Large ground-mounted solar plants cover wider areas, use extensive metal structures, and are often built in open, high-exposure terrains. This combination increases the likelihood that lightning strikes or nearby strikes will induce larger transient voltages across the site, making step-voltage risks more significant unless the earthing design is carefully engineered.

With this in mind, a group of researchers from the Graz University of Technology in Austria has investigated the transient behavior of ground-mounted photovoltaic installations under lightning conditions using detailed case studies. By calculating surface potentials and step voltages, they evaluated how different earthing and equipotential bonding strategies perform in maintaining safety limits defined by IEC standards.

“We calculated the 2D transient step-voltage distribution of a large PV plant subjected to a lightning strike,” the research’s lead author, Benjamin Jauk, told pv magazine. “Unlike much of the existing literature, we did not rely on a single representative lightning frequency. Instead, we applied a Fourier-based approach to resolve the lightning signal into multiple frequency components, calculating the corresponding voltage responses for each component, and then reconstructing the time-domain behavior via inverse Fast Fourier Transform approach.”

Gauk explained that this approach was motivated by two main considerations: first, the IEC standard recommends that step voltages remain below 25 kV for human safety; and second, there was the need for more detailed representation of the transient behavior than what a single-frequency approximation can provide. In this context, the lightning current was treated as a broadband signal whose frequency-dependent effects could significantly influence local voltage peaks.

A central engineering question in large PV installations is how to effectively reduce hazardous touch and step potentials. Meshing is a commonly used mitigation technique, but its application in large-scale PV systems is not straightforward. These systems often rely on rammed steel pipe foundations that already provide distributed grounding paths and can conduct lightning current into the earth. As a result, there are differing views: some argue that no additional meshing is necessary, since “nothing needed” due to the extensive grounding network and relatively low earth impedance, while others expect further measures to still be required.

“To investigate this, we simulated different equipotential bonding configurations and various strike locations, including corner strikes that are typically more critical,” Gauk further explained. “We also varied meshing widths, since traditional building-based mesh densities would be impractical for large PV fields due to cost, construction effort, and environmental impact. The intention was to test whether such invasive measures are truly necessary or whether the existing grounding infrastructure is sufficient under realistic conditions.”

In the paper “Effects of equipotential bonding strategies on the transient step voltage in ground-mounted PV power plants,” published in Electric Power System Research, Jauk and his colleagues explained that they modeled and analyzed lightning-induced step voltages in a PV plant using a commercial simulation tool based on the partial element equivalent circuit (PEEC) method, a full-wave 3D electromagnetic modeling technique that translates electromagnetic field equations directly into the electrical circuit domain.

The PV system model consisted of 15 rows over an 84 m × 84 m area with dense steel pile foundations, with different lightning strike locations being considered to capture worst-case scenarios. Equipotential bonding strategies range from no connection to mesh-like and grading-ring configurations with varying mesh widths between 40 m and 80 m.

Step voltages were computed on a fine grid using the PEEC-calculated surface potentials, and evaluated as the maximum potential difference between opposing points around each location. The impedance to earth was analyzed in both transient and frequency-domain forms, showing that transient impedance is generally higher than frequency-domain values. Corner strikes consistently produce higher surface potentials and step voltages due to faster wave propagation across the grounding network and fewer nearby electrodes.

The analysis showed that equipotential bonding significantly reduces surface potentials and step voltages, but its effectiveness strongly depends on soil resistivity. In low-resistivity soil, safety limits can be met, while in high-resistivity soil additional measures are required. Smaller mesh spacing, meanwhile, was found to improve performance, but meshing alone was insufficient in all cases.

“The simulation results show several consistent trends,” Jauk explained. “Lightning strikes at PV array corners generally lead to higher step and touch potentials. Moreover, a strict 10 Ω grounding resistance limit, as suggested in IEC 62305, is not sufficient on its own to guarantee compliance with the 25 kV step-voltage threshold. Equipotential bonding clearly reduces both grounding impedance and resulting potentials.”

“At this stage, we deliberately neglected frequency-dependent soil effects, even though they are known to be relevant in detailed lightning analyses,” he stated. “This simplification was chosen to isolate the influence of system geometry and grounding configuration, and because including dispersion and frequency-dependent soil parameters is expected to reduce peak potentials. As such, the current results should be interpreted as conservative worst-case estimates.”

Future work will extend the model by incorporating frequency-dependent soil behavior, which is already used in advanced lightning calculations. “The broader research direction aligns with our institute’s focus on electrical power systems, particularly high-voltage substations,” Jauk concluded.