Have you ever been shocked near a swimming pool when you touched a metal fixture, or even in your home when you touched the showerhead fixture? And for dairy farmers, have you noticed a reduction in milk production? These may all be symptoms of stray voltage.
What is stray voltage?
Stray voltage is a small voltage (less than 10 volts as defined by the U.S. Department of Agriculture) that can be measured between two possible contact points. When these two points are connected together by an object, such as a person or an animal, a current will flow. The amount of current depends on the voltage and the circuit impedance, including the source, contact, and body impedances. People and animals respond to the resulting current flow and not to the applied voltage. Sometimes, it just takes a few milliamps to create a mild sensation.
Where does stray voltage come from?
Due to the common grounding of the utility system and the customer electrical system, any neutral-to-earth voltage (NEV) on the utility system can be transferred to any grounded objects in a building, such as metal water pipes. Other possible sources of NEV can be the customer’s wiring system, a neighbor’s wiring system, another utility such as the phone, cable, pipeline, or any combination of the above.
Load, leakage, and fault currents flowing through the impedances of the neutral or grounding conductors to earth, produce NEV. There are multiple paths from the neutral or grounding system to earth such as ground rods, metallic water lines, or other ground electrodes. This means that there is always voltage to earth. Any metallic structure connected to the neutral or grounding system will also be at the same NEV. So, the question is not if there is stray voltage, but what is a safe level.
What is the problem and why is it important?
Because stray voltage is normally related to very low voltage and current, sometimes it is not detectable and therefore not a problem. When people start to get shocked, however, it can become a safety-related issue and if not corrected, has the potential to become a problem.
Stray voltage has become an issue with dairies when cows feel the effects, resulting in lower milk production. This has become an important issue in states like Wisconsin where there are now more installation and detection guidelines available to help customers.
When should I be concerned?
There is no standard for stray voltage, but the U.S. Department of Agriculture recommends that actions be taken to reduce NEV when it is higher than the normal range level of 2 to 4 volts at the service entrance or between contact points. There are on-going efforts to adopt this recommendation as part of the National Electric Code (NEC).
When measuring for voltage and current, you need to stimulate the impedance of what you are measuring for—in the case of a cow, a typical impedance in the range of 500 ohm resistance. The accuracy of the current transformer at very low magnitudes can be an issue when measuring for current. Some handheld volt/ammeters can measure up to 10 amps directly without using a current transformer. Please refer to the U.S. Department of Agriculture for more detail and information on this subject.
How to resolve a stray voltage problem
- Reduce the potential voltage source to an acceptable level. This includes correcting bad neutral connections and removing faulty loads; improving or correcting wiring and grounding; balancing load or controlling leakage current by cleaning, re-insulating, or grounding.
- Use active suppression of the voltage by a nulling device. A current is delivered to a remote grounding electrode to null out the measured voltage.
- Use gradient control by use of equipotential planes and transition zones to maintain the animals’ step and touch potential at an acceptable level. Also, in the National Electrical Code, Article 547, which covers Agricultural Buildings, Section 547.10 Equipotential Planes (EP) and Bonding of Equipotential Planes states that EP shall be installed in all concrete floor confinement areas of livestock buildings that contain metallic equipment that is accessible to animals and likely to become energized.
- For some utility ground-neutral distribution systems, there may be current flowing from the customer grounded neutral system to the utility grounded neutral system. To reduce and
minimize the impact of this, installation of spark gap, saturable reactor, or a solid-state switch can be used.
For more information on stray voltage, please check out the following resources:
- Effects of Electrical Voltage/Current on Farm Animals (PDF) by the U.S. Department of Agriculture
- Equipotential Plane in Livestock Containment Areas by the American Society of Agricultural and Biological Engineers
- The Public Service Commission of Wisconsin has a variety of documents and reports on stray voltage.
- Equipotential Planes for Stray Voltage Reduction (PDF) and Stray Voltage Detection: A Self-Help Guide by the Midwest Rural Energy Council.