Question How touch voltages are to be computed from earth potentials?
Answer Touch voltages are encountered by a person whose feet are touching the earth at some point and whose hands are touching an aboveground metallic structure. Generally, only buried grounding conductors are modeled by the software in a grounding study, so earth potentials are computed and available, but the potentials of aboveground metallic structures are unknown. Most of the time, it is fair to assume that the potential of an aboveground metallic structure located anywhere in the grounding system under study will be at a potential which is similar to that of the nearest buried grounding conductor, since this conductor usually provides the most convenient grounding point. In such cases, the default touch voltage options proposed by the graphics software are the most appropriate.
On the other hand, there are situations in which an aboveground metallic structure may not be grounded to the nearest metallic structure and this, in an area where the grounding grid potential is varying rapidly from one point to another: this may be the case close to the fault location, particularly for a large grounding grid in low resistivity soil, for structures such as aboveground piping, ground conductors of power tools, etc. In such cases, you may wish to extend the conductor GPR search radius, to account for exposed conductors that may be grounded further away. Note that only MALZ and HIFREQ include voltage drop effects in grounding grids: MALT does, however, reflect GPR differences between different grounding electrodes modeled.
In order to compute the touch voltage at each observation point, the graphics software subtracts the computed earth potential from a reference GPR value. The reference GPR value used can be a single user-specified value applicable to all observation points or can be determined by the program on an individual basis for each point, based on the location of the observation point with respect to buried conductors that have been modeled, and based on the selection criteria specified by the user. This location-specific option is required, as indicated above, because the GPR of conductor segments may vary significantly from one point to another of a conductor network, particularly at high excitation frequencies or for large network dimensions (this is true of MALZ and HIFREQ). The presence of multiple electrodes with different energizations (including return electrodes and non-energized buried structures) will also result in the presence of conductors exhibiting different GPR values (this is true of MALT, MALZ and HIFREQ).
Note that for observation points at which no conductor segment is found within the user-specified radius (Average Spherical, Closest Spherical, Worst Spherical options), two outcomes are possible: either the touch voltage at the observation point is set to 0 (if the Reach Touch Voltages option has been selected from the Determine dropdown box) or the GPR of the nearest conductor segment outside the user-specified radius is used (if the Touch Voltages option has been selected from the Determine dropdown box).