Er. Binod Ghimire
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Kathmandu ; In its weekly educational column 'Urja Gyan,' Urja Khabar has published a technical and educational article titled 'Step Potential, Touch Potential, and Reduction of Ground Resistance' authored by Engineer Binod Ghimire, Lecturer at Manmohan Technical University and Purbanchal University's Department of Electrical Engineering.
Step potential and touch potential are significant safety hazards in electrical systems that take place under line-to-ground faults. To counter these dangers, the reduction of ground resistance becomes vital. Such dangers are most observable during monsoon seasons, when wet conditions offer favorable routes for the path of current through the earth. In Nepal, such incidents mark the rising need for awareness among the public and technical safety attributes.
What is Step Potential?
Step potential is the dangerous voltage that occurs across two feet of a person standing on the ground near a fault point. When a fault current travels to the ground, there occurs a voltage gradient around the grounding site. When the feet of a person touch two places with different voltages, there is current flow through the body, which could lead to ventricular fibrillation or even death.
Suppose a high-voltage tower has a fault and permits 5,000 amperes of current to the ground. With the soil resistivity being 100 ohms-meters and ground resistance equal to 10 ohms, the Ground Potential Rise (GPR) is:
GPR = Current × Resistance = 5,000 × 10 = 50,000 volts
If a person's feet are 1 meter apart, and the voltage gradient is 1,000 volts/meter, the step potential will be 1,000 volts, which will cause heart fibrillation and even death.
What is Touch Potential?
Touch potential is when a person touches an energized metal object (e.g., transformer or tower leg) with one hand while on the ground at the same time. The current travels from hand to foot through the body, causing severe damage or death.
During safety evaluation, a 1-meter reach for touch potential and a 2-meter span if two objects are touched simultaneously at the same time.
Example:
If a tower has an impulse voltage reading of 50,000 volts, and someone touches the tower 1 meter away from it, the touch potential can be almost 50,000 volts. If the voltage difference between the tower and ground point is 40,000 volts, that is the same voltage which will pass through the body – extremely hazardous.
Soil Resistivity and Fault Duration
Soil resistivity will greatly affect step and touch potentials. Dry soil (such as 1,000 ohm-meters) with high resistivity would result in elevated step voltages. A profiled soil with high-resistivity top and low-resistivity bottom (such as 100 ohm-meters) will charge step voltage near the grounding electrode.
Example:
Soil resistivity: 1,000 ohm-m (top), 100 ohm-m (bottom)
Fault current: 5,000 A
Step voltage at 1m distance: ~1,500 V (near electrode)
Fault clearing time is also critical. When the fault is 0.5 seconds, the hazard for heart fibrillation increases. Automatic reclosers, which are widely used by utilities to momentarily reopen power after a fault, in the process unwittingly increase the hazard for step potential exposure.
Mitigating Step and Touch Potentials
Professional grounding electrical consultant recommends countermeasures if high-risk zones are identified by Rise of Earth Potential (ROEP) studies. Countermeasures are intended to use grounding and bonding techniques to equate potential at ground surfaces. It is always challenging to implement solutions that are economical and efficient and keep surface voltage below human safe levels, especially for the heart.
Key Mitigation Measures:
1. Ground Resistance Reduction
Ground resistance reduction immediately follows the reduction of GPR and consequently step/touch potential.
Example:
Original resistance: 10 ohms, Fault current: 5,000 A → GPR = 50,000 V
Reduced resistance: 5 ohms, Fault current: 7,000 A → GPR = 35,000 V
Recommended Solutions:
Use deep ground rods or ground grids (e.g., 10 meters deep)
Use low-resistivity materials (e.g., bentonite, resistivity ~2.5 ohm-m)
Perform periodic ground system testing
2. Installation of Proper Ground Conductor
Ground conductors shall encircle metal structures with 2/0 AWG copper wire, 3 feet depth and 18 inches below grade.
Example:
Conductor at or near surface (6 inches): Touch potential ~1,000 V, Step potential ~1,500 V
Conductor 3 feet deep: Touch potential ~500 V, Step potential ~800 V
Adjust burial depth based on soil resistivity.
3. Surface Resistive Layers
Crushed rock: ~3,000 ohm-m resistivity, attenuates current flow by 50%
Asphalt: ~10,000 ohm-m resistivity, overcomes vegetation hazards
Electrical hazard shoes: ~1 MΩ resistance dry, ~100 Ω wet
Heightened Risk During Monsoon
Wet ground during monsoon season decreases ground resistance considerably and facilitates current to move more easily. This increases the extent and impact of step and touch potentials.
This is especially dangerous in rural areas with ungrounded poles, exposed transformers, and bare wires. People may unknowingly lean against metal objects or stand without footwear, facing electrocution.
Safety Recommendations:
