Guide for Measurement of Impedance and Safety Characteristics of Large, Extended or Interconnected Grounding Systems
|Publication Date:||1 January 1991|
(This foreword is not a part of IEEE Std 81.2-1991, IEEE Guide for Measurement of Impedance and Safety Characteristics of Large, Extended or Interconnected Grounding Systems.)
During the late 1970's, in an effort to increase its usefulness, this guide was divided into two parts. The first part is entitled IEEE Std 81-1983, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System. It covers the majority of field measurements that do not require special high-precision equipment and measuring, and that do not encounter unusual difficulties such as may be found with extensive grounding systems, abnormally high stray ac or dc currents, etc. IEEE Std 81 (Part I) has been extensively revised and updated. Part I was approved in 1983 and reaffirmed in 1991. This part of the guide (Part II) is entitled IEEE Std 81.2- 1991, IEEE Guide for Measurement of Impedance and Safety Characteristics of Large, Extended or Interconnected Grounding Systems. This new part covers measurement of very low values of ground impedance (less than 1 Ω ). The extensive use of specialized instrumentation, measuring techniques, and safety aspects are incorporated.
This guide was prepared by the Measurement of Impedance and Safety Characteristics of Large, Extended or Interconnected Grounding Systems Working Group of the RLC Measurements Subcommittee of the Power Systems Instrumentation and Measurements Committee of the IEEE Power Engineering Society.
Test methods and instrumentation techniques used to measure the ac characteristics of large grounding systems include the following topics:
1) Measurement safety
2) Earth-return mutual errors
3) Low-current measurements
4) Power-system staged faults
5) Communication and control cable transfer impedance
6) Current distribution (current splits) in the grounding system
7) Step, touch, mesh, and profile measurements
8) The foot-equivalent electrode earth resistance
9) Instrumentation characteristics and limitations
Grounding electrodes consisting of a single ground rod, arrays of ground rods, tower footings, and many grids (if no external grounding is connected) can be measured, interference voltages permitting, with methods outlined in IEEE Std 81-1983  1 . Even if a large grid has an impedance phase angle of 18 ° the resistance component will be only 5% lower than its impedance. However, for large grounding grids in low-resistive earth (<75 Ω -m) and for grounding systems that have numerous extended grounding conductors, the impedance could be significantly greater than the resistive component measured with the conventional test sets of IEEE Std 81-1983 .
Measurement of low-impedance grounding-system characteristics, with injection currents between 0.1-100 A, using techniques described in Section 8. is generally preferred. Or, grounding-system parameters may be measured or verified under power-system conditions with the staged-fault methods of Section 9. Due to the diversity in station grounding configurations and the variety of possible test connections, this guide cannot narrowly define the test method for each application. The user must take responsibility for the adequacy of the method selected (see Section 13. for a review of instrumentation components).