Thank you for your questions regarding IEEE 80. It is our pleasure to help.
In regards to your first question, how to calculate ground electrodes in an earth pit, the best way to do this is to use a computer program. We use and recommend the CDEGS program, the same software that was used to generate the graphs and charts in IEEE 80 standard. However, if you are trying to do the calculations by hand, then you are in for a much bigger task. In either case, you would first need to gather soil resistivity data your site. The Wenner 4-pin Method is the preferred way to gather soil resistivity data. The data gathered will of course need to be processed in order to determine an accurate soil model. The IEEE 80 standard is primarily based on uniform soil as it is far easier to do hand-calculations. However the standard highly recommends that two-layer soil models be used. We often use 3 and/or 4 layer models, particularly when frost-line considerations could impact the soil during winter months.
Once you have a good soil model, you can start making electrode and grid resistance calculations. IEEE 80 Section 14 has a number of equitation’s that will relate to your task. Keep in mind that the formulas that are listed are primarily designed for equal length electrodes. If you are mixing a deep ground well with typical ground rods and ground grids, than you will need to calculate the deep well electrode separately and then apply it back into the final equation. This is easier said than done, as hand computations will undoubtedly be complex and tedious. IEEE 80 Annex B & C will provide you with some guidelines on how to calculate the current splits. That said, we highly recommend using a computer program.
In regards to your second question about the 70Kg weight when doing Step & Touch Voltage Hazard calculations, we actually use the 50Kg value. IEEE 80 Section 8.3 gives formula’s for both 70Kg and 50Kg. The 50Kg standard is far stricter and provides better protection, particularly for the public which may be touching the outside fence or walking near your substation. IEEE Figure 15 graphically shows the difference between the two body weight standards, which can translate into an acceptable clearing time difference for the 50Kg standard (0.8s) that is nearly half the time that is acceptable for 70Kg (1.46s). It should be noted that the IEEE standard gives you the option as to which weight standard you should use. Please check with your local authorities, as some countries have different mandates.
We hope you find this information useful. You have a very complex question that we could never fully do justice in a simple email response. Please feel free to call us from 8am to 5pm California time to discuss this project.
The Engineering Department at E&S Grounding Solutions
Photo Credit: E&S Grounding Solutions