What are Touch and Step Potential Requirements?
To grasp the concept of step and touch potential as per IEC 62305-3 (EN 62305-3) how energy dissipates over conductive objects. Some good conductors exist that provide a path to the ground during broken pole or downed wire conditions, such as metal fences, damp soil, and puddles. Other conductors, such as trees, wood fences, and utility poles, may not be as good as metal conductors but still allow electricity to flow to the ground. Although wood is considered an insulator, damp wood will carry electricity.
The object and nearby vicinity become activated when an energised conductor falls across a chain-link fence or directly to the ground, generating high voltage in proportion to the earth. The voltage is determined by the source, the object's resistance, and the soil conditions, which includes substance and moisture.
The ground potential gradient is the voltage dissipation from a grounded conductor – or the grounded end of an electrified grounded item. Ground potentials are voltage decreases related to this dissipation of voltage. With increasing distance from the grounded end, the voltage drops fast.
What is Step Potential?
A high-voltage condition is when current flows from an overhead conductor through a chain-link fence to the earth, resulting in a voltage gradient based on the soil resistivity, resulting in a voltage differential – also known as a potential difference – between two sites on the ground.
The voltage between the feet of a person standing near an electrified earthed object is known as step potential. It's also equal to the voltage difference determined by the voltage distribution curve between two sites separated by a significant distance from the electrode. Only standing near the connected device puts a person in danger during a fault. A part of the earth potential that can be spanned by a human taking a 1 m step is called step voltage.
The form of the potential gradient area determines the step voltage. The step voltage drops as the distance from the structure increases, as illustrated in the diagram. As a result, the risk to people decreases as they move away from the structure.
The following steps can reduce the step voltage:
1) Using barriers or fences to keep people from entering potentially dangerous places.
2) The contact resistance of the floor layer within a 3 m spacing around the down conductors is not less than 100 k (IEC 62305-3 (EN 62305-3), chapter 8.2).
3) Reducing mesh size of the earthing network for potential control.
What is Touch Potential?
It's the difference between the maximum Earth Potential Rise and the touch potential. Also, within a 1 m radius of the earthed plant, the minimal surface potential. In some circumstances, the contact potential may be virtually equal to the whole voltage across the earthed object.
If the thing is earthed at a location other than the person's point of contact with it. A crane, for instance, is earthed to the system neutral. And if that contacted an energised line, anyone associated with the crane would be exposed. Its uninsulated load line will also be exposed to a touch potential approximately equivalent to the fault voltage.
The following steps can reduce the touch voltage:
1) Insulating material (at least 3 mm cross-linked polyethene with an impulse withstand voltage of 100 kV (1.2/50 s)) is used to cover the down conductor. The position of the down conductors can be altered so that they are not, for example, in the structure's entrance area, and the likelihood of people accumulating can be decreased by using notes or prohibition signs. There's also the potential of encountering roadblocks.
2) Within a radius of 3 m surrounding the down conductors, the contact resistance of the floor layer is not less than 100 k.
3) Note: A 5 cm thick layer of insulating material, such as asphalt (or a 15 cm gravel layer), reduces the danger to a level that is tolerable (IEC 62305-3). (EN 62305-3).
4) Potential control is used for compressing the earth-termination system's meshed network.
Importance of Potential Control:
If a big group of people routinely congregate in an area dangerously close to a structure that needs to be protected, they should be offered control for being safe.
If the resistance gradient on the earth's surface in the area to be protected does not exceed 1/m, potential control is sufficient. To do this a ring earth electrode put at a distance of 1 m and a depth of 0.5 m should be added to an existing foundation earth electrode. This is "the first ring" of the potential control if the structure already has an earth-termination system in the form of a ring earth electrode.
If a structure has potential control, it must be installed as shown in Figure 2. The down conductors must be linked to all of the potential control rings.
Potential Hazards due to Step and Touch Potential:
Step and Touch voltage safety has become the driving criterion for safe Earthing/Grounding Design, thanks to recent modifications to BS EN 50522 and IEEE Std.81. Safety used to be ensured by using a 1-ohm earth mat, but that is no longer the case.
Both the IEEE and IEC bodies have accepted current wisdom and best practices. Accept that the natural danger posed by a Rise of Earth Potential is whether the human heart (or any given animal) can endure (survive) the current flow caused by a difference in potential when touching a piece of equipment or standing close, such as the step and contact voltage or potential.
How to reduce the risk caused due to Step Potential and Touch Potential?
After identifying the hazards through a Rise of Earth Potential Study, the specialised Electrical Earthing Consultant has access to several vital fundamental measures to decrease the possibility of a step and the risk of a touch (mitigation).
However, knowing how to apply, combine, and arrange these into a reliable electrical earthing solution is difficult. The surface voltages are controlled and maintained such that the heart's permitted thresholds don't exceed and are within a budget's practical financial restrictions.
The top resistivity surface layers should be considered as a supplementary mitigating strategy in most circumstances. For example, the goal of the earthing should be to provide a safe base design when it's possible, without the use of soil conditioning chemicals or surface layer mitigation.
Withstanding touch voltage on a lightning protection system's down conductor:
For people outside a building, the area at risk of touch and step voltages is within 3 m of the building and at a height of 3 m. The height of this area is equivalent to a person raising their hand to their highest reaching height plus a separating distance.
Measures against impermissibly high touch voltages may be required in the case of exposed structures (structures prone to lightning strikes, that are accessible to the general public (e.g., shelters). In addition, a risk analysis according to IEC 62305-2 (EN 62305-2) includes the risk to people in the form of the parameter L1 (injury or death of persons).
Touch voltage is abridged during lightning protection by taking the following steps:
1) The down conductor is insulated (at least 3 mm cross-linked polyethene with a 100 kV (1.2/50 s) impulse withstand voltage). The position of the down conductors is altered (for example, there are no down conductors in the structure's entrance area).
2) The earth's surface layer resistivity must be at least 100 k within 3 m of the down conductor (IEC 62305-3 (EN 62305-3)).
3) Notes or restriction notices can minimise the likelihood of people accumulating. There's also the potential of encountering roadblocks.
Most important criteria of personal safety of human being against the Step and Touch Potential:
Do not assume that a conductor that has fallen to the ground or is not sparking is safe.
It's a common misconception that all coated, waterproof, or insulated wire is telephone, television, or fibre-optic cable.
Even though low-hanging wires are not contacting the earth, they still contain voltage potential to avoid touching them. Everything is energised until it is checked to see if it needs to be de-powered.
Do not approach a downed or falling power line. Assume that it is energised at all times. It might be fatal if you touch it.
From the point of contact, electricity can flow outward in a circular pattern through the earth. Travelling out from the centre can form voltage discrepancies. It's never a good idea to drive over downed power lines. Assume that they're pumped up. Even if they aren't, deadlines can coil up in your machinery or vehicle.
If you come into contact with an electrical power line while driving, stay calm and do not exit the vehicle unless it is on fire. If at all feasible, seek assistance.
If you have to escape any equipment due to a fire or other safety concern, try to jump entirely clear, avoiding touching the equipment and the ground at the same moment.
To reduce electric current and avoid electrical shock, land with both feet together and shuffle away in tiny steps. Maintain your balance at all times.
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