What is Step and Touch Potential And How Can You Reduce its Risk?

Awareness of step potential and touch potential risks, caused by an earth potential rise. Is vital for anyone working on high-voltage power transmission. Also, distribution systems above 1 kV.

Earth Potential Rise

Earth potential rise (EPR) is caused by an imbalance of electrical charges between the earth and an electrical system, it can occur at:

• electrical substations,
• power plants,
• or high-voltage transmission lines.

Therefore short-circuit current flows through the plant structure and equipment and into the earthing electrode. 

This imbalance can be caused by a variety of factors, including:

• High resistance in the earth-grounding system, which can be caused by poor soil conditions or corrosion in the grounding system
• High current flow through the system, which can occur during a fault or heavy load conditions
• A large number of electrical connections to the earth, such as in a densely populated area with a high concentration of electrical equipment
• A lack of proper bonding and grounding between different electrical systems, which can cause potential differences between them.

Since the Soil Resistivity is not zero. Therefore, any current injected into the earth at the earth electrode produces a Rise of Earth Potential. This EPR earth potential rise is concerning an infinitely distant reference point. The resulting EPR earth potential rise can cause hazardous voltage, many hundreds of metres away from the actual fault location. However, many factors determine the level of hazard. Including available fault current. Also, soil type, soil moisture, temperature, underlying rock layers. And the clearing time to interrupt the fault.

EPR can also be caused by natural phenomenon such as lightning strike and thunderstorm. These events can generate significant electric current and voltage that can cause damage to electrical equipment and can be a safety hazard to people.

Earth potential rise is a safety issue in the coordination of power and telecommunications services. So, an EPR event at a site. Such as an electrical distribution substation. May expose personnel, users or structures to hazardous voltages. These hazards are referred to as Step Potential and Touch Potential risks.

Step Potential Definition

Step Potential is the voltage between the feet of a person standing near an energised earthed object. And, it is equal to the difference in the voltage given by the voltage distribution curve. Between two points at different distances from the electrode. A person could be at risk of injury during a fault only by standing near the connected object.

The magnitude of the step potential depends on the current flowing through the ground and the electrical resistance of the soil. Dry, sandy soils have a lower resistance, which can result in a higher step potential, while wet, clay soils have a higher resistance and a lower step potential.

Touch Potential Definition

Touch potential is defined as the difference between the maximum Earth Potential Rise (EPR). And, the minimum surface potential within a 1 m radius of the earthed plant.   Also, there are cases where the touch potential could be nearly the full voltage across the earthed object. If that object is earthed at a point remote from the place where the person is in contact with it. For example, a crane earthed to the system neutral. And, that contacted an energised line would expose any person in connection with the crane. Along with, its uninsulated load line to a touch potential nearly equal to the full fault voltage.

Touch potential can be a safety hazard to people and animals because it creates an electric field that can cause electrical shock or injury. Again, the magnitude of the touch potential depends on the current flowing through the ground and the electrical resistance of the soil. Dry, sandy soils have a lower resistance, which can result in a higher touch potential, while wet, clay soils have a higher resistance and a lower touch potential.

Charles Dalziel

The person who pioneered much of the early work into how the human body responds to electrocution was Charles Dalziel, pictured below. He conducted experiments studying his body responses to electrocution; presumably, there were not many volunteers for the task!!!  Fortunately, he did survive (86 yrs young), and the results from his experiments formed the basis of IEC 60479-1, Effects of Current on Human Beings and Livestock.

What Dalziel was able to determine is that response to stress voltages is probabilistic, which means that once accepting a given threshold as tolerable, not everyone in a given general population is going to survive!  This understanding is because every individual is different, has differing tolerance to the stress voltage before the heart fibrillates.  For example, an elderly person with a heart condition or very young child is likely to be more ‘at risk’ than say a fit/healthy adult.

To this day there remains some controversy between EU regions and authorities regarding where the touch and step threshold voltages should sit.  However, with recent amendments to the IEC standards, the remaining area of ambiguity largely centre’s on the selection of appropriate fault clearance times.

Step and Touch Potential Hazards

Based on the recent amendments to BS EN 50522 and IEEE Std.81, Step and Touch voltage safety has become the driving criterion for safe Earthing/Grounding Design. It used to be the 1-ohm earth mat that would ensure safety, but this is no longer the case.  Current wisdom and best practices adopted by both the IEEE and IEC bodies. Agree that the natural hazard posed by a Rise of Earth Potential is whether the human heart (or given animal) can withstand (survive) the current flow resulting from a difference in potential when touching a piece of equipment or standing nearby, e.g. the step and touch voltage or potential.

You can see in the image above. That the heart is further away from the body currents in the Step Potential case. Whereas, in the Touch Potential scenario the electrons flow almost directly through and around the heart itself.  Ignoring shoe resistance. This is a fundamental reason why the permissible voltage thresholds for Step Potential can be far higher. Than those for Touch Potential.

Reducing the Risks of Step Potential and Touch Potential

Once a Rise of Earth Potential Study has identified the risks. Also, there are many necessary essential measures available to the specialist Electrical Earthing Consultant. Touch & step potential can be reduced by providing proper grounding and bonding of electrical systems, which can help to equalize the potential between different points on the ground. However, the complexity comes in knowing how to apply, combine and configure these into a robust electrical earthing solution. That controls and maintains the surface voltages. And, in such a way as to not exceed the permissible thresholds of the heart. AND within the practical financial constraints of a budget.  

Some ways to reduce the risk of step and touch potential include:

• Providing proper grounding and bonding of electrical systems: This helps to equalize the potential between different points on the ground, reducing the risk of electrical shock.
• Using low-resistance grounding systems: Soil conditions can play a major role in the magnitude of step and touch potentials, so using low-resistance grounding systems in soils with high resistance can help to reduce the potential difference between different points on the ground.
• Implementing protective measures: Such as installing insulated barriers around electrical equipment, or using insulating mats to protect workers from electrical hazards.
• Increasing the safety distance: Increasing the distance between people and electrical equipment can reduce the risk of electrical shock by reducing the strength of the electric field.
• Proper maintenance and inspection of equipment: Regularly inspecting and maintaining electrical equipment and grounding systems can help to identify and repair any defects that could contribute to step and touch potentials.
• Proper training of the personnel: Providing training and education to workers about the hazards of step and touch potentials can help to raise awareness and reduce the risk of injury.

Some of the hardware to incorporate in design includes:

1. Grading conductors
2. Conductor meshes
3. Vertical electrodes
4. Horizontal electrodes
5. Deep bore electrodes
6. Counterpoise electrodes
7. Earthed planes
8. Rod groups
9. Proper Bonding arrangements
10. Ground conditioning agents*
11. High resistance surface layers, such as crushed rock, stone, rubber, tarmac, etc.*

* For most cases, the top resistivity surface layers should be considered more as a secondary mitigation method. E.g. the earthing strategy should be to achieve a safe base design. Where practicable. Also, without surface layer mitigation or soil conditioning agents.

As a result, other non-hardware related mitigation may include a Risk Management Approach. Where the risks are ‘managed’ through the application of processes and or procedures to avoid injury.