What is Earth Potential Rise (EPR)? Technical Guide [PDF]

Earth Potential Rise (EPR), also known as Ground Potential Rise (GPR) or Ground Potential Gradient (GPG), is a phenomenon in electrical systems where the ground potential at a specific location rises concerning a reference point. This rise in ground potential can occur due to various factors, primarily during fault conditions in power systems. Here’s a more detailed explanation:

1. Fault Conditions: The most common scenario where EPR occurs is during ground faults in electrical power systems. A ground fault typically involves a phase-to-ground fault or short circuit, where one of the current-carrying conductors (e.g., a phase conductor) comes into contact with the earth or a grounded object. During such a fault, a significant amount of fault current flows into the ground.
2. Soil Resistivity: The soil or earth in which the fault current dissipates plays a crucial role in determining the magnitude of EPR. The resistivity of the soil influences how easily or difficultly the fault current spreads and dissipates. Soils with higher resistivity can experience more substantial EPR because they resist the dissipation of fault current.
3. Distance from Fault: The distance from the point of the fault to the location where EPR is measured is also a critical factor. The potential rise is most significant near the fault and decreases with distance.
4. System Voltage and Fault Current: The system voltage and the magnitude of the fault current also influence EPR. Higher voltage systems and higher fault currents can lead to more substantial potential rise.
5. Grounding Systems: The design and quality of the grounding system used in the electrical installation can affect EPR. A well-designed grounding system helps to disperse fault current and minimize EPR.

The primary concern with EPR is its potential to create hazardous conditions, especially in areas with grounded structures or equipment. When EPR occurs, the ground potential can rise to a level that poses a safety risk to people, animals, and nearby structures. It can lead to electric shock hazards, equipment damage, and even fires or explosions in extreme cases.

To prevent the risks associated with EPR, electrical systems are designed with various protective measures, including proper grounding, fault current limiting devices, and insulation coordination. Additionally, engineers and operators perform EPR studies to assess and minimize potential risks in specific installations, especially in high-voltage or critical facilities like power plants, substations, and industrial complexes.

1. Introduction to Earth Potential Rise (EPR):

Earth Potential Rise (EPR) occurs when fault currents in electrical systems flow through the ground back to their source, typically the grounding point of a transformer. This results in a rise in the ground potential at both the fault location and the source transformer’s grounded star point. This rise in potential is due to the resistance of the earth.

2. EPR Hazards for Telecommunication Cables:

When telecommunication cables pass through areas with EPR hazards, there is a risk that hazardous voltage (EPR) may be induced onto the conductors within the cable. This can happen if:

• A telecommunication conductor is grounded within the EPR hazard zone.
• The EPR in the ground surrounding the telecommunication cable exceeds the cable’s insulation rating.

3. Power Systems and EPR Hazards:

Some power systems, even if properly grounded as per regulations, can still generate EPR values of several thousand volts. These systems may be in close proximity to telecommunication infrastructure or indirectly affect it through the Multiple Earthed Neutral (MEN) system.

4. Risks Associated with EPR on Telecommunication Systems:

EPR from power systems can pose serious risks to telecommunication systems, users, staff, and equipment. When telecommunication users or staff are exposed to EPR voltage, it can lead to significant damage to telecommunication plant, potentially affecting large areas and causing costly repairs. Damage is often widespread, making repairs challenging and expensive, and service interruptions can affect a broad geographical area.

5. EPR Caused by Power Line Contacts:

EPR resulting from power lines making contact with the ground, such as due to falls or insulation failures, in areas not linked to a power system ground typically does not pose a hazard to telecommunication systems. This is because:

• Such contacts are unlikely to occur in proximity to telecommunication infrastructure.
• The EPR levels decrease rapidly, becoming safe within a few meters of the contact point.

6. Protective Measures for Telecommunication Systems:

Given the potential hazards associated with EPR, protective measures are essential for telecommunication systems. These measures can include grounding strategies, insulation rating assessments, and careful consideration of proximity to power systems and MEN systems. Protecting telecommunication infrastructure from EPR is critical to ensure the safety and reliability of services to users and minimize costly damage and repairs.

Earth Potential Rise Case Study:

Ground Potential Rise (GPR) or Earth Potential Rise is a phenomenon that occurs when a substantial amount of electricity is discharged into the earth. This often happens during lightning strikes, especially in the vicinity of cell towers. When such large currents enter the ground through a grounding system, it not only causes an increase in electrical potential within the grounding system itself but also elevates the electrical potential of the surrounding soil. This elevated potential can pose significant hazards to people and their homes nearby.

The voltage levels generated during a GPR event can be extremely dangerous. As mentioned earlier, soil possesses a property known as soil resistivity, which results in an electrical potential gradient or voltage drop along the path of the lightning strike’s current through the soil. These potential differences can induce currents to flow into any nearby conductive objects that are grounded, including structures like concrete buildings, pipes, copper wiring, residences, and even individuals.

The direct consequences of GPR can extend from the cell tower’s antenna down to its base station ground and equipment ground bus. These grounding systems are also connected to the service neutral and ground from the incoming power service. In the event of a lightning strike on the cell tower, there is a real risk of damaging GPR effects, including the transfer of current and voltage into the ground, potentially affecting electric power lines, underground telephone lines, and the service neutral. This, in turn, can impact nearby homes and pose safety hazards.

In the case of our family’s home located just 850 feet from the Mobility cell tower site, at coordinates 35° 58′ 49.7″ N and 092° 33′ 00.3″ W, there is a pressing safety concern due to the effects of GPR. In the absence of actual test results, calculated GPR levels can be used to assess the need for proper isolation of nearby homes, people, and livestock from the potentially damaging effects of GPR. These calculated GPR levels can reach magnitudes in the thousands of volts. Cell tower companies have the capability to mitigate these levels by improving their grounding systems.

During a lightning strike, the energy is conducted down through the cell tower into the ground. Under worst-case theoretical conditions, GPR levels in such scenarios could reach a maximum of 85,000 Volts at your home. Achieving a grounding system with less than 5 ohms of resistance in soils with poor resistivity, as is likely the case at your home’s cell site, may prove to be a considerable challenge and may require improvements in the grounding infrastructure to ensure safety and reduce GPR risks.

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