Section 12/Grounding Grid Design Procedure
IEEE 80 Section 12 outlines the step-by-step procedure for designing a substation grounding grid including soil resistivity and conductor sizing.
The grounding grid design procedure includes: determining the maximum grid current, measuring soil resistivity, selecting the grid conductor material and size, designing the grid layout and depth, calculating the grid resistance, computing the ground potential rise (GPR), and verifying that step and touch potentials are within safe limits. The grid conductor must be sized to carry the maximum fault current for the duration of the fault without fusing. Ground rods supplement the grid to reduce resistance in high-resistivity soils.
Why this section exists
Substations handle fault currents that can reach tens of thousands of amperes. When a fault occurs, the grounding grid must safely dissipate this current into the earth without creating dangerous voltage gradients on the surface. A person standing near the substation during a fault can be electrocuted by step potential (voltage between feet) or touch potential (voltage between hands and feet when touching grounded equipment). The design procedure ensures these potentials remain within survivable limits.
What plan reviewers look for
Plan reviewers check the grounding study for soil resistivity data, grid conductor sizing calculations, GPR computation, and step and touch potential analysis. They verify the grid layout on the site plan matches the study assumptions. They check that the conductor cross- section is adequate for the fault current magnitude and clearing time.
Common violations
Related IEEE 80 requirements
Section 14 covers the step and touch potential criteria. Section 11 covers soil resistivity measurement methods. NEC Section 250.50 covers grounding electrode systems. NESC covers grounding requirements for utility installations.