The term stray voltage describes a special case of voltage developed on the grounded-neutral system of a farm. According to a USDA publication, “Stray voltage is a small voltage (less than 10 Volts) measured between two points that can he contacted simultaneously by an animal. Because animals respond to the current produced by a voltage and not to that voltage directly, the source of the voltage must be able to produce current flows greater than the threshold current needed to elicit a response from an animal when an animal, or an equivalent electrical load, contacts both points.”
Contact voltages less than 10 volts are not lethal to farm animals or people in normal farm contact situations. Neutral-earth voltage levels >10 volts may be indicative of a fault in the farm electrical system and require careful evaluation by a competent electrician to identify and mitigate any potential safety hazards.
Because stray voltage is defined as a voltage developed between two points that can be contacted simultaneously by an animal, it is also referred to as animal contact voltage. The grounding and neutral systems on a farm or in a home wiring system should be properly bonded to ensure electrical safety. As a result, some level of voltage between the grounded-neutral system and the earth (neutral-to-earth voltage) is always present as a normal consequence of the operation of properly installed electrical equipment.
Animals respond to the current flowing through their bodies rather than the contact voltage. Stray voltage is therefore also referred to as stray current. Ohms law describes the relationship between voltage exposure and current conducted through an animal: [current = voltage / resistance] or [Amps = Volts / Ohms]. This simple relationship has been a source of much confusion and resulting controversy (see Diagnosis). The current measurement most commonly used is the milliamp (mA), or 1/1,000th of an amp. Applying Ohm’s law to a voltage difference of 1 volt across a resistance of 500 ohms results in current flow of 2 mA through the resistor.
The term stray voltage is often applied incorrectly to other electrical phenomena, such as electric fields, magnetic fields, electric current flowing in the earth (earth currents), or electric current flowing on a grounding conductor (ground currents). The 1998 general finding of the Science Advisors to the Minnesota Public Utilities Commission was that "We have not found credible scientific evidence to verify the specific claim that currents in the earth or associated electrical parameters such as voltages, magnetic fields and electric fields are causes of poor health and milk production in dairy herds."
Over the past 70 years, a great deal of research on the effects of stray voltage on dairy cows has been conducted. A variety of behavioral responses has been reported in cows exposed to voltage and current. No one sign is pathognomonic, however, because these behaviors can also be caused by other factors in the animal's environment. The only way to determine whether stray voltage is a potential cause of abnormal behaviors is to perform electrical testing.
At current exposure levels that are just perceptible to animals, behaviors indicative of perception (eg, flinches) may result, with little change in normal routines. At higher exposure levels, avoidance behaviors may result. The severity of response depends on the amount of electric current flowing through the animal’s body, the pathway it takes through the body, and the sensitivity of the individual animal. The indirect effects of these behaviors can vary considerably depending on the specifics of the contact location, level of current flow, body pathway, frequency of occurrence, and many other factors related to the daily activities of animals. All of the documented effects of excessive voltage exposure have been behaviorally mediated.
The most sensitive cows (<2%) begin to show mild behavioral responses for exposures of 2 mA of current applied from muzzle to hooves or from hoof to hoof (~1 volt in wet locations or >2 volts in dry locations). For muzzle-hooves exposure of 4 mA (2 volts cow-contact voltage in wet locations or 4 volts in dry locations), approximately 30% of cows will show behavioral responses but not avoidance behaviors. Avoidance behaviors begin to be exhibited for the most sensitive 2% of cows at muzzle-hoof exposure of about 5 mA.
Increased levels of stress-related hormones have been documented in some, but not all, cows at voltage/current exposures substantially higher than the threshold required to elicit avoidance behaviors. There have been no studies in the large body of research that support the hypothesis that voltage exposures of up to 8 V will result in increased somatic cell counts or incidence of mastitis.
Swine respond to voltage/current exposure in a way similar to that of cows. Behavioral modification in swine has been observed for exposures of greater than 5 volts, with avoidance behaviors at exposures greater than 8 volts. The body plus contact resistance for swine appears to be somewhat higher than for cows, and 1,000 ohms appears to be a conservative value for measurement purposes. Ewes have been shown to avoid electrified feed bowls when exposure levels exceed 5.5 volts, whereas lambs showed this same preferential behavior when exposure levels exceeded 5 volts. Exposures to voltages as high as 18 volts had no effect on hens’ production and behavior. This is likely because of the very high electrical resistance of poultry, which has been documented to be between 350,000 and 544,000 ohms.
If animal contact voltage reaches sufficient levels, animals coming into contact with grounded devices may exhibit avoidance behaviors. At watering locations, these behaviors manifest as decreased number of drinks of water per day along with increased length of time per drink. Exposures at feeding locations are uncommon unless metallic feed bowls are used. Hoof-hoof exposures at building transitions may result in reluctance to move in or out of buildings or building sections. If avoidance behaviors are observed, the application of industry-recognized electrical test methods is necessary to confirm a stray voltage diagnosis.
Standardized test protocols have been developed to determine stray voltage exposure levels at locations that are accessible to animals. Measurements at animal contact locations are required to determine the level of voltage/current that animals may experience. Industry-recognized field investigations include voltage measurements at animal contact locations (cow contact voltage) with an appropriate "shunt" resistor to estimate current exposure levels (animal contact current). The standard method to estimate the amount of electrical current flowing through an animal uses a 500-ohm shunt resistor across animal contact points to represent the resistance of a cow’s body plus the contact resistances at the muzzle and hooves. This is meant to estimate the current flowing across the muzzle-to-hooves pathway for a cow standing on a wet floor. Cows standing on a dry surface typically have body plus contact resistance of ≥1,000 ohms. Cows standing or lying on dry bedding have a resistance many times higher than this.
Industry standard protocols will also include measurements that allow for determination of the source resistance in the animal contact circuit as well as the contribution to animal contact voltage from both on- and off-farm sources. Neutral-to-earth voltage measurements are useful for diagnostic purposes. Animal contact measurements are typically about one third of neutral-earth voltage levels. Animal-contact and neutral-to-earth voltage levels are typically monitored for 24 hours or more to capture the range of normal electrical system operating conditions on a farm.
All of the voltage and current levels cited above are for steady-state 60 Hz alternating (AC) voltage/current reported as root mean square (rms) average values. As the AC frequency increases or the duration of a current pulse gets shorter, more voltage and current are required to cause the same response. The measurement of high-frequency events requires specialized equipment and careful measurement technique consistent with industry-recognized stray voltage testing methods.
The main cause of high-frequency/short-duration electric pulses on farms is improperly installed electric fences and electrified crowd gates. These devices are designed to produce a powerful electric impulse that is used to control animal behavior. Improper installation of these devices can cause these pulses to appear in unintended areas on the farm. The other common source of high-frequency events is a switching transient that occurs when electric equipment is turned on or off. These high-frequency pulses decay quickly and do not travel far from their source, and it is extremely rare for them to reach problematic exposure levels.
The best way to prevent stray voltage problems is to ensure that the farm's electrical system meets electrical code requirements. Equipotential planes are required at critical animal contact locations in animal confinement facilities and effectively eliminate contact voltage even if substantial levels of neutral-to-earth voltage are present. Additionally, all farm equipment requires maintenance and repair when damaged, and the farm electrical system is no different. Periodic inspection and testing of voltage/current levels by the electric power supplier or a qualified electrician is recommended, especially when there are changes to the farm's electrical system.
Most states and provinces use 2 mA at cow-contact locations as the level of concern for stray voltage exposure. This 2 mA standard is further defined to include a standardized measurement of 1 volt of steady-state 60 Hz AC voltage/current measured across a 500-ohm resistor at animal contact locations. It is recognized that this exposure level is a conservative, preventive level, below the point at which moderate avoidance behavior is likely to occur and well below the level at which a cow's behavior or milk production would be affected. This level is not a damage level. As stated in the USDA Stray Voltage publication, “Recent research indicates that current levels below 6 mA have no direct effect on production, reproduction, or animal health; furthermore, there is no evidence that hormones naturally released during milking and stress are adversely impacted by elevated current levels. Some moderate behavioral changes are seen in cows exposed to currents of between 3 mA and 6 mA.”
The summary recommendation from the USDA is that contact voltages should be kept below 2–4 volts (steady-state 60 Hz measured across a 500-ohm resistor) on dairy farms to avoid any adverse effects on animal behavior, milk production, and animal health.
The only studies that have documented reduced water or feed intake in cows had both sufficient current applied to cause avoidance behavior and forced exposures (ie, cows could not eat or drink without being exposed to aversive voltage and current). It is typical for voltage levels to vary considerably through the normal daily operation of electrical equipment on a farm. Decreased feed and/or water intake will result only if current exposure levels at watering and feeding locations are sufficient to produce avoidance at these locations and they occur often enough to interfere with drinking and eating behaviors. If an avoidance level current occurs only a few times per day, it is not likely to have an adverse effect on cow behavior. The more often avoidance-level current exposures occur in areas critical to drinking or feeding, the more likely it is to affect the cows.
Common deficiencies in farm electrical systems include:
Faults or electric code violations that could pose an electrocution hazard should be corrected immediately to protect both animals and people. If stray voltage levels are excessive, a competent investigator should assess the situation to determine the most practical, safe, and efficient way to reduce animal contact voltage.
The causes and solutions for stray voltage are well understood, as are the effects of electrical exposure on farm animals.
Research shows that dairy herds display no adverse responses when cow contact voltage levels are below 2 volts (60 Hz rms) in low-resistance environments or 4 volts (60 Hz rms) in typical farm environments. These voltage exposure levels are equivalent to 4 mA of current passing through a cow.
Investigative techniques are well-developed, and the vast majority of stray voltage problems will be solved by applying accepted codes and practices on farm wiring and electrical distribution systems.
Effects of Electrical Voltage / Current on Farm Animals: How to Detect and Remedy Problems. USDA-ARS Agriculture Handbook No. 696. AM Lefcour, editor, 1991.
MINNESOTA STRAY VOLTAGE GUIDE: A Guide Addressing Stray Voltage Concerns, 2015.
Stray voltage and milk quality: a review. D.J. Reinemann, Clin North Am Food Anim Pract. 2012 Jul;28(2):321–45. doi: 10.1016/j.cvfa.2012.03.008