stray alternating current can be mitigated in groundresistance measurements by utilizing a frequency that is not present in the stray current or by using arandom noisc signal. Most measuring devices use frequencies within a range of 50 Hz to 3400 Hz.Theuse of filters or narrow band measuring instruments, or both, is often required to overcome the effects ofstray alternating currents.