# What Is Ground Potential Rise (GPR)? A Complete Technical Guide **Author:** David R. Stockin, PE — President, E&S Grounding Solutions, Inc. **Published:** May 2026 **Category:** Grounding Engineering **Tags:** ground potential rise, GPR, substation grounding, IEEE 80, step voltage, touch voltage, fault current, grounding design --- ## Introduction Ground potential rise (GPR) is one of the most consequential — and most misunderstood — phenomena in electrical power system engineering. It is the reason substation grounding systems must be carefully engineered rather than simply installed. It is the reason telecommunications cables entering a substation require special protective equipment. And it is, ultimately, the reason people can be killed by touching a fence post or a metal pipe hundreds of feet away from a substation during a ground fault. This guide provides a complete technical explanation of ground potential rise: what it is, why it occurs, how it is calculated, what its consequences are, and how engineers design grounding systems to manage it safely. --- ## Definition **Ground potential rise (GPR)** is defined by IEEE Std. 80-2013 as: > "The maximum electrical potential that a substation grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth." In simpler terms: when a ground fault occurs at a substation, a large current flows from the faulted conductor into the earth through the substation grounding grid. This current flow through the resistance of the grounding grid causes the entire grid — and everything connected to it — to rise in electrical potential above the surrounding earth. The magnitude of this potential rise is the GPR. The GPR is not a steady-state condition. It exists only during the fault event, which typically lasts from a few cycles (for fast-clearing protection systems) to several seconds (for slower systems). But even a fraction of a second at a high GPR can be lethal. --- ## Why GPR Occurs: The Physics To understand GPR, it is necessary to understand how fault current flows through the earth. When a phase conductor contacts the grounded structure of a substation — whether through insulation failure, equipment breakdown, or a direct contact event — the fault current takes the path of least resistance back to the source. In a substation, that path runs through the grounding grid, into the earth, and back to the neutral of the source transformer through the earth itself. The earth is not a perfect conductor. It has a finite resistivity, measured in ohm-meters (Ω·m), that varies widely depending on soil type, moisture content, temperature, and depth. Typical soil resistivities range from less than 10 Ω·m for saturated clay to more than 10,000 Ω·m for dry granite. When fault current flows from the grounding grid into the earth, it encounters this resistivity. By Ohm's Law, the voltage across any resistance is equal to the current times the resistance. The grounding grid has a finite resistance to remote earth — typically expressed as the **ground resistance** (R_g) in ohms. The GPR is simply: > **GPR = I_g × R_g** Where: - **I_g** is the maximum grid current (the portion of the total fault current that flows through the grounding grid into the earth) - **R_g** is the resistance of the grounding grid to remote earth For a large substation with a well-designed grounding grid, R_g might be 0.5 ohms. If the maximum grid current is 20,000 amperes, the GPR would be 10,000 volts — ten kilovolts above the potential of the surrounding earth. --- ## GPR, Step Voltage, and Touch Voltage: The Relationship GPR is the "headline number" — the maximum potential of the grounding grid relative to remote earth. But the quantities that actually determine whether a person is in danger are **step voltage** and **touch voltage**, which are derived from the GPR and the distribution of current through the earth. ### Step Voltage Step voltage is the potential difference between two points on the earth's surface, separated by a distance of one pace (typically defined as 1 meter in IEEE Std. 80). A person standing with one foot at a higher potential and one foot at a lower potential will experience a current flowing through their body from one foot to the other. Step voltages are highest near the perimeter of the grounding grid, where current density in the earth is greatest. They can be dangerous even to people standing entirely outside the substation fence. ### Touch Voltage Touch voltage is the potential difference between a grounded metallic structure (such as a substation fence, equipment enclosure, or surge arrester base) and the earth surface at the point where a person is standing. A person touching the structure while standing on the earth will experience a current flowing from their hand through their body to their feet. Touch voltages are typically higher than step voltages for the same grounding system because the metallic structure is at or near the full GPR, while the earth surface potential at the person's feet may be significantly lower. ### The Danger Zone The danger from GPR, step voltage, and touch voltage is not simply a matter of the voltage magnitude. It is a function of the current that flows through the human body — specifically through the heart — and the duration of that current flow. The **Dalziel equation**, which forms the basis of the permissible voltage limits in IEEE Std. 80, expresses this relationship: > **I_b = 0.116 / √t_s** (for a 50 kg person) Where I_b is the permissible body current in amperes and t_s is the fault clearing time in seconds. For a fault clearing time of 0.5 seconds, the permissible body current is approximately 164 mA — a level that can cause ventricular fibrillation. IEEE Std. 80 uses this equation, combined with a body resistance model of 1,000 ohms and a surface layer resistance model (for crushed rock), to calculate permissible step and touch voltages. The grounding system is then designed so that the actual voltages do not exceed these limits. --- ## Consequences of Uncontrolled GPR The consequences of a high GPR event extend well beyond the substation fence. ### Personnel Safety The most immediate consequence is the risk of electric shock to personnel working in or near the substation during a fault event. Step voltages outside the fence and touch voltages on the fence itself can be lethal even to people who are not in direct contact with any energized equipment. This is why substation grounding design is not a commodity engineering service. A poorly designed grounding system can kill people — not just during the fault, but potentially during maintenance activities if the grounding system has degraded over time. ### Telecommunications and Control Systems GPR is a major concern for telecommunications cables, fiber optic cables with metallic sheaths, control cables, and any other conductor that enters the substation from outside. When a ground fault occurs, the GPR is impressed on any metallic conductor connected to the substation grounding system. If that conductor exits the substation and connects to equipment at a lower potential (such as a telephone exchange or a SCADA system), the GPR can damage or destroy the equipment — and can create a shock hazard for personnel at the remote location. IEEE Std. 367 (Recommended Practice for Determining the Electric Power Station Ground Potential Rise and Induced Voltage from a Power Fault) provides guidance for calculating GPR for telecommunications protection purposes. Telecommunications engineers use this data to design protective equipment (isolation transformers, surge arresters, gas tubes) for cables entering substations. ### Pipeline and Structural Integrity GPR can also affect buried metallic structures — pipelines, cable sheaths, and reinforcing steel — that pass through or near the substation ground grid. The impressed voltage can accelerate corrosion and, in extreme cases, can create a shock hazard for personnel working on the pipeline at a remote location. This is why corrosion control engineering and grounding system design must be coordinated on projects where both are present. --- ## How GPR Is Calculated The calculation of GPR for a real substation involves several steps. ### Step 1: Soil Resistivity Measurement The first step is to measure the soil resistivity at the substation site using the **Wenner four-pin method** (per IEEE Std. 81). This involves driving four equally spaced electrodes into the ground and measuring the resistance between the outer pair while passing a known current through the inner pair. By varying the electrode spacing, the engineer can develop a resistivity profile that shows how resistivity varies with depth. The measured data is then used to develop a **soil model** — typically a two-layer model with a surface layer resistivity and a deep layer resistivity — that is used in the grounding system calculations. ### Step 2: Fault Current Determination The maximum grid current (I_g) is determined from a fault current study of the power system. The total fault current at the substation must be calculated for the maximum credible fault condition (typically a three-phase-to-ground fault or a single-line-to-ground fault, depending on which produces the higher ground current). Not all of the total fault current flows through the grounding grid. Some returns through overhead ground wires, shield wires, and the neutral conductors of connected lines. The **current division factor** (S_f) accounts for this, and the grid current is: > **I_g = S_f × I_f × D_f** Where I_f is the total fault current and D_f is the **decrement factor** that accounts for the DC offset in the fault current waveform. ### Step 3: Ground Grid Resistance Calculation The resistance of the grounding grid to remote earth (R_g) is calculated using the Schwarz equations or, for complex geometries, using computer software such as CDEGS. The resistance depends on the grid geometry (conductor spacing, depth, total length), the soil model, and the presence of any ground rods. ### Step 4: GPR Calculation The GPR is then simply: > **GPR = I_g × R_g** ### Step 5: Step and Touch Voltage Calculation The distribution of current through the earth — and therefore the step and touch voltages — is calculated using the Schwarz equations or CDEGS. The calculated voltages are compared to the permissible limits derived from the Dalziel equation. ### Step 6: Grid Design Iteration If the calculated step or touch voltages exceed the permissible limits, the grid design is modified — by adding conductors, reducing conductor spacing, adding ground rods, or adding a crushed rock surface layer — and the calculations are repeated until compliance is achieved. --- ## Role of CDEGS in GPR Analysis For complex substation grounding projects, the simplified Schwarz equations are not adequate. CDEGS (Current Distribution, Electromagnetic Fields, Grounding, and Soil Structure Analysis) is the industry-standard software for rigorous GPR analysis. CDEGS can model: - Multi-layer soil structures (not just two-layer models) - Complex grid geometries with irregular shapes and variable conductor depths - The effects of nearby metallic structures (pipelines, cable sheaths, fences) on the grounding system - Inductive coupling between the grounding system and nearby conductors - The distribution of step and touch voltages across the entire substation yard CDEGS is used by E&S Grounding Solutions for all complex substation grounding projects, and its results are accepted by utilities, regulatory agencies, and telecommunications companies worldwide as the basis for GPR studies and grounding system design. --- ## GPR Mitigation Strategies When the calculated GPR or the resulting step and touch voltages exceed permissible limits, engineers have several options for mitigation. **Increase the grounding grid area.** A larger grid has a lower resistance to remote earth, which reduces both R_g and the resulting GPR. This is the most effective mitigation strategy for new substations where the site area is not constrained. **Add ground rods.** Ground rods extend the grounding system into deeper, lower-resistivity soil layers, reducing R_g and GPR. **Add a crushed rock surface layer.** A 4–6 inch layer of crushed rock (with a high resistivity of 3,000 Ω·m or more) on the substation yard surface increases the contact resistance between a person's feet and the earth, reducing the body current for a given step or touch voltage. This is one of the most cost-effective mitigation strategies for existing substations. **Reduce fault clearing time.** Faster protection systems reduce the permissible body current threshold (via the Dalziel equation), which allows higher step and touch voltages to be tolerated. Upgrading protection systems to reduce fault clearing time is sometimes more cost-effective than expanding the grounding grid. **Install equipotential bonding.** Bonding all metallic structures within the substation yard to the grounding grid ensures that they are all at the same potential, eliminating touch voltage hazards between structures. **Install gradient control conductors.** Conductors buried just outside the perimeter of the grounding grid can reduce the step voltage gradient at the fence line, protecting people outside the substation. --- ## Frequently Asked Questions **Q: What is a dangerous level of GPR?** A: There is no single "dangerous" GPR level — the danger depends on the resulting step and touch voltages, which depend on the soil resistivity distribution, the grid geometry, and the fault clearing time. A GPR of 5,000 V at a substation with a well-designed grid and fast protection may produce step and touch voltages well within safe limits. A GPR of 1,000 V at a substation with a poorly designed grid and slow protection may produce dangerous voltages. The IEEE Std. 80 calculation methodology accounts for all of these factors. **Q: How is GPR different from step voltage and touch voltage?** A: GPR is the potential of the entire grounding grid relative to remote earth. Step voltage is the potential difference between two points on the earth's surface one pace apart. Touch voltage is the potential difference between a grounded metallic structure and the earth surface at the point where a person is standing. Step and touch voltages are derived from the GPR and the current distribution through the earth. **Q: Does GPR affect telecommunications cables?** A: Yes. GPR is a major concern for any metallic conductor that enters the substation from outside. Telecommunications cables, control cables, and fiber optic cables with metallic sheaths can all conduct GPR to remote locations, creating shock hazards and equipment damage. IEEE Std. 367 provides guidance for GPR calculations for telecommunications protection purposes. **Q: How often should a GPR study be updated?** A: A GPR study should be updated whenever there is a significant change in the substation's fault current level (due to system expansion or reconfiguration), whenever the grounding system is modified, and whenever the protection system's fault clearing time changes. As a general rule, GPR studies should be reviewed every 5–10 years or whenever a major system change occurs. **Q: Can GPR affect people outside the substation fence?** A: Yes. Step voltages can be dangerous outside the substation fence, particularly near the perimeter of the grounding grid. This is why gradient control conductors are sometimes installed outside the fence, and why the crushed rock surface layer is sometimes extended beyond the fence line. --- ## Conclusion Ground potential rise is not an abstract engineering concept — it is a real, measurable, and potentially lethal phenomenon that occurs every time a ground fault happens at a substation. Understanding GPR, and designing grounding systems that keep step and touch voltages within safe limits, is one of the most important responsibilities of the substation grounding engineer. The calculation of GPR requires accurate soil resistivity data, a rigorous fault current study, and — for complex sites — sophisticated computer modeling using tools like CDEGS. It is not a task for a spreadsheet or a rule of thumb. It requires engineering expertise, site-specific data, and a thorough understanding of IEEE Std. 80 and the applicable protection system parameters. E&S Grounding Solutions provides complete GPR studies, IEEE Std. 80 substation grounding design, CDEGS analysis, and soil resistivity testing for substations, renewable energy facilities, and mission-critical infrastructure. [Contact us](/contact) to discuss your project. --- ## References 1. IEEE Std. 80-2013 — IEEE Guide for Safety in AC Substation Grounding. Institute of Electrical and Electronics Engineers. 2. IEEE Std. 81-2012 — IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System. Institute of Electrical and Electronics Engineers. 3. IEEE Std. 367-2012 — IEEE Recommended Practice for Determining the Electric Power Station Ground Potential Rise and Induced Voltage from a Power Fault. Institute of Electrical and Electronics Engineers. 4. Dalziel, C.F. (1972). "Electric Shock Hazard." IEEE Spectrum, 9(2), 41–50. 5. Sverak, J.G. (1984). "Sizing of Ground Conductors Against Fusing." IEEE Transactions on Power Apparatus and Systems, PAS-100(1), 51–59. 6. EPRI TR-100622 (1992). Substation Grounding. Electric Power Research Institute.