Stray voltage is an unwanted electrical potential difference in the environment, often found around equipment or grounding systems in dairy farms. This phenomenon is dangerous for animals because even very small unintended electrical currents can pass through the ground, metal equipment, or water sources in a stable and be picked up by an animal’s body.
Livestock such as dairy cows are especially sensitive because they frequently stand on damp concrete floors and have large contact areas with milking machines, water bowls, or metal stalls, which can complete an electrical path. Cows are more sensitive that other animals due to their body mass. Because they are large animals, they often have greater total contact with conductive surfaces, which increases the likelihood that small voltage differences can drive current through their bodies.
When stray voltage is present, an animal may receive a mild but repeated electric shock when it drinks, eats, or is milked. This can cause pain, stress, and anxiety, leading to behavioral changes such as reduced feeding, avoidance of water, and reluctance to enter milking parlors. Over time, this stress can lower milk production, affect growth, and increase the risk of health problems.
Barn incident
Swiss broadcaster RTS recently reported that around 300 cows died in the canton of Vaud, with stray voltage suspected to be the cause in a barn where a PV system had been installed. The report indicates the issue is more likely linked to faulty or poorly designed electrical installations rather than the solar panels themselves. The case was also taken up by the right-wing Swiss People’s Party, which submitted a motion to the National Council calling for a temporary moratorium on the installation of solar panels on barns.
“So far, PV can be excluded as a cause in this specific case,” Swiss PV association Swissolar told pv magazine. “Stray currents can occur with all electric installations, like milking machines. The cause must be analyzed and eliminated in each case. We have heard of other cases in Switzerland, none of them as serious as the one mentioned in RTS.”
Studies suggest that a current level of around 2 mA can sometimes trigger behavioral responses such as startled movements, reluctance to drink, or avoidance of specific areas of a barn. However, according to the MSD Veterinary Manual, only about 2% of cows show mild behavioral responses at this level, with the proportion increasing to approximately 30% at 4 mA muzzle-to-hoof exposure, although these responses do not necessarily translate into broader health or productivity effects.
Vet perspective
“I have never encountered any cases related to solar power units,” Ronald J. Erskine, the author of the stray voltage entry in the MSD Veterinary Manual told pv magazine. Erskine, a professor at the Michigan State University College of Veterinary Medicine, explained that solar panels generate DC current, while AC which is the current that cows are most sensitive to. “Installing a solar system is not more dangerous then installing a new milk pump or water heater in a barn. One just need to make sure that the inverter, which transforms the DC to AC current, is properly wired and grounded by a qualified electrician,” he added.
Erskine, which also represents defendants in stray voltage cases as an expert witness, says that the true occurrence of the phenomenon is very low. “Stray voltage is sometimes blamed for problems that may actually have other causes,” he said. “It's important to conduct a careful differential diagnosis – essentially detective work – especially when dealing with complex, multifactorial issues such as lower milk production or lameness.”
However, he still emphasized that even if rare, stray voltage does exist and in certain cases has caused problems in cattle. “My experience is that for most stray voltage cases, if the cows are exposed to levels of AV voltage above 2 volts, there is often an issue with the wiring and grounding, especially of the water system. Another cause occurs in retrofitted old barn, where the floor wasn't properly grounded for step potentials.”
In 2021, a Dutch field study investigated reports of unusual grouping behavior among dairy cows in the Netherlands, with farmers frequently suspected causes such solar panels, high-voltage lines and automated milking systems, or stray voltage. After comparing 31 affected farms with 62 control farms, the researchers found associations between the behavior and recently built barns, the presence of fans, and reports of stray-voltage investigations. However, causality for these risk could not be proven.
“In our study we did not find an association with solar panels on the roof. Farms with cows with abnormal behavior were equally likely to have solar panels on their roofs as farms where cows behaved normally,” said corresponding author of that research Gerdien van Schaik, a Professor in Veterinary Epidemiology in Utrecht University. “However, that does not mean that solar panels can never cause stray voltage problems.”
According to the owner of Elektro-Solar(Munich) PTIA Consultant(Phnom Penh), Andreas Iliou, the animal health issues associated with stray voltage and current are not caused by PV modules themselves, but by the broader electrical infrastructure, including grounding design, bonding, and interactions between multiple systems such as inverters, milking machines, and building earthing networks. “Inadequate, corroded, or inconsistently implemented grounding configurations may create voltage gradients across conductive surfaces, which can under certain conditions allow small leakage currents to pass through animals acting as conductive paths,” he told pv magazine.
The PV expert further emphasized that modern agricultural buildings often rely on highly interconnected metal structures and reinforced concrete foundations, which are intended to create equipotential zones. However, when additional electrical systems such as PV arrays are introduced without careful system integration, the overall balance of the earthing network may be altered. In such cases, currents may redistribute through unintended paths, particularly where moisture and metal contact increase conductivity.
Other cases
“I have experienced some cases of cow health issues associated with stray voltage in Israel, Ireland, Germany and Switzerland,” Iliou said. “But in the Israel case, the issue already existed before the PV system was installed. A house located around 50 m to 100 m from the milking parlor had a corroded grounding system, which allowed small leakage currents to travel through the farm’s electrical network toward the cowshed. Because all household appliances and electrical equipment were still functioning normally, the problem initially went unnoticed.”
According to the expert, the situation changed after PV inverters were added. While small leakage currents may remain relatively limited in conventional installations, each inverter can introduce additional leakage current depending on system size and design. In larger PV systems, this can significantly increase the amount of current circulating through the earthing network. “If the grounding system is not properly engineered, these currents may follow unintended paths through conductive structures connected to the milking parlor, including metal flooring grids and reinforced concrete foundations used as equipotential grounding systems,” he explained.
The expert also described a case in Ireland involving a livestock shed fitted with rooftop PV. According to his account, the grounding system had not initially been properly implemented, and later modifications focused mainly on the PV array itself without fully assessing the building’s underlying conductive structure, including reinforced concrete flooring and metal support pillars.
He explained that many modern livestock buildings rely on large steel or iron frames that are interconnected with the earthing system. In this case, leakage currents associated with the electrical installation were believed to travel through the metal pillars and structural beams into the ground. The areas most affected appeared to be sections of the shed where cows typically gathered to rest before returning to the milking parlor or sleeping areas.
The expert said he observed that cows consistently avoided certain corners of the building, which he associated with higher current concentrations around the structural supports. According to his interpretation, conductive metal beams and grounding paths may under some conditions create localized areas with elevated electrical activity if the overall earthing system is not properly balanced and monitored.
Iliou also pointed to the increasing use of stainless steel drinking troughs in modern livestock facilities. While these systems are often adopted for hygiene reasons, he suggested that conductive metal surfaces may under certain conditions make animals more sensitive to small voltage differences if grounding systems are not properly balanced. In such cases, cows may become reluctant to drink, enter milking parlors, or remain in specific sections of the barn.
The expert further noted that moisture can amplify these effects by lowering electrical resistance. In the Israel case, cooling systems using water sprays and large ventilation fans were introduced to improve animal comfort during hot weather. However, he claimed that wet conditions may also increase conductivity and make low-level electrical sensations more noticeable to animals already exposed to grounding irregularities. According to him, some cows subsequently showed signs of stress or avoidance behavior in affected areas of the facility.
Proper PV deployment
Iliou stressed that PV systems themselves are not inherently problematic, but they introduce additional electrical components, including inverters and long cable runs, that must be carefully integrated into the building’s grounding network. He argued that improper cable routing, inconsistent grounding practices, or insufficient equipotential bonding may contribute to elevated electromagnetic fields or unintended leakage current paths within the structure.
“Stray currents can originate from many conventional farm systems unrelated to PV, including milking pumps, lighting circuits, ventilation equipment, and variable-frequency drives used in modern milking systems,” he said. “These devices continuously modulate electrical loads and may under certain conditions introduce harmonic currents or leakage currents into the earthing system.”
In his view, the key issue is ensuring that all conductive elements within the barn remain at the same electrical potential through properly designed mesh grounding systems and coordinated earthing layouts. He warned against practices such as disconnecting grounding conductors to reduce measured leakage values, arguing that this may simply redirect currents through alternative conductive paths rather than eliminate the underlying issue.
The expert further claimed that some farmers in southern Europe and the Middle East use water-spraying systems to cool PV modules and improve energy yield during hot weather. While this practice may increase module performance under certain conditions, he suggested that moisture can also increase conductivity around electrical structures and grounding systems, potentially amplifying existing grounding deficiencies if installations are not carefully engineered and maintained.
He also referred to separate cases involving horses housed in buildings with rooftop PV installations. In those situations, he attributed unusual animal behavior to elevated electromagnetic fields generated by cable layouts and inverter configurations rather than to the PV modules themselves. Modifying cable routing and improving grounding reduced these effects and appeared to change how the animals used different parts of the stable. “Achieving a properly balanced system is technically demanding and requires careful planning and coordination of all conductive elements within the building,” he stressed.
Broad perspective
The PV expert also referred to historical cases involving stray voltage in urban infrastructure. He described situations in which railway systems, nearby watercourses, and building grounding networks were interconnected, unintentionally allowing electrical currents to spread through earthing systems and conductive utilities. In some older installations, metallic water pipes had been used as part of the grounding system, leading to significant current flow between buildings.
He explained that these systems were later redesigned after incidents involving corrosion and unintended electrical shocks during maintenance work on water and electrical infrastructure, noting that such experiences led to stricter rules prohibiting the use of water pipes as grounding electrodes and encouraged the development of dedicated earthing systems in building foundations.
The expert concluded that these historical examples illustrate how interconnected grounding networks can unintentionally carry significant currents across buildings and infrastructure. In his view, similar principles apply today in agricultural PV installations, where multiple conductive systems must be carefully integrated to avoid unintended current pathways.
Recommendations
“Installing PV systems on stables is absolutely possible, but it requires careful design. It’s not something to be done casually, because there are multiple interacting electrical and grounding effects that can impact both the installation and the animals,” he said. “When PV systems are added to a barn, the situation becomes more complex. The entire electrical environment of the site changes.”
Another contributing factor is design inconsistency. Some installations separate positive and negative conductors physically or modify grounding practices in ways that do not necessarily reduce leakage but instead redistribute it through different paths. “This can unintentionally worsen local field effects rather than solve them,” Iliou highlighted. “The correct approach is not to remove grounding, but to improve it. A properly designed PV installation on a stable should use a consistent equipotential bonding system—essentially a well-designed mesh grounding structure that connects roof, inverters, metal infrastructure, and earth into one controlled system.”
In many cases, this does not require expensive upgrades. Often it can be achieved using existing surplus cabling and minor additional connections during installation or maintenance visits. The cost impact is typically small compared to the overall system cost, but the effect on system stability can be significant.
The industry challenge is that PV technology evolves quickly, especially inverter and power electronics design, while installation practices and cabling standards often lag behind. This mismatch can lead to outdated grounding concepts being applied to modern systems.
“So my conclusion is not that PV systems should be avoided on stables,” Iliou said. “Rather, they should be installed with proper attention to grounding, equipotential bonding, and system-wide electrical behavior. With the right design, the risks can be managed and the system can operate safely for both production and animal welfare.”
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