Overview of Stray Voltage in Animal Housing
The term stray voltage describes a special case of voltage developed on the grounded-neutral system of a farm and is defined as <10 volts (measured as the root-mean-square value of 50 or 60 Hz alternating voltage, Vrms) between two points that can be contacted simultaneously by an animal (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. 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). Electric currents flowing in the earth or on grounded metal objects will affect animals only if sufficient animal contact voltage is developed.
If stray 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. Contact voltages <10 Vrms are not lethal to farm animals or people. If neutral-to-earth voltages >10 Vrms are measured on a farm, the safety of the farm wiring system should be carefully evaluated by a competent electrician.
Animals respond to the current flowing through their bodies. 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. Ohm's law indicates that if the voltage across animal contact points is increased, the current flowing through the animal will increase. If the resistance of the circuit is increased, the current flowing through the animal will decrease. The current measurement used in most studies is the milliamp (mA) or 1/1,000th of an amp. Applying Ohm’s law to a circuit with contact voltage of 1 Vrms across a resistance of 500 ohms results in current flow of 2 mA.
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 often 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 estimated as 500 ohms 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. 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 standard measurement circuit used for field investigations uses a 500-ohm "shunt" resistor to simulate the combined resistance of a cow’s body plus a conservative (lowest or worst case) estimate of the resistance of the two contact points (cow + contact resistance).
A great deal of research on the effects of stray voltage on dairy cows has been conducted throughout the past 50 yr. The most sensitive cows (<1%) begin to react to 60 Hz electrical current of 2 mA (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 in wet locations or >2 Vrms in dry locations. Numerous studies have documented that avoidance behaviors occur at exposure levels well above this first behavioral reaction threshold. As the current dose increases above 2 mA, an increasing percentage of cows show mild behavioral responses, and some cows start to show avoidance behaviors. The median avoidance threshold for 60 Hz current flowing through a cow is ~8 mA (or 4 Vrms contact voltage in wet locations and >8 Vrms in dry locations). 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 resumed normal behaviors within 1 day of removal of aversive voltage and current levels.
The only studies that have documented reduced water or feed intake in cows had both sufficient current applied to cause aversion 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 aversion at these locations and they occur often enough to interfere with drinking and eating behaviors. 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 aversive current exposures occur in areas critical to drinking or feeding, 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 behavioral responses. The main cause of 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.
Research suggests that swine respond to voltage/current exposure in a way similar to that of cows. Behavioral modification in swine has been seen above 60 Hz exposures of ~5 Vrms, with avoidance behaviors at exposures >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, whereas 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.
A wide variety of behavioral signs have 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 environment. The only way to determine whether stray voltage is a potential cause of abnormal behaviors is to perform electric testing.
The direct effects of stray voltage 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 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.
Results of studies to investigate direct physiologic effects that may be produced at levels both above and below those required to produce behavioral responses have shown that blood cortisol concentrations and other stress hormones do not increase at levels below behavioral response levels. Increases in stress-related hormones have been documented in some, but not all, cows at voltage/current exposures substantially higher than the threshold required for behavioral modification. A large body of research clearly indicates that at levels of voltage exposure typically used as regulatory limits (1 volt at cow contact locations, or 2 mA of current flow through a cow) will not result in increased somatic cell counts or incidence of mastitis.
Several common situations are 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 locations that may result in reduced feed or water intake; and 3) difficulty of moving or handling animals in some areas. If these behaviors are seen, electric testing indicating cow contact exposure in excess of 4 mA (2 Vrms in wet locations with a 500 ohm cow + contact resistor, or 4 Vrms in dry locations with a 1,000 ohm cow + contact resistor) is necessary to confirm a stray voltage diagnosis. Signs in pigs, ewes, and poultry are similar, although threshold response levels are higher than for cows.
It is critically important to use a realistic value of animal resistance to relate voltage exposures to the level of current conducted through an animal and the resulting effects on nerve stimulation, sensation, and behavioral reaction. A competent field investigation will include voltage measurements at cow contact locations both with an appropriate "shunt" resistor (cow contact voltage) and without a shunt resistor (open circuit voltage). The shunt resistor is meant to represent the resistance of the body resistance of a cow plus the contact resistance representative of the exposure location. Both animal-contact and open circuit voltage measurements are required to determine the "source resistance" of the electric parts of the circuit. Note that source resistance is different from the cow contact resistance and can be used as a diagnostic to determine the voltage source.
Animal-contact voltage levels should be monitored at different times of the day and on different days to represent the normal operating conditions of the farm electrical system. Point-to-reference ground measurements can be useful for diagnostic purposes. A reference ground can be established with a copper-clad rod driven into the ground at least 25 ft (8.5 m) from any part of the grounding network on the farm. The other contact point is typically the ground-neutral interconnection in the barn service entrance panel or some other part of the grounded-neutral system. Cow contact measurements are typically ½ to ⅓ of point-to-reference voltage levels.
Long, insulated meter leads (6–10 ft [2–3 m]) facilitate measurements on the farm and give a reasonable estimate of power frequency (60 Hz and 50 Hz) 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.
Off-farm sources of stray voltage are most often related to improper function of the grounded-neutral system of the electric utility. On-farm sources of stray voltage are most often 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.
The first step in a competent stray voltage investigation is to determine the source of neutral-to-earth voltage. 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 electrician should assess the situation to determine the most practical, safe, and efficient way to reduce neutral-to-earth voltage. 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.