Thank you for your question regarding the required resistance-to-ground (earth to mass resistance) of earth matt’s (ground grids) installed at substations. It is our pleasure to help.
The short answer to your question is that there are no requirements for a specific resistance-to-ground for substation grids (matt’s), as the purpose of these grids is for Human Safety by reducing hazardous Step & Touch Voltages.
The resistance-to-ground of a grounding system is primarily a function of the resistivity of the soil/earth the grounding system is placed in, and the relative size of the grounding system itself. In most cases, we as engineers have no control over the available size of the substation, and we certainly have no control of the soil/earth conditions at the site. As such, we really can’t adjust the resistance-to-ground without installing deep ground wells. See our blog: http://www.esgroundingsolutions.com/blog/ground-potential-rise/how-can-i-reduce-the-grid-ground-potential-rise-without-increasing-the-physical-area-of-my-site
That said, for substation ground grids, the actual goal of the grid is the reduction of hazardous Step & Touch Voltages. This critical human safety issue has to do with the reduction of the Difference in Potential during a Ground potential Rise (GPR) event and is called the Ground Potential Difference of GPD. For your substation, the key issues would be the size of the wires used for the grid, the depth at which they are buried, the spacing of the grid, and any insulative layers that may or may not be placed over the grid such as crushed rock. The actual resistance-to-ground of the grid is NOT relevant to human safety, which is the ONLY concern one should have when dealing with these matt’s. Please refer to Title 29 of the Code of Federal Regulations Part 1910.269, this code (29 CFR 1010.269) makes it Federal Law that Human Safety be your primary concern , and specifically requires that Step & Touch Voltages be reduced below fibrillation current thresholds. Appendix C lists this even more clearly: http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=9871&p_table=STANDARDS
The most commonly used voluntary standard designed to meet the requirements under 29 CFR 1910.269 is IEEE Std.-80. Here is some information regarding the calculations found in this standard: http://www.esgroundingsolutions.com/ground-potential-rise/how-do-you-calculate-ground-electrodes-in-an-earth-pit-what-is-the-recommended-weight-calculation-for-step-50kg-or-70kg
Occasionally, we have seen an arbitrary specification listed in a blue-print requesting a specific resistance-to-ground for a grounding system; and of course with firm specifications detailing exactly how to install the ground grid. With no means of making adjustments to the ground grid design to lower the resistance, the contractor installing the system is stuck between a rock and a hard place. How could you possibly be expected to meet a specific earth resistance when you have no control over the design or the local soil conditions? When we see these arbitrary specifications it is generally due to a lack of understanding by the A&E firm as to how electrical grounding actually works. Grounding Engineers can predetermine the resistance-to-ground of a grid by using actual measured soil resistivity data taken at the site, and using computer models to calculate the resistance or impedance in advance. Sometimes these predicted resistances are listed on blue-prints and are misunderstood to be specifications. We would be happy to look at any specifications you may be needing to meet and perhaps assist you further.
In conclusion, modifications to the ground grid at any substation is a serious event and new engineering studies must be made to insure compliance with 29 CFR 1910.269 and IEEE std.-80. The resistance-to-ground of the earth matt (ground grid) is only one factor of many in determining human safety thresholds; as such there is no required resistance-to-ground for ground grids at substations. We are of course more than happy to assist you with any Human Safety studies you may need for your site to comply with 29 CFR 1910.269 requirements. But if not us, please get someone with the proper computer modeling software and experience to deal with these life-threatening issues.
We hope you find this information useful. Should you have any further questions or concerns, please do not hesitate to call us and we will be glad to discuss your issues with you free of charge.
The Engineering Team at E&S Grounding Solutions
Photo Credit: E&S Grounding Solutions