The term stray voltage has been used to describe a special case of voltage developed on the grounded neutral system of a farm. If this voltage reaches sufficient levels, animals coming into contact with grounded devices may receive a mild electric shock that can cause a behavioral response. At voltage levels that are just perceptible to the animal, behaviors indicative of perception (eg, flinches) may result with little change in normal routines. At higher exposure levels, avoidance behaviors may result. The term stray voltage is often applied incorrectly to other electrical phenomena such as electric fields, magnetic fields, and most recently, electric current flowing in the earth.
A great deal of research on the effects of stray voltage on dairy cows has been conducted over the past 40 yr. The most sensitive cows (<1%) begin to react to 60 Hz electrical current of 2 milliamps (measured as the root mean square average or rms) applied from muzzle to hooves or from hoof to hoof. This corresponds to a contact voltage level of ~1 Vrms (Vrms = alternating voltage measured as its rms average). As the voltage and current increase, a larger percentage of cows react with behavioral responses that become more pronounced. Numerous studies have documented avoidance behaviors at levels above the first reaction threshold. The median avoidance threshold for 60 Hz current flowing through a cow is ~8 milliamps (4–8 Vrms). This response assumes that the cow comes into contact with objects that have different voltages and that this voltage causes sufficient current to flow through the cow. Even when the threshold is exceeded, all cows do not respond behaviorally all the time, nor do they exhibit the same signs; however, as the voltage increases, signs in the herd become more widespread and uniform. Cows have been shown to resume normal behaviors within 1 day of removal of adverse voltage and current levels. Conditions produced by abnormal behaviors may take longer to resolve but should improve within 1 mo.
In most situations, cows are less sensitive to current and more sensitive to voltage than are people. While the resistance of bovine and human tissues is similar, the contact resistance is generally lower for cows than for people, particularly in wet environments. The resistance of a cow's body plus the contact resistance with the floor is commonly estimated as 500 ohms for a cow standing on a wet floor. Cows standing on a dry surface typically produce ≥1,000 ohms resistance. Cows standing or lying on dry bedding have a resistance many times higher than this. The resistance of a person can be as low as 1,000 ohms for wet hand-foot contact to >10,000 ohms for dry hand-foot contact. The contact voltage to produce sensation can therefore be higher for people than for cows, depending on the conditions of the contact points.
The only studies that have documented adverse effects of voltage and current on cows had both sufficient current applied to cause aversion and forced exposures (ie, the cows could not eat or drink without being exposed to voltage and current), and all of the indirect responses (reduced feed or water intake and milk production) were behaviorally mediated. It is typical for voltage levels to vary considerably at different locations on a farm. Decreased feed and/or water intake or undesired behaviors result only if current levels are sufficient to produce aversion at locations that are critical to daily animal activity, eg, feeders, waterers, and milking areas. If an aversive current occurs only a few times per day, it is not likely to have an adverse effect on cow behavior. The more often an aversive voltage occurs in areas critical to normal feeding, drinking, or resting, the more likely it is to affect the cows. Studies investigating the effects of high frequency or short duration transient voltages on cows clearly indicate that as the duration of a current pulse gets shorter (or the frequency increases), more voltage and current is required to cause a behavioral response.
The main cause of short-duration electrical 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 electrical 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.
Research suggests that swine respond to voltage/current exposure in a way similar to that of cows. Behavioral modification in swine has been observed above 60 Hz exposures of ~5 Vrms with avoidance behaviors at exposures above 8 Vrms. 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 60 Hz exposure levels exceed 5.5 Vrms, while lambs showed this same preferential behavior when exposure levels exceeded 5 Vrms. Exposures to voltages as high as 18 Vrms 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.
No one sign is pathognomonic; a wide variety of signs has been reported in cows exposed to different levels of voltage. Intermittent periods of poor performance, poor milk letdown and incomplete or uneven milk-out, increased milking time, refusal of feed or water, increased somatic cell counts in milk, and increased mastitis are signs often attributed by farmers to stray voltage; however, none of these signs were evident in numerous controlled studies. These signs are often caused by other factors, such as abusive cow handling, faulty milking machines, poor milking techniques and hygiene, and nutritional deficiencies. Therefore, animal behavior or other signs cannot be used to diagnose stray voltage problems. The only way to determine if stray voltage is a potential cause of abnormal behaviors or poor performance is to perform electric testing (see Diagnosis).
The direct effect of animal contact with electrical voltage and the resulting current flowing through the animals' bodies can range from mild behavioral reactions indicative of sensation to intense behavioral responses indicative of pain. The severity of response depends on the amount of electrical 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 signs of excessive voltage exposure have been behaviorally mediated. There are several common situations of concern in animal environments, including 1) changes in drinking behavior, such as decreased number of drinks of water per day and increased length of time per drink; 2) avoidance of certain exposure locations that may result in reduced feed or water intake that may result if painful exposure (>8 Vrms) is required for cows to access feeding or watering devices or locations; and 3) difficulty of moving or handling animals in areas of annoying voltage/current exposure. Signs in pigs, ewes, and poultry are similar, although threshold response levels are higher than for cows.
Studies have been done to investigate direct physiologic effects that may be produced at levels above those that produce behavioral changes, as well as potential detrimental physiologic responses that may result from animals' exposure to voltage/current below levels that may produce sensation and behavioral response. Results have shown that concentrations of cortisol are not increased at levels below behavioral response levels and only become apparent in some, but not all cows, at substantially higher voltage/current exposures than the threshold required for behavioral modification, and typically at levels that produce severe behavioral changes and probably at current levels that produce discomfort and/or pain. Furthermore, the lack of detrimental effects of current exposure on the incidence of mastitis and immune response seen in several experimental and field studies indicate that the levels of voltage/current exposure that elicit behavioral changes do not compromise the immune function of dairy cows.
For confirmation, a potential of 2–4 volt (60 Hz) must be measured between 2 points that an animal might contact (animal contact measurement), and some animals should exhibit signs of avoidance behavior. The animal contact voltage measurement with an appropriate shunt resistor value provides the only reliable indication of exposure levels. Voltage readings at cow contact points should be made with a 500 or 1,000 ohm resistor across the 2 measuring leads to the cow contact points in addition to open circuit measurements. Readings without the use of a shunt resistor are meaningless. Voltage levels should be monitored at different times of the day and on different days, because the threshold level may be exceeded intermittently. When exposure during milking is suspected, measurements should be made with all electrical milking equipment turned on (both 110 and 220 volt). Although levels of 60 Hz exposure up to 2–4 Vrms are not detrimental, farms on which these levels have been detected should be monitored to ensure that higher levels do not occur intermittently, and an investigation by a qualified electrician and/or the local power supplier is advisable.
Point-to-reference ground measurements can be useful for diagnostic purposes. Animal contact measurements are typically ½ to ⅓ of point-to-reference voltage and current levels. A reference ground is established with a 4 ft (1.3 m) copper-clad rod driven into the ground 25 ft (8.5 m) from any grounding rods or electrical equipment. The other contact point is typically the secondary neutral buss in the service entrance panel to the barn or some other part of the grounded neutral system.
Long, insulated meter leads (6–10 ft [2–3 m]) facilitate measurements on the farm and give a reasonable estimate of 60-Hz electric events but introduce considerable noise to higher frequency measurements.
The measurement of high frequency events requires proper equipment and careful measurement technique. Details on measurement techniques are available through electric power suppliers and extension publications.
Prevention and Control
Most on-farm sources of stray voltage are due to wiring systems that do not meet wiring codes and standards. Deficiencies may include loose or corroded connections, ground faults (shorts), undersized wiring, or wiring damaged by animals, accidents, moisture, or corrosion. A licensed electrician should examine the system and repair any defects. Voltage produced by nonfaulty 240-volt equipment usually indicates the distribution system as the source and is the responsibility of the utility company to examine and correct. Electric systems should comply with wiring codes and standards at all times to protect both animals and people.
The first step in a competent stray voltage investigation is to determine the major sources of neutral-to-earth voltage. Any major faults or code violations in the wiring system that could pose an electrocution hazard or are a major source of neutral-to-earth voltage should be corrected immediately. If the wiring systems (farm and utility) are operating correctly, an assessment should be made to determine the most practical, safe, and efficient way to reduce neutral-to-earth voltage. Equipotential planes are effective in eliminating contact potentials even if substantial levels of neutral-to-earth voltage are present.
Last full review/revision July 2011 by Douglas J. Reinemann, PhD