What Is Earth Potential Rise (EPR)? A Practical Guide
Earthing · 6 July 2026 · For contractors and project engineers
If a DNO, principal contractor or designer has asked you for an "EPR assessment" or an "earthing study", this guide explains what they're asking for, why it matters, and what information the study needs — in practical terms.
The Definition
When an earth fault occurs on a high-voltage system, fault current flows through the site's earthing system and into the surrounding soil. Because soil has resistance, the earthing system — and everything bonded to it — rises in voltage relative to the general mass of earth far away ("remote earth"). That voltage is the earth potential rise (EPR), sometimes called ground potential rise (GPR).
In simple terms: EPR is the earth fault current flowing into the ground multiplied by the impedance of the earthing system. A site with a 10 kA earth fault and a combined earthing impedance of 0.5 Ω could rise to several kilovolts during a fault. The fault only lasts until protection clears it — typically fractions of a second — but during that time the voltages around the site can be dangerous.
Why EPR Matters: Touch, Step and Transferred Potentials
EPR itself is not the hazard — the hazard is the fraction of it a person can be exposed to:
- Touch voltage — the voltage between a bonded metal part someone is touching and the ground they are standing on (hand-to-feet).
- Step voltage — the voltage between a person's feet as the ground potential falls away with distance from the electrode (foot-to-foot).
- Transferred potential — the EPR exported away from the site by metallic services: cable sheaths, pipework, fences, rails or telecoms circuits. A person far from the substation can be exposed to a large fraction of the full EPR.
An earthing study exists to prove these voltages stay within permissible limits — or to redesign the earthing system until they do.
What Determines the EPR
Three things dominate the result. First, the earth fault current — and specifically how much of it actually returns through the soil rather than through cable sheaths and overhead earth wires; this split is calculated in the study (we use SES FCDIST for the current distribution). Second, the earthing system impedance, set by the electrode geometry and the soil resistivity — which is why a proper Wenner or Schlumberger soil resistivity survey is the first input every study needs. Third, the fault clearance time: it doesn't change the EPR, but it sets the permissible touch and step voltage limits, because the human body tolerates higher voltages for shorter durations.
The Limits: BS EN 50522 and ENA TS 41-24
In the UK, two documents set the assessment framework. BS EN 50522 (earthing of power installations above 1 kV) provides the permissible touch voltage curves as a function of fault duration. ENA TS 41-24 is the Energy Networks Association specification applying those principles to UK distribution networks — it's the document DNOs assess connection designs against. The study also classifies the site for telecoms purposes (the traditional "hot/cold site" assessment), because a high EPR site restricts what telecoms equipment can be installed and how it must be protected.
What an EPR Study Actually Delivers
A competent earthing study takes the soil survey, site layout and DNO fault data, builds a computer model of the electrode system (we use SES MultiFields, part of the CDEGS suite), and calculates the EPR, touch, step and transferred potentials. Results are compared against the BS EN 50522 / ENA TS 41-24 limits. Where limits are exceeded, the design is optimised — additional electrodes, grading conductors, high-resistivity surface layers such as crushed rock, or restricting access — and the final report presents the compliant design with the evidence DNOs and approval authorities expect.
Need an EPR assessment for a connection, substation or generation site? See our earthing study design service for what we deliver and what inputs we need.