During a major ground fault in a substation, the earth itself transforms from a safe haven into a map of varying potentials: a touch on a structure, or a long stride in the wrong direction, could place a person's body between two points with a lethal potential difference. These are touch voltage and step voltage.
How Does the Danger Arise?
When a large fault current flows to ground through the earthing network, the potential of the network and the surrounding soil rises relative to remote earth, and this rise is distributed in a gradient around the dissipation points — areas near the electrode are at a higher potential than areas farther away. From this gradient, two hazards arise:
Touch Voltage
- The potential difference between an equipment body touched by a person (hand) and the position of their feet on the ground.
- It is most dangerous when earth resistance is high: the body of the faulty equipment rises in potential while the ground under the feet remains lower, so current flows through the body from the hand to the feet — through the heart.
Step Voltage
- The potential difference between a person's two feet spaced apart on ground with a potential gradient during a fault.
- The wider the stride near the current dissipation point, the greater the difference — which is why the recommendation in hazard areas is: short, shuffling steps, or jumping with feet together during evacuation.
How Does a Substation Protect Against Them?
| Method | Its Role |
|---|---|
| Mesh/grid earthing network beneath the substation | Equalizes potential across the site, reducing differences between points |
| Lowering earth resistance | Reduces the overall potential rise during a fault — improvement methods |
| Surface gravel layer | Raises the contact resistance between foot and ground, reducing current through the body |
| Bonding all structures to the grid | Makes whatever the hand touches and whatever the foot stands on roughly the same potential |
| Fast protection tripping | Shortens exposure time — the decisive factor in survival |
Sample answer: Touch voltage is the potential difference between an equipment body touched by a person's hand and the position of their feet during a ground fault, while step voltage is the difference between a person's two spaced feet on ground with a potential gradient around a current dissipation point. Design reduces both through a mesh earthing grid that equalizes potential across the site, lowering earth resistance, bonding all structures to the grid, and a surface gravel layer that raises contact resistance — combined with fast-tripping protection that shortens exposure time.
Assuming that touching "earthed" objects is always safe during a fault. Earthing reduces the hazard but does not eliminate it — while dissipating a large fault current, the entire grid's potential rises, and safety lies in keeping distance and letting protection trip, not in excessive trust in the earthing system.
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