IEEE 81-1983 pdf free.IEEE Guide for Measuring Earth Resistivity, Ground Impedance, andEarth Surface Potentials of a GroundSystem.
IEEE 81 the purpose of this guide to describe and discuss the present state of the technique of measuring ground resistanceand impedance, earth resistivity, potential gradients from currents in the earth, and the prediction of the magnitudes ofground resistance and potential gradients from scale model tests. Factors influencing the choice of instruments and thetechniques for various types of measurements are covered. These include the purpose of the measurement, theaccuracy required, the type of instruments available,possible sources of error, and the nature of the ground orgrounding system under test.
lt should be strongly impressed on all test personnel that a lethal potential can exist between the station ground and aremote ground if a power-system fault involving the station ground occurs while ground tests are being made.
Since one of the objectives of tests on a station-ground system is to establish the location of remote earth for bothcurrent and potential electrodes, the leads to these electrodes must be treated as though a possible potential could existbetween test leads and any point on the station ground grid. Some idea of the magnitude of this possible potential maybe gained from the consideration that even in the larger stations the ground grid shall have an impedance in the orderof 0.05 Q to 0.52.Assuming for this example that the ground-fault current through the grid is in the order of 20 kA thepotential to remote earth(ground potential rise) will be in the order of 1.0kV to 10 kV.For higher ground impedanceor greater fault currents, the rise of station-ground voltage may exceed 10 kV.The preceding discussion points to the necessity of caution when handling the test leads, and under no circumstancesshould the two hands or other parts of the body be allowed to complete the circuit between points of possible high-potential difference. It is true that the chances are remote that a station-ground fault will occur while test leads arebeing handled, but this possibility should not be discounted and therefore the use of insulating shoes, gloves, blankets,and other protection devices are recommended whenever measurements are carried out at an energized power station.
In all cases, safety procedures and practices adopted by the particular organization involved shall be followed.
These grounds fall in a special category because of the extremely high short-duration lightning currents carried bysurge-arrester grounds. These currents may be in excess of 50 000 A for surge currents,with a possibility of fault-system currents in the case of a defective surge arrester. An isolated surge arrester ground should never bedisconnected to be measured, since the base of the arrester can be elevated to the line potential.A surge-arrester groundcan be tested as long as precautions axe taken to minimize arrester discharge.
Another precaution concerns possible high-potential gradients around the current electrode. If current is passed into aremotely located electrode, as in the fall-of-potential method, it is worthwhile to ensure against a curious person beingallowed near the current electrode while tests are in progress. Similarly, in rural areas grazing animals should not beallowed near the test current electrode.IEEE 81 pdf download.