A colleague recently asked me a critical question: "Why do you design the earth grid to extend from fence to fence, even across areas where there is no equipment to earth?"
This highlights one of the most dangerous misconceptions in power engineering. Many feel that the earth grid is solely a "base" for connecting equipment to provide a low-resistance path for residual currents. While true, that is only half the story.
What is Step Voltage?
Step voltage is the potential difference that exists between a person's feet while walking during an earth fault. When a fault occurs, massive amounts of current return to their source. Because the earth’s soil is not homogenous (it has varying levels of resistivity), the current does not take a straight path.
Instead, it takes the path of least resistance. If you are walking in a switchyard during a fault, your two feet may be standing on two different points of the earth with different soil resistivity. This creates a potential difference.
The Human as a Conductor
When this potential difference occurs, the human body becomes the path of least resistance. Current can flow up one leg and across the body to the other leg. This is why we design a full mesh grid. The earth grid creates a low equipotential path across the entire substation footprint.
By ensuring the grid is fence-to-fence, we ensure that regardless of where a technician is standing or walking, the potential difference between their feet is minimized, allowing the fault current to return safely to its source through the copper grid rather than the person.
The Jeffen Reds Standard
At Jeffen Reds Ltd, we don't just follow "the rules"—we understand the physics behind them. Our earthing designs prioritize safety above all else, ensuring that every substation we commission is a safe environment for operators.