EARTH POTENTIAL RISE

'Earth Potential Rise (EPR)' also called Ground Potential Rise (GPR) occurs when a large current flows to earth through an earth grid impedance. The ground potential at the current injection site increases relative to a remote earth, with the potential decreasing with increasing distance from the injection point.

Contents

Causes

Safety

Step and Touch Potentials

Mitigation

Calculations

Standards & Regulations

References

External links

Causes

Earth Potential Rise (EPR) has typically been caused by lightning strikes and/or electrical faults (ie short circuits) that occur at electrical substations, power plants, or high-voltage transmission lines. 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, X/R Factor, clearing time, and the amount of current entering the earth. The voltage of the fault is not a major factor in determining EPR. Thus, even low-voltage (120/240 Volt) sources handeling as little as 1,000-amps, such as switches, transformers, and load centers, can generate very significant EPR events resulting in dangerous Step & Touch Potentials.

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 under 29 CFR 1910.269, with a specific addition under Appendix C. http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9868

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.

Mitigation

OSHA 29 CFR 1910.269 Appendix C states that "Protection From the Hazards of Ground-Potential Gradients" requires nn 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.
Appendix C of 29 CFR 1910.269 provides information primarily with respect to employee protection from contact between equipment being used and an energized power line. The information presented is also relevant to ground faults to transmission towers and substation structures; however, grounding systems for these structures should be designed to minimize the step and touch potentials involved.

Calculations

The potential of the earth grid can be calculated using Ohm's Law if the fault current ('I_f') and resistance of the grid ('Z_grid') are known.
:$mathbf\{V\}\_\{grid\}\; =\; mathbf\{I\}\_f\; imes\; mathbf\{Z\}\_\{grid\}$
At points outside the earth grid, the earth potential rise decreases. The simplest estimate of the EPR at a distance from an earth grid involves the analysis of a driven rod electrode, assuming the rod radiates contours of equipotential voltage in a homogeneous earth environment. The voltage profile is given by the following equation.
:
ho 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.
:$mathbf\{$
ho} is the resistivity of the earth, in Ω·m.
:$mathbf\{I\}$ is the earth fault current, in amperes.
This calculation provides an estimate only. A real earthing system is unlikely to be a single earth electrode, and the soil may have varying resistivity.

Standards & Regulations

The Earth Potential Rise (EPR) phenomenon is regulated by US Federal Law under 29 CFR 1910.269 which mandates the elimination of hazardous voltages created by EPR, such as Step & Touch Potentials, in the work place.
IEEE Std. 80-2000 is the only known standard in north America that addresses the elimination of Step & Touch Potentials. While all IEEE standards are voluntary and Std. 80 was specifically designed for use in substations, it has become the defacto standard for use in EPR mitigation the world over.

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

External links

★ http://www.acif.org.au/__data/page/15836/S009_2006r.pdf AS/ACIF S009:2006 Installation Requirements for Customer Cabling (Wiring Rules).

★ http://esgroundingsolutions.com/ Information about Ground Potential Rise Studies

★ http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9868 OSHA 29 CFR 1910.269