Earth potential rise

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In electrical engineering, Earth Potential Rise (EPR) also called Ground Potential Rise (GPR) occurs when a large current flows to earth through an earth grid impedance. The potential relative to a distant point on the Earth is highest at the point where current enters the ground, and declines with distance from the source. Ground potential rise is a concern in the design of electrical substations because the high potential may be a hazard to people or equipment. The potential gradient (drop of voltage with distance) may be so high that a person could be injured due to the voltage developed between two feet, or between the ground on which the person is standing and a metal object. Any conducting object connected to the substation earth ground, such as telephone wires, rails, fences, or metallic piping, may also be energized at the ground potential in the substation. This transferred potential is a hazard to people and equipment outside the substation.

1 Causes
2 Safety
3 Step and Touch Potentials
4 Mitigation
5 Calculations
6 Standards & Regulations
7 References
8 External links

[edit] Causes

Earth Potential Rise (EPR) is caused by electrical faults that occur at electrical substations, power plants, or high-voltage transmission lines. Short-circuit current flows through the plant structure and equipment and into the grounding electrode at station. The resistance of the Earth is finite, so current injected into the earth at the grounding electrode produces a potential rise with respect to a distant reference point. The resulting EPR or GPR can cause hazardous voltage in the form of Step & Touch Potentials, many hundreds of feet away from the actual fault location. Many factors determine the level of hazard, including: soil conditions, clearing time, and the amount of current entering the earth.

[edit] Safety

EPR, along with other interference phenomena such as low frequency induction, 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 telecommunications personnel, users or plant to hazardous voltages.

The US Occupational Safety and Health Administration (OSHA) has designated EPR as a "known hazard" and has issued regulations governing the elimination of this hazard in the work place. ^[1]

[edit] Step and Touch Potentials

"Step potential" is the voltage between the feet of a person standing near an energized grounded object. It is equal to the difference in 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 simply by standing near the grounding point.

"Touch potential" is the voltage between the energized object and the feet of a person in contact with the object. It is equal to the difference in voltage between the object (which is at a distance of 0 feet) and a point some distance away. The touch potential could be nearly the full voltage across the grounded object if that object is grounded at a point remote from the place where the person is in contact with it. For example, a crane that was grounded to the system neutral and that contacted an energized line would expose any person in contact with the crane or its uninsulated load line to a touch potential nearly equal to the full fault voltage.

[edit] Mitigation

OSHA 29 CFR 1910.269 Appendix C states that "Protection From the Hazards of Ground-Potential Gradients" requires an engineering analysis of the power system under fault conditions can be used to determine whether or not hazardous step and touch voltages will develop. The result of this analysis can ascertain the need for protective measures and can guide the selection of appropriate precautions.

Several methods may be used to protect employees from hazardous ground-potential gradients, including equipotential zones, insulating equipment, and restricted work areas.

1. The creation of an equipotential zone will protect a worker standing within it from hazardous step and touch potentials. Such a zone can be produced through the use of a metal mat connected to the grounded object. In some cases, a grounding grid can be used to equalize the voltage within the grid. Equipotential zones will not, however, protect employees who are either wholly or partially outside the protected area. Bonding conductive objects in the immediate work area can also be used to minimize the potential between the objects and between each object and ground. (Bonding an object outside the work area can increase the touch potential to that object in some cases, however.)

2. The use of insulating equipment, such as rubber gloves, can protect employees handling grounded equipment and conductors from hazardous touch potentials. The insulating equipment must be rated for the highest voltage that can be impressed on the grounded objects under fault conditions (rather than for the full system voltage).

3. Restricting employees from areas where hazardous step or touch potentials could arise can protect employees not directly involved in the operation being performed. Employees on the ground in the vicinity of transmission structures should be kept at a distance where step voltages would be insufficient to cause injury. Employees should not handle grounded conductors or equipment likely to become energized to hazardous voltages unless the employees are within an equipotential zone or are protected by insulating equipment.

[edit] Calculations

In pricniple, the potential of the earth grid V_grid can be calculated using Ohm's Law if the fault current (I_f) and resistance of the grid (Z_grid) are known.

$\mathbf{V}_{grid} = \mathbf{I}_f \times \mathbf{Z}_{grid}$

While the fault current from a distribution or transmission system can usually be calculated or estimated with precision, calculation of the earth grid resistance is more complicated. Difficulties in calculation arise from the extended and irregular shape of practical ground grids, and the varying resistivity of soil at different depths.

At points outside the earth grid, the potential rise decreases. The simplest case of the potential at a distance is the analysis of a driven rod electrode in homogeneous earth. The voltage profile is given by the following equation.

$\mathbf{V}_{r} = \frac{\mathbf{\rho I}}{\mathbf{2 \pi r_x}}$

where

$\mathbf{r_x}$ is a point beyond the edge of the earth grid.

$\mathbf{V}_{r}$ is the voltage at distance $\mathbf{r_x}$ from the earth grid, in volts.

$\mathbf{\rho}$ is the resistivity of the earth, in Ω·m.

$\mathbf{I}$ is the earth fault current, in amperes.

This case is a simplified system; most earthing systems are more complex than a single rod, and the soil will have varying resistivity. It can, however, reliably be said that the resistance of a ground grid is inversely proportional to the area it covers; this rule can be used to quickly assess the degree of difficulty for a particular site.

[edit] Standards & Regulations

US Federal Law ^[2] mandates the elimination of hazardous voltages created by EPR, such as Step & Touch Potentials, in the work place.

IEEE Std. 80-2000 is a standard that addresses the mitigation of Step & Touch Potentials to acceptable levels. It has become widely applied for EPR mitigation.

[edit] References

[1] ACIF Working Committee CECRP/WC18, AS/ACIF S009:2006 Installation Requirements for Customer Cabling (Wiring Rules), Australian Communications Industry Forum, North Sydney, Australia (2006) ISBN 1-74000-354-3 <reflist/>