Q124 : Conductor Segmentation in MALT and MALZ

Question
How to segment conductors in MALT and MALZ.

How can earth potentials exceed the GPR of the grounding system?

This typically happens when inadequate conductor segmentation is specified in the conductor input. The MALT/MALZ software use numerical methods to compute conductor leakage currents and earth potentials, based on the fragmentation of a system of conductors into a large number of smaller segments, each injecting current uniformly along its entire length into the earth. When segments are sufficiently small, a uniform current distribution in each segment successfully represents the current distribution in the entire conductor by a staircase function. When segments are too long, the uniform current distribution along the segment may represent a leakage current density that is too high in some locations and too low in others. For example, a single bare wire buried horizontally in the earth will have a large leakage current density at its ends, where exposure to "open" soil without interference from other sources of current is a maximum; at the center of the wire, the leakage current density will be lowest. If you run MALT/MALZ for a single long horizontal conductor, first with no segmentation, then with, say, 20 segments specified for Nseg (in the conductor coordinates table, next to the conductor radius), then check the output for the current density, you will see what I mean. In a more complex system, with intersecting conductors, the leakage current in reality decreases significantly at conductor crossings, to account for the fact that the two intersecting conductors impede the expulsion of current from one another. If the conductors have not been segmented enough, then MALT/MALZ will compute excessive current densities at the intersection locations, since it has not been given the freedom to compute small densities near the intersections and larger densities elsewhere. As a result, excessive earth potentials will be computed at these locations. The error will be most apparent very close to the intersections; further away, there may be no error, since the average current density computed along each segment may be correct. To ensure accuracy, proceed as follows:

1. As a rule of thumb, ensure each long or average-sized conductor in your system has been subdivided into 5 to 20 segments: either via MALT/MALZ's default automatic node subdivision process, which cuts conductors where they contact one another, or via the Nseg specification in the conductor coordinates table, or via specification
of the total number of conductors desired in the grounding system (i.e., Main electrode, Return electrode, or Buried Structures).

2. After MALT/MALZ has run, check how many segments were created by MALT/MALZ. Next re-run MALT/MALZ, requesting twice as many (or more) segments in each of your electrodes (i.e., Main, Return, Buried Structures). Compare your key results from the two runs: computed ground resistance, unsafe touch voltages near grid conductors, maximum step voltages. If they are close, then your segmentation is adequate for your application. Note that the more accuracy you require at very short distances from conductor nodes, the more segments you will need.

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