Causes, Concerns and Remediation of Stray Voltages on Distribution Systems

Nuisance shocking — the unpleasant sensation that a person or animal can experience when they inadvertently get between an electrically energized point and ground — can be something of a mystery. Stories are told of cows that won't give milk, dogs that avoid metallic grates and manhole covers, and folks getting an unusual “wake-up call” when they step into their hot tub or backyard pool. However, despite the esoteric nature of nuisance shocking, the science necessary for understanding it — and mitigating it — has advanced significantly in recent years. Also, the various mechanisms that can lead to nuisance shocking are now better recognized. This article provides insights on the utility distribution system aspects of nuisance shocking, first defining the sources of these voltages and then discussion of both traditional and new techniques for mitigating the problem once it has been identified.

Understanding Stray Voltage

To make sense of this sometimes-vague electrical phenomenon, investigators use the following four terms for describing the sources of nuisance shocking: remote earth, neutral-to-earth voltage, metallic-object-to-earth voltage and stray voltage.

Each of these terms is defined in the following paragraphs, along with some insight into their most common causes. With an understanding of the terminology and what causes each condition, useful mitigation techniques can be discussed.

Remote earth is defined as an earthing or grounding point that is at the same voltage potential as other points on the earth in the surrounding area. Electrical current flowing through grounding or neutral conductors, or even through earth itself, will cause voltage variation from point to point. The remote earth point is hypothetically “beyond” or outside of the influence of these current paths. Thus, the remote earth point is at zero potential with respect to the voltage source and provides a consistent and repeatable reference point.

Neutral-to-earth voltage (NEV) is a measure of the voltage potential between a neutral-to-ground bonding point and a remote earth point. In essence, any time current is flowing through a neutral conductor, there will be a voltage potential with respect to the earth. This voltage potential can be metallically transmitted over to a remote earth point through code-required grounding and bonding of water pipes, neutrals and other metallic objects. It is important to note that NEV is a normal occurrence caused by the intentional grounding of the power system.

Metallic-object-to-earth voltage (MOEV) is caused by an accidental or unintentional energization of a metallic object. The most common scenario for MOEV occurs when an energized electrical conductor comes in direct contact with a metallic object, such as a streetlamp, service box, manhole cover or virtually any type of metallic object, thereby energizing the object as well. MOEV potentials to remote earth can range anywhere from just a few volts to 120 V or more, depending on the source. MOEV can also occur when a pipeline or other insulated metallic object is close enough to an electric field from a power line to receive an induced voltage from that electric field.

Stray voltage from intentional actions. The U.S. Department of Agriculture Publication 696 defines stray voltage as “a small voltage (less than 10 V) that can develop between two possible contact points.” Contact points are generally considered to be points close enough between the voltage source and a remote earth path that would allow a current to flow through any human, animal or other object that contacts both points simultaneously. Another important note about Publication 696 is that the document summary states: “While stray voltage cannot totally be eliminated, it can certainly be reduced to an acceptable level.” While this definition focuses primarily on NEV sources, which result from the intentional grounding of the power system neutral conductor, many cases have been identified where improper wiring and load faults on the customer side of the meter contribute to the measured voltages.

Stray voltage from unintentional actions. The New York Public Utility Commission uses the term “stray voltage” to describe the unintentional or accidental energization of manhole covers, street lamps and other urban street-level metallic objects. This definition focuses on MOEV sources such as contact with the power system phase conductor, or in some cases as the result of induced voltages from electric fields.

While the previous two definitions have created some confusion, the common element is that stray voltage could be considered an undesirable voltage potential across any two points that can be simultaneously contacted by an animal or a human. In summary, to develop a stray voltage we can consider the following sources: 1) currents flowing on primary and secondary neutral conductors; 2) faulted-phase conductors; and 3) induced voltages from currents flowing through power lines.

Remediation

There is a distinct difference between the causes of an elevated NEV concern and the causes of an energized metallic object concern. For energized metallic objects, there are obviously no mitigation techniques, as the objective is to identify and isolate the problem and remove or repair the source of the energization. Research is presently underway in this area to develop consistent and repeatable measurement protocols to determine where energized objects are present so monitoring and protection devices can detect the problem using new diagnostic instruments such as pen lights and electric field detectors.

For higher-than-acceptable NEV, the mitigation solutions are well understood but can be challenging to ascertain. Each method has its proven success stories as well as potential shortcomings. Traditional mitigation techniques include: load balancing, resizing neutral conductors, isolation, improved grounding techniques and equipotential planes.

Load balancing. On three-phase, grounded-wye distribution systems with equally balanced 60-Hz phase currents, the net neutral current should be zero. That is, the neutral current from the three phases effectively cancels out. Unfortunately, in the real world, perfect balancing can be upset by many factors such as phase shift, load unbalance and harmonic currents. These phenomena can cause current to flow in the neutral conductor and into the ground rod at each of the neutral-to-ground bonding points, which creates a proportional NEV. Balancing the phase currents can reduce the 60-Hz component of NEV across the entire distribution system. Load balancing at a customer's facility can also reduce NEV, but only at their location.

Resizing neutral conductors. Currents returning on grounded-wye power systems cause a voltage drop across the impedance of the neutral conductor. Because the neutral conductor is grounded, the impedance of the earth return path in parallel with the impedance of the neutral return path dictates the percentage of earth current and the corresponding NEV at that neutral-grounding point. A very simplified way to look at this is to examine a circuit with a current source (neutral return current) and two current paths (neutral path and earth path). All else being equal, current will follow all return paths in proportion to the conducting path impedances. Therefore, reducing the impedance of the neutral effectively reduces the amount of current flowing through the earth path and lowers corresponding NEV at that neutral-to-ground bonding point.

Isolation A simple and effective means of keeping stray voltage from getting to animal and human contact points would be to not jumper the primary and secondary neutral conductors at the service transformer. If there is no metallic connection, there can be no voltage and no opportunity for current flow from the primary neutral. The key considerations in removing the primary to secondary neutral jumper would be the adverse effect on protection of the load-side of the transformer in the event of a lightning strike, system fault or wiring error. Therefore, isolation is best accomplished through the use of a separate isolation transformer or use of neutral isolation products that allow voltages greater than 10 V to be equalized via low-voltage clamping devices.

Improved grounding techniques. In terms of NEV and corresponding stray voltages, it is generally accepted that reducing the impedance of all current return paths — neutral or to earth — can reduce voltage levels. From the standpoint of reducing the voltage at remote earth, any impedance reduction will provide a corresponding reduction in NEV because of a reduced voltage drop, given the same currents.

Equipotential planes. Similar to the ground-reference structures used for computer rooms and the ground mats that minimize step potentials at utility substations, the equipotential plane is a useful means of minimizing nuisance shocking at animal contact points. An equipotential plane typically consists of a conductive wire mesh installed under the area where nuisance shocking has been reported, and attaching most (if not all) conductive materials in the area directly to the mesh. The technique does not reduce NEV levels, but rather moves the problem away from areas where animals are likely to insert themselves into the conducting path.

Innovative Mitigation Techniques

While the traditional mitigation techniques discussed above have been successfully and consistently applied in specific applications, two experimental techniques are noteworthy. These include distribution level harmonic filters and insulated five-wire distribution systems.

Passive harmonic filters A recent area of research into NEV concern relates to triplen harmonic currents flowing on distribution system neutral conductors. These odd multiples of the fundamental 60-Hz current add instead of canceling out on the neutral conductor, thereby creating harmonic NEV levels. The causes of these harmonic currents include harmonic generating equipment owned by the end user and circuit resonances created by distributed capacitor banks. Harmonics due to customer loads is expected to increase over time as more equipment such as variable frequency drive washers and air conditioning equipment proliferate and as more televisions, PCs and other home entertainment equipment use increases.

While filters for harmonic load mitigation is not necessarily a new concept, applying passive harmonic filters to mitigate NEV impacts on the entire utility distribution circuit is new. In under an ongoing research project, EPRI is working with some U.S. electric utilities to determine how well filters tuned to the third harmonic reduce NEV levels on circuits where the neutral-to-earth voltages have been found to be nearly all harmonic in makeup. The approach used is to convert existing distribution capacitor banks to filter banks by employing a combination of capacitors and one or more inductors. Modeling results demonstrate up to a fivefold improvement with installation of harmonic filters in conjunction with grounding modifications. Harmonic filters appear to be a promising way to reduce NEV impacts on the entire utility distribution circuit instead of at the traditional single point of interest.

Five-Wire Distribution System

The so-called “five-wire design” is a concept that has been demonstrated to evaluate the potential to reduce stray voltages and magnetic fields and also make high impedance faults more easily detectable. The first four wires of this system are the familiar three-phase conductors plus a neutral. The fifth wire is a new isolated neutral that carries all of the unbalanced return current, relieving the original neutral of this burden. The multigrounded ground wire continues to perform the safety functions associated with a multigrounded system. The five-wire system was tested by EPRI at New York State Electric and Gas (NYSEG), which converted a section of a circuit in Cooperstown, New York, U.S., to a five-wire configuration. Benefits included reductions in stray voltages and magnetic fields along with promising results that high-impedance faults could be detected. Drawbacks included lack of ability to convert underground systems to the five-wire topology, concerns that an open neutral would cause overvoltages for customer and utility system equipment, and the inability to control stray voltage unless the entire circuit is converted over to the five-wire design.

Ongoing Stray Voltage Research

Although utilities, the government and research organizations have accomplished much to minimize undesirable voltages with respect to remote earth, additional research and development will promote a better understanding of the effects of stray voltages and the methods to minimize these phenomena. To help further this research and to supply credible and unbiased information on the subject, EPRI has identified a number of different concerns and topics of interest to include:

• Animal contact areas' health and productivity concerns
• Residential contact concerns for swimming pools, hot tubs and metallic objects
• Power-circuit resonance and harmonic conditions creating magnified stray-voltage potentials
• Voltages induced onto insulated metallic pipelines
• Metallic pipe corrosion
• Questions regarding impacts of power-line-carrier communication technologies
• Proper specification of measurement equipment, measurement protocols and measurement durations
• Harmonic neutral-current interference with telecommunication circuits and railroad-crossing signals
• Modeling and simulation challenges for complete distribution circuits
• Contributions of transmission lines to elevated NEV levels at human and animal contact areas
• Understanding of comparative costs and benefit of various mitigation techniques
• The energization of metallic objects in urban areas
• Impacts of nonlinear voltage waveforms and voltage magnitudes on humans and animals
• Better understanding of the voltage levels that warrant investigation and remediation.

Of these concerns, the majority can be distilled down into four areas of research related to:

• Measurement devices and measurement protocols
• Modeling and simulation guidelines
• Testing and demonstration of mitigation technologies
• Regulatory information and support.

Work at EPRI is presently ongoing in each of these areas to supplement and support the needs of utilities and end users. The largest challenges to date are not in understanding the issues but in prioritizing the concerns and funding the research. Stray-voltage issues are analogous to power-quality-related concerns in that when problems are identified, limited resources are applied to investigate and resolve the concerns. The IEEE has started a working group under the power engineering society that will be focused on instrumentation and measurement protocols to standardize efforts in these areas. For more information on the ongoing efforts, visit https://strayvoltage.epri.com/.

Douglas S. Dorr is a director of project development at EPRI Solutions Inc. He coordinates EPRI efforts in areas of stray and elevated neutral-to-earth voltage research as well as metallic-object-to-earth voltage research. He is vice chair of IEEE's Surge Protective Devices Main Committee and chairs the Low Voltage AC Surge Protective Device Working Group. He has been involved in the development of more than a dozen standards and currently chairs the 2005 revision to the IEEE Emerald Book Recommended Practice on Power and Grounding Electronic Equipment. Dorr is a senior member of the IEEE and received the BSEE degree from Indiana Institute of Technology in Fort Wayne, Indiana, U.S. ddorr@eprisolutions.com

Ashok Sundaram is a project manager in the Power Delivery and Markets Division at EPRI in Palo Alto, California, U.S. He joined EPRI in April 1993 and has been responsible for management of projects related to power electronics and distribution systems. Sundaram received the BSEE degree from the University of Madras, India, in 1978 and the MSEE degree from Southern Illinois University in 1984. He also has completed his course requirements for his Ph.D. in electrical engineering. Sundaram specializes in power system analysis, electrical machines, control systems, power electronics and power quality. asundara@epri.com