Grounding Grid Design: Step & Touch Potential Explained
Many engineers obsess over achieving a grounding resistance of less than 1 ohm. But in a fault scenario, low resistance is not what saves lives. We deep dive into Ground Potential Rise (GPR), specifically the lethal "Step" and "Touch" potentials that can electrocute a person walking near a substation, even if the grounding resistance is perfect.
Imagine a high-voltage fault occurs at a substation. A lightning arrestor fails, or a line drops onto the gantry structure. Thousands of amps of fault current instantly rush down the steel structure and into the earth grid.
The common engineering myth is: "If my earth grid resistance is below 1.0 Ohm, everything is safe."
This is dangerously incorrect. While a low resistance is good for the equipment, it tells you nothing about the safety of the human standing next to it. In fact, you can have a grid with 0.1 Ohm resistance that kills someone, and a grid with 50 Ohms resistance that is perfectly safe. The real killers are Step Potential and Touch Potential.
The Physics of GPR (Ground Potential Rise)
When current ($I_{fault}$) flows into the earth resistance ($R_{g}$), Ohm's Law takes over:
$V_{GPR} = I_{fault} \times R_{g}$
If you have a 10,000A fault and a 0.5 Ohm grid, the entire substation earth rises to 5,000 Volts relative to "remote earth" (zero volts far away). The dirt under your feet is now energized at 5,000V. This voltage creates a gradient (a funnel shape) radiating outwards from the substation.
What is Step Potential? (The Silent Killer)
Step potential is the voltage difference between a person's feet when walking. When fault current flows into the soil, the voltage is highest at the injection point and drops off as you move away.
If you are walking away from the substation during a fault, your left foot might be on ground at 4,000V, while your right foot (one meter away) is on ground at 3,500V.
The difference is 500 Volts. This voltage drives current up one leg, through your pelvis, and down the other leg. While this path misses the heart, significant current can cause muscle contractions that cause you to fall. Once you fall, your head and feet are now further apart, exposing your heart to a much higher voltage difference.
What is Touch Potential? (The Deadly Contact)
Touch potential is almost always more dangerous than step potential. This is the voltage difference between a metallic object you are touching (like a fence or panel) and the ground you are standing on.
During a fault, the metallic structure (bonded to the grid) rises instantly to the GPR voltage (e.g., 5,000V). However, the soil you are standing on—perhaps just 1 meter away from the grounding rod—might only be raised to 4,000V due to the resistance of the soil in between.
The difference is 1,000 Volts. This voltage drives current through your hand, down your arm, across your heart, and out through your feet. This is the "Touch Voltage," and because it crosses the heart, the tolerable limit for touch potential is much lower than for step potential.
Designing for Safety: The Grid Mesh
So, how do we fix this? We cannot easily change the fault current or the soil resistivity. The solution lies in the Grid Geometry.
By installing a mesh of copper conductors buried in the ground (typically spaced 3m to 10m apart), we force the ground potential to rise uniformly. It’s like standing on a metal plate: if the whole plate is energized to 5,000V, there is no voltage difference between your feet. You are safe (like a bird on a wire).
The Goal of IEEE 80 Design:
- Mesh Density: Add enough conductors so the voltage difference between any two points in the substation (Mesh Voltage) is lower than the tolerable Touch Voltage.
- Corner Rods: The corners of the grid leak the most current and have the highest gradients. Deep vertical rods are used here to push current deeper into the earth, smoothing out the surface voltage.
The Role of Surface Material (Gravel)
You may have noticed that substations are covered in a layer of crushed rock (blue metal/gravel). This is not just for drainage or weed control. It is a critical electrical safety component.
Gravel has a very high electrical resistance (3,000 Ω-m wet) compared to soil (100 Ω-m). By standing on a 150mm layer of gravel, you are adding a large series resistance between your feet and the earth. This resistance limits the current that can flow through your body, effectively increasing your "Tolerable Step and Touch Voltage" limits.
Without gravel, a grid might need twice as much copper to be safe.
Calculation Workflow (IEEE 80 Simplified)
- Soil Resistivity Test: Perform a Wenner 4-Point test to measure Ohm-meters ($\rho$) of the soil.
- Calculate Tolerable Limits: Based on body weight (50kg or 70kg), fault duration (e.g., 0.5s), and surface layer (gravel).
- Design Initial Grid: Lay out a rectangular mesh and ground rods.
- Calculate GPR: $I \times R_g$.
- Calculate Mesh & Step Voltages: Use the complex formulas in IEEE 80 to find the worst-case voltage difference inside the grid.
- Compare: Is Mesh Voltage < Tolerable Touch Voltage? If yes, you pass. If no, add more conductor mesh.
Conclusion: It's About the Gradient, Not the Resistance
Stop focusing solely on the "1 Ohm" target. A grid can fail safety checks at 0.5 Ohms if the conductors are spaced too far apart. Conversely, a small distribution pad-mount transformer might have 10 Ohms resistance but be perfectly safe because the GPR is low.
Grounding design is about equipotential bonding—ensuring that everything a person can touch and stand on rises in voltage together. Use the simulation tools to visualize the voltage funnel and design a grid that protects the operator, not just the transformer.
Verify Your Substation Safety
Don't rely on rules of thumb. Use our IEEE 80 compliant calculators to verify your design:
- Grounding System Design - Calculate Mesh & Step Voltages.
- Earthing Fault Simulator - visualize current flow.
- Earth Conductor Sizing - Size the riser cables correctly.