Q224 : Distribution of Observation Points for the Computation of Touch-Voltages.

Question
Where should I specify the observation points in order to get accurate values for the touch voltages over a large grid?

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Answer
By definition, the touch voltage is the difference between the earth potential at a given point and the potential of a nearby conductor connected to the grounding grid. Now, the earth potential will usually reach its maximum just above a buried conductor and can decrease quite fast (depending on the type of soil) as you move away from it. This is what explains why the touch voltage gets to be very large for points far away from any buried conductor.

Typically, the region of concern for touch voltages extends about 1 meter (3 feet) away from an energized, buried conductor. Moreover, the program can reliably interpolate the results to a distance of 1 m of a conductor only if the actual computation point is not much farther than 1 m, since the earth potential is not a "smooth function" ( i.e. it is full of spikes) and the interpolated results would be extremely sensitive to the location of the computation points.

The best solution is therefore to specify computation points in the MALZ input so that:
- they cover the region of interest
- they are no more than 1 m. apart in both the X and Y directions

When the grid is large, it can be difficult to cover the entire grid with observation points at that density. You could proceed by:
- specifying a few observation surfaces in the corners of the grid and other "hot spots"
- specifying individual computation profiles running parallel to the grid conductors and about 1 m away from them (on both sides of the conductors)

These last two options are more time consuming from the point of view of data entry, but they can help reduce the computation time tremendously. Note that the "Generate Profiles" feature of SESCAD (available from the Tools | Generate Profiles menu item) can help specify such irregular profile structures. Consult SESCAD's on-line help for details.

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