IEEE 81-2012 pdf download

IEEE 81-2012 pdf download.IEEE Guide for Measuring Earth Resistivity, Ground Impedance,and Earth Surface Potentials of a Grounding System.
b) Exposed test leads and electrodes are isolated from workers and the general public prior to applying test voltages. Exposed test leads and electrodes are also isolated from workers and the public prior to connecting the leads to a station ground grid or other grounding systems that might be exposed to system ground fault currents.
c) A signal is applied for short test periods, and all test leads are promptly removed after the test is completed.
d) If remote current and potential probes are not within sight of test personnel or if the test leads are located in an area accessible to the public, then these points are under continuous observation using a spotter in radio contact with the test equipment operator as long as the test signal is applied or remote potentials are over 50 V. One or more test leads that are electrically connected to a ground grid can cause a transfer of potential under fault conditions that would far exceed 50 V, and a spotter would be necessary as long as the test leads are connected to the ground grid.
e) If the ungrounded ends of test leads parallel an energized line for several hundred feet, then a hazardous voltage can be induced into the test leads if large currents are flowing in the energized line. This issue can sometimes be mitigated by the physical orientation of test leads, grounding, or both.
5.2 Surge arrester ground continuity tests
Surge arrester ground continuity tests require special care for the following reasons:
a) The base of the surge arrester can approach line potential if the ground lead is disconnected while the primary terminal remains energized. Never disconnect the ground of a surge arrester for any reason while the primary terminal remains connected to an energized line.
b) Extremely high, short-duration lightning or switching currents can be discharged into the ground and can exceed 50 000 A.
c) A system fault can occur if a surge arrester fails during testing.
Surge-arrester ground leads can be tested as long as sufficient safety precautions are implemented.
5.3 Neutral and shield wire ground tests
Disconnecting neutral and shield wires can generate hazardous voltages. This hazard can occur whether the line is energized or not, due to current flow through the interconnected shield wires, or mutual coupling with other energized circuits. Appropriate work procedures can mitigate these hazards.
5.4 Equipment neutral ground test
High voltages can occur and possibly harm personnel and/or damage equipment, if neutrals are disconnected from energized equipment.
6. General considerations on the problems related to measurement
6.1 Complexities
The measurements of soil resistivities, ground impedances, and potential gradients of the earth introduce a number of complexities not encountered in other resistance, impedance, and potential measurements. In some situations, it might be necessary to perform several measurements to plot trends and analyze the situation. Stray currents and other factors can interfere with the measurements.
With development and industrial growth adjacent to power substations, choosing a suitable pattern or location for test probes to make a resistance test is becoming increasingly difficult. The connection of overhead ground wires, buried water pipes, cable sheaths, adjacent railroad tracks, conveyor systems, and so on, can all have an effect on the electrical circuit being tested and can introduce significant errors. It should also be noted that overhead ground wires might he insulated either deliberately or by poor connections, and therefore, low-voltage tests might give answers different from actual fault tests.
To improve the accuracy of the measurement for comparison with calculated values, ground impedance measurements can be performed prior to interconnection of external shield wires, metallic pipes, and other external interferences.
The impedance of a ground grid will usually decrease slightly as the earth settles to a uniform compactness a year or more after installation.
6.2 Test electrodes
Several ground-impedance measurement methods described in Clause 7 through Clause 12 require the use of current and voltage test electrodes.
The impedance of the test electrodes can have a significant effect on the accuracy of impedance measurements. If a ground test is performed using the two-point test method, the measurement error can be minimized if the impedance of the test electrode is negligible with respect to the ground being tested. Conversely, for the three-point test method, the measurement error can be minimized if the impedance of the test electrode is similar in magnitude to the impedance of the ground under test.