Awareness of step potential and touch potential risks, caused by an earth potential rise is vital for anyone working on high-voltage power transmission and distribution systems above 1 kV.
Earth Potential Rise
Earth Potential Rise (EPR) is caused by electrical faults that 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.
Since the Soil Resistivity is not zero, 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. Many factors determine the level of hazard, including available fault current, soil type, soil moisture, temperature, underlying rock layers, and clearing time to interrupt the fault.
Earth potential rise is a safety issue in the coordination of power and telecommunications services. 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 Body Current
Step Potential is the voltage between the feet of a person standing near an energised earthed object. 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.
Touch Potential Body Current
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. 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 or its uninsulated load line to a touch potential nearly equal to the full fault.
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/around the heart itself. Ignoring shoe resistance, this is a fundamental reason why the permissible voltage thresholds for Step Potentials 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 – There are many necessary essential measures available to the specialist Electrical Earthing Consultant to reduce step potential and touch potential risks (mitigation). 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 in such as way as to not exceed the permissible thresholds of the heart AND within the practical financial constraints of a budget.
Some of the hardware to incorporate in design includes:
- Grading conductors
- Conductor meshes
- Vertical electrodes
- Horizontal electrodes
- Deep bore electrodes
- Counterpoise electrodes
- Earthed planes
- Rod groups
- Proper Bonding arrangements
- Ground conditioning agents*
- 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) without surface layer mitigation or soil conditioning agents.
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.
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