IEEE 81-2012 pdf free download – IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System.
4. Test objectives
4.1 Earth resistivity measurements
Earth resistivity measurements are used to perform the following:
a) Estimate the ground impedance of a grounding system
b) Estimate potential gradients including step and touch voltages
c) Compute the inductive coupling between neighboring power and communication circuits
d) Design cathodic protection systems
e) Design alternating current (ac) mitigation for coupling between transmission lines and
pipelines
f) Conduct geological surveys
4.2 Impedance and potential gradient measurements
Measurement of ground resistance or impedance and potential gradients on the surface of the earth due
to ground currents are used to perform the following:
a) Verify the adequacy of a new grounding system
b) Detect changes in an existing grounding system
c) Identify hazardous step and touch voltages
d) Determine GPR to design protection for power and communication circuits
5. Safety precautions while making ground tests
5.1 Ground electrode tests
A lethal voltage can exist between the ground electrode under test and a remote ground during routine conditions or if a power-system fault involving the station ground occurs while ground tests are being conducted. The ground potential rise can be in the order of several thousand volts. Step and touch voltages around the ground electrode under test, test equipment, and remote grounds can also be lethal.
A test plan is typically developed and reviewed with applicable test personnel. Appropriate safety rules need to be followed.
Since the current and potential electrodes are located at points that represent remote earth, the leads to these electrodes are treated as though a possible voltage could exist between the test leads and any point on the station ground grid. The main area of concern involves system faults or lightning strikes, which can cause voltages as high as several thousand volts to occur between station ground and remote points.
It is also important to understand that the test signal injected into a remote current electrode can also result in significant touch voltages. The following precautions can reduce these hazards although other  precautions might also be necessary:
a) Hands or other parts of the body are not allowed to complete the circuit between points of possible high-potential difference. Gloves and dielectrically rated footwear can reduce the hazards associated with handling test leads that extend outside the station ground grid.
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.IEEE 81 pdf pdf download.