Soil Resistivity Measurement

[timm33333]

I have a question regarding the soil resistivity measurement. Soil resistivity for the ground grid system of high voltage (HV) substations is measured as per IEEE-81. How about transmission lines, is it required that the soil resistivity be measured at the base of each tower of high voltage transmission line? Thanks.

 

[9wearSuave]

I do not believe it is required. When calculating step and touch potential formulas at the substation, sometimes a split factor estimation is used for 4 wire distribution lines or transmission lines with OPGW's, which is where pole grounding resistance would come into play. But these are just conservative estimations mostly. I usually just look at what is the type of soil along the terrain that produces the worst resistivity values and just use the value for that from a table. If someone is looking for a more accurate idea of how the current splits for certain faults, the proper way to quantify it would be to actually measure it using clamp on CT's.

 

[jghrist]

If you are designing the transmission line, it would be helpful to measure soil resistivity at a few points along the line route. The readings would be used to design structure grounding to meet the requirements for lightning protection. You don't need a measurement at each pole, but enough to get a good average along the line.

As 9wearSuave said, if you are designing the substation grounding, what is needed is structure ground resistance to determine fault current return splits.