The stray voltage phenomenon has been studied over the past 30 years. Research funded by federal and state agencies, electric utilities, private consultants and others holds the common goal of determining the effect stray voltage has on livestock and farm operations.
Before stray voltage was studied, there were no clear answers to farmers' questions: How does stray voltage affect livestock? What causes stray voltage? Is there a way to prevent it?
Today there are answers, but too often those answers fall short of reaching everyone. To many farmers, stray voltage is still an unknown. Our intent is to offer research findings to farmers — the people who need answers.
One thing is certain: if a farmer looks for stray voltage on his property, chances are good that he will find it. The key is understanding what level of stray voltage does and does not impact his operation. This summary of up-to-date research findings by U.S. government agencies and major universities provides clear, objective answers to questions about stray voltage.
Stray voltage is a natural phenomenon that can be found at low levels between two contact points at any property where electricity is grounded.1
Electrical systems — both farm systems and utility distribution systems — are grounded to the earth to ensure safety and reliability. Inevitably, some current flows through the earth at each point where the electrical system is grounded and a small voltage develops. This voltage is called neutral-to-earth voltage (NEV). When NEV is found at animal contact points, it is frequently called stray voltage.
In other words, stray voltage is this small voltage that is measured between two points that livestock can simultaneously touch. If these points are simultaneously contacted by an animal, a current will flow through the animal.2
Although it may seem unacceptable that small amounts of current can flow through an animal when two contact points are touched simultaneously, most research points to the fact that stray voltage is a natural phenomenon on farms powered by electricity. According to a report by the WPS Commission of Wisconsin, "The occurrence of neutral-to-earth voltage and current flow on neutral and grounding conductors are unavoidable consequences of the use of electrical power. Complete elimination of these phenomena is an unreasonable and costly goal."3
If a farmer tests for stray voltage, he should expect to find low levels of voltage throughout his property.
Herd problems often can be difficult to diagnose. There are many factors to consider, such as the herd's environment, diet and health. Often, stray voltage is considered a last resort when other potential causes have been eliminated.
If stray voltage becomes the suspect, the only way to know if it is a problem is to have the farm tested. "It is not possible to tell with any accuracy if a farm has stray voltage or not without appropriate electrical testing," according to John Roberts, D.V.M., of the Wisconsin Department of Agriculture, Trade and Consumer Protection.4
But it is important for farmers to know that research repeatedly states this: if a stray voltage test is conducted on a property, some stray voltage will almost always be found. The key is understanding what levels of stray voltage do and do not affect an operation.
The mere presence of stray voltage does not necessarily affect livestock. What is important is the level of stray voltage present between two livestock contact points. Much research has been done to determine the level of stray voltage needed to affect dairy cattle in different ways. Following are the key findings:
The USDA has found that the level of electric current that elicits the first signs of awareness by livestock is 1 to 3 milliamps (mA), with no milk production loss in this range.5
Other studies have found the USDA response levels to be conservative and claim that perception at 1 to 3 mA will occur only for a small percentage of cows.6
Moderate behavioral changes in livestock are expected in the range of 3 to 6 mA (at a frequency of 60 hertz).7 An animal may be more difficult to manage when the current exceeds 4 mA. 8
Many studies and field observations have noted that stray voltage can cause changes in animal behavior with no measurable decline in water/feed intake or milk production.
In fact, research has shown no significant changes in feed/water consumption or milk production for currents at or below 4 mA. One study9 measured a decrease in milk production at 5 mA, but all other studies found that currents of 5 and 6 mA had no effect on long-term milk production.10
When current levels were tested at 1.5 times the amount that caused behavioral response, a short-term reduction in water/feed intake and milk production was measured. The researchers continued to test current levels at 1.5 times the behavioral response level applied to water bowls for three weeks. Reductions in water/feed intake and milk production were measured on the first three days of exposure. From the fourth day on, cows resumed their regular feed/water intake and milk production levels for the remainder of the three-week study.11
A 305-day study was conducted to measure feed/water intake and milk production loss during very long-term exposure to stray voltage. Four groups of 10 cows were analyzed and the groups were exposed to 0, 1, 2 or 4 volts. Results showed no significant difference in milk weights, milk composition or feed/water intake between groups not exposed and those exposed to 1, 2 or 4 volts.12
Since the late 1980s, the PSCW has collected data from about 1,700 Wisconsin farms at which stray voltage was suspected. Of these farms, the PSCW found that more than 90 percent had "cow contact currents" — currents flowing when a cow simultaneously contacts two conducting surfaces — less than 2 mA and more than 70 percent had currents less than 1 mA.13
As stated above, stray voltage at 1-2 mA may cause slight behavioral changes, but research has found that feed/water intake and milk production are not affected at these levels.
Electrical systems — both on-farm systems and utility distribution systems — are grounded to the earth. From the grounding points, some current flows through the earth and a small voltage develops. This voltage is called neutral-to-earth voltage (NEV). When NEV is found at animal contact points, it is frequently called stray voltage.14
The NEV measured at any point on the system can come from two sources:
NEV can be due to either or both of these sources. If a farm has 10 service drops, then there are 10 potential secondary sources and one potential primary source.
However, research has shown that secondary NEV is the major source of stray voltage.15
Voltage can be categorized as either steady or transient. Steady voltages are normally caused by current being used to operate an electrical motor and generally run at the same level for several minutes to several hours.
Transient voltages are generally caused when an improperly grounded electrical device is started or stopped or by improperly grounded devices that operate intermittently, such as a fencer. Transient voltages may occur as infrequently as once per day or as frequently as once per second.16 Both steady and transient voltages have common sources that should be checked in a stray voltage investigation.
Since transient currents are more common on farms than steady currents, cows were exposed to the transient current (1 cycle, 60 Hz) whenever they attempted to drink during most of the studies reviewed above.
In practice, however, transient currents are not always present,17 meaning that researchers often provide the worst-case scenario when studying the effect of stray voltage on livestock.
The PSCW and a researcher from UW — Madison joined up to publish a report in 1995 about the cause of stray voltage on farms, using findings from PSCW investigations and investigations by Wisconsin's largest investor-owned electric utilities.
The data show that cow contact current is dependent on many physical factors stemming from both on-farm and off-farm electrical power systems. Specific measurement of (stray voltage) on each farm is required to determine the potential impact on cows on that farm. Because of the wide variation in the data, gross indicators — such as grounds per mile, secondary and primary neutral to reference voltages, etc. — are not good predictors of cow contact currents.18
Following are some attributes of farms and electrical systems that are commonly believed to affect the level of stray voltage, along with research findings that provide answers. Data were derived by the PSCW Stray Voltage Analysis Team's database of stray voltage investigations.
Over the years there have been concerns that the location of the farm along the electric distribution line determines the probability that a farm is affected by stray voltage. Research by the PSCW and the University of Wisconsin — Madison shows that there is no significant relationship between cow contact current and distance from the substation.19
Although circuit theory states that the primary neutral voltage will be reduced as more grounds are placed between the farm and substation, this theory is true in less than one percent of the farms studied. The researchers concluded that there are factors of far greater importance influencing cow contact currents than grounds per mile between the farm and substation.20 In rural areas, new standards require each pole to be grounded when new construction takes place.
Based on 1,089 sites, the primary neutral conductor size explains only one percent of the variation in cow contact currents.21
A trend indicates that higher primary line voltages actually produce lower cow contact current measurements, according to data from 1,089 sites.22
The larger the herd size, the larger the transformer kilovolt ampere (kVA) rating needed to supply power to a farm. The data consists of 854 sites with transformer sizes ranging from 10 kVA to 75 kVA. This is not a good indicator of stray voltage problems.23
Of the cases from the Stray Voltage Analysis Team (SVAT) investigations database where the primary distribution system contributed more than 1 mA of stray voltage, 69 percent were served by an electric cooperative and 23 percent were served by an investor-owned utility. In those cases where on-farm sources caused more than 1 mA of stray voltage, 62 percent of the farms were served by an investor-owned utility and 31 percent were served by an electric cooperative. In those cases where the stray voltage exceeded 1 mA and a combination of the two sources was identified, 69 percent were served by an electric cooperative and 25 percent were served by an investor-owned utility.24
According to a study commissioned by the Minnesota Public Utilities Commission25, some of the common herd problems that farmers mistake for stray voltage are reduced milk production, increased somatic cell counts, reduced water consumption and hocks that are swollen or have sores and abrasions.
There are known causes for these adverse health and production outcomes that are not electrical.
The main risk factors for lowered milk production include:
Water intake is primarily determined by four factors: dry matter intake, milk production, air temperature and sodium intake. According to the study, "Exceptions occur when cows are not able to swallow water because of some physical disability or when the cow cannot reach a water source. Over a period of several days, a cow must consume enough water either through drinking or in forages to maintain a balance in body constituents. A change in water intake can occur whenever one or more of the four major determinants of water needs is altered."
"An elevated SCC in milk indicates that the mammary gland has an inflammatory condition and is shedding more cells than normal in the milk. Inflammation of the mammary gland can be caused by many factors such as traumatic injury or bacteria that originate in the environment or come from another cow through contamination of milking machine equipment, defective milking equipment or improper milking techniques. Improper nutrition can also lead to deficiencies in the immune system which are often associated with high SCC. In addition, poor environmental conditions such as dirty stalls and muddy pastures can produce high levels of contamination in the teat openings," according to the study.
"A lack of cow comfort and rumen acidosis are the primary causes of swollen hocks, abscesses, sore feet and nose bleeding. Small stalls and little or no bedding commonly cause increased standing time and sore feet and swollen hocks. Rumen acidosis typically results from feeding high carbohydrate rations that, in addition to the signs listed above, increase the production of lactic acid, cause a metabolic acidosis, and reduce rumen motility, feed intake and milk production," as stated in the Minnesota PUC study.
As long as electricity is supplied to farms, the issue of stray voltage will exist. Although research has found that stray voltage is rarely strong enough to affect the behavior or production of dairy cattle, many farmers still consider its mere existence to cause herd problems. Today's perception of stray voltage may be compared to the suspicion of the microwave oven when it was first introduced to consumers two decades ago. Microwave ovens were once feared for possibly transmitting dangerous waves but were deemed safe after much research was conducted and results were widely communicated. Stray voltage, too, may be better understood once more and more people learn what little effect it has on livestock today.
Remember, the key to ensuring that your farm is safe from the negative effects of stray voltage is to: