Electrical professionals work toward universal definitions and testing and mitigation techniques for stray and contact voltages
Responding to testing requirements imposed after a woman was electrocuted while walking her dog in the East Village in 2004, this March New York City's Consolidated Edison found 1,214 instances of stray voltage during a year-long examination of electrical equipment on city streets. The stray voltage was detected from December 2004 through November 2005 on 1,083 streetlights, 99 utility poles, and 32 power-distribution structures like manholes, service boxes, and transformer vaults, according to test results submitted to the state's Public Service Commission, which regulates utilities. In total, 728,789 pieces of equipment were tested, which was required under state rules adopted in January 2005 — a year after the woman, Jodie S. Lane, a 30-year-old Columbia University graduate student, was killed when she stepped on a metal plate.
In all, Con Ed expects to spend $100 million this year toward reducing the risk of stray voltage. Yet, incidents involving shock and electrocution persist in the city. On Feb. 12 of this year, four people were shocked after a frayed cable energized the cover of a service box near the Port Authority Bus Terminal. Five days later, a dog was electrocuted on a patch of concrete in Park Slope, Brooklyn, where Con Ed had continued to supply electricity to a nonexistent streetlight. In March, a nine-year-old boy was briefly hospitalized after he reported an electric jolt while walking over a metal plate at West 127th Street and Lenox Avenue in Harlem. But is this a matter of “stray voltage”? Despite the fact that electrical professionals say otherwise, The New York Times, the New York City Council, and even representatives of Con Ed insist these incidents are a matter of stray voltage.
“The real confusion lies with faults on manhole covers where you've got real electricity — not just a couple of volts — that can shock dogs and humans,” says Douglas Dorr, power systems engineer for Electric Power Research Institute (EPRI) Solutions, Knoxville, Tenn. “It's not stray voltage: It's contact voltage.”
Defining terms. Under the strictest definition, the term “stray voltage” is used to describe persistent, elevated levels of neutral-to-earth voltage that do not exceed 8V and are not lethal. Stray voltage originates from an electric utility power supply system when the current running on the grounded neutral uses the earth as a return path to the substation, resulting in ground currents that cause voltage gradients in the earth's surface. This is mostly due to return currents on unbalanced loads. In addition, voltage between the neutral conductor and the earth increases with increased distance from the substation. Stray voltage may also originate on a utility customer's premises. When a neutral conductor is grounded at more than one location, the electric current will split. Part of the current will stay in the neutral wire, while the other part of the current flow generates elevated voltage levels on building components, piping, and other conductive structures, causing a ground loop.
The most common type of stray voltage exposure occurs in rural areas when a “nuisance shock” is delivered to confined livestock that become part of an electrical pathway through physical contact with a contact point. This situation has resulted in decreased milk production and other behavior abnormalities among the livestock. There are also increased reports of stray voltage affecting backyard swimming pools and spas, fences, appliances and equipment housings, and water faucets in suburban areas, as well as corruption to instrumentation and control systems at hospitals, industrial plants, and control centers. Other industries characterize similar incidents in their fields as stray voltage: Aerospace electro-magnetic interference (EMI), electronic loops and noise, and microshock hazards in the medical sector.
Even for electrical professionals, the terms can be confusing. According to Dorr, contact voltage occurs when a human or animal comes into contact with two points of different voltage potential. When that occurs, it pushes a current through the body. Under Dorr's meaning, stray voltage falls under the umbrella of contact voltage.
In simpler terms, Dorr says that contact voltage is when an energized conductor is causing the voltage; not stray voltage. “If you've got ‘neutral-to-earth’ voltage, where the power system neutral — not the energized conductor — creates this contact voltage problem, that can be a stray voltage,” he explains.
Under Dorr's definition, contact voltage usually originates with the electric utility, but in less common cases the problem can arise from a simple household appliance. “It could happen if you touched some frayed wires going to your toaster or if you have bad insulation on a wire in your house,” Dorr says. “There are a lot of ways that contact voltage can happen, but I don't think any of them are common. There're a lot of situations where we have a stray voltage, but it's not large enough in voltage to cause any problems for anybody. It's almost never 0V though.”
To clear up some of this confusion regarding the definitions of these phenomena, members of the Institute of Electrical and Electronics Engineers, Inc., (IEEE), Piscataway, N.J., have formed a Working Group on Voltages at Publicly and Privately Accessible Locations. The impetus for starting the group was to work toward a more universal, or national, language for speaking about stray voltage versus contact voltage.
“Contact voltages are related to electrical faults that haven't cleared that are potentially hazardous depending upon the exposure specifics,” says Chuck DeNardo, principal engineer for We Energies, Milwaukee, instructor for the University of Wisconsin Extension Stray Voltage Investigator Training courses, and the working group's committee chair. “Because the media was talking about stray voltage in terms that weren't consistent with how it was being used in the rest of the country and because there's been some legislation as well as a public service commission and regulatory activity referring to this more hazardous voltage as “stray voltage” — because of that confusion — it was felt that we needed to create some definitions explaining what the differences are between these types of voltages.”
According to DeNardo, the stray voltage of the backyard swimming pool incidents are fairly consistent with the situation experienced by confined livestock, with regard to the sources of the voltage and the exposure mechanisms. The charged manhole covers, however, are an entirely different story. “That's essentially a high-impedance fault that could be extremely hazardous,” he says. “Because this is no longer just a Midwest issue — it's a national issue — and the term ‘stray voltage’ has different meanings for different people in different parts of the country, we have to have some sort of a national dialog to decide exactly how we're going to define ‘stray voltage,’ or even if we're going to use the term ‘stray voltage’ to define those electrical potentials that exist in publicly and privately accessible locations. Our job in this committee is to put together a guide that talks about what the proper instruments are to measure for them, what those measurement protocols might be, and what the mitigation measures might be.”
Measurement instruments and protocolsIEEE isn't the only organization concerned with measurement protocol. “We would like to have a consistent and repeatable way to actually measure or quantify the concern,” Dorr says. “If you're looking at these problems that are 2V, it's kind of hard to measure it because that's a pretty low voltage. To do it correctly, there are some procedures that you need to take into consideration. And if it's a 120V source or other high voltage, there are some safety concerns.”
One way to test for stray voltage, according to the “Protocols and Practices for Stray Voltage Testing,” published by the Agricultural Engineering Department at Michigan State University, is to apply a known burden load. According to the report, a voltmeter can be connected neutral-to-earth at the building end of the conductor. Under Ohm's law, current times resistance yields voltage. If there is excessive resistance at some point along the neutral conductor, the voltmeter will register a significant change in the level of neutral-to-earth voltage when the burden load is applied. This type of test can also be applied to a utility primary distribution system neutral.
A digital multimeter (DMM) with the capability to store events can also be used to measure stray voltage. The meter can be set up at the site to record the electrical activity over an extended period of time, without anyone having to watch the meter or write down the readings. This data can be saved and viewed at the test site on the DMM display or downloaded to a computer where it can be viewed or printed out graphically and numerically. The test setup consists of selecting the AC volts measurement function, connecting the common test lead to an isolated ground rod away from farm buildings or other location to be tested, and then connecting the volts measurement lead to a point of interest. Depending on the application, the connections can be made between any two points of interest.
Mitigation techniques. Once the voltage is measured and is determined to be a problem, the next step is mitigation. However, with issues of stray voltage, sometimes there isn't one clear answer. “Most of the time, there are a dozen different solutions,” Dorr says. “You've got to know which one to use in which situation. If it's coming from the utility system, there's one solution. If it's coming from equipment in the house, that's another solution.”
The one reliable way to control stray voltage is to design the electrical system to prevent ground loops, as well as stop corruption from outside sources. AC and DC isolating devices can be used to isolate transformer primary and secondary neutrals to prevent stray voltages to the customer neutral from the utility system. With regard to swimming pools, the electrical contractor installing the system should inspect the electrical bonding grid to make sure it's intact and meets the requirements of the National Electrical Code. If the meter identifies a problem under the deck or pool shell, concrete may be chipped away to expose the steel. A clamp is then attached to the steel, and a bonding conductor is installed from the clamp to the existing bond grid, reconnecting the spot back into the grid.
Grounding the neutral in only one place may also help control stray voltage, although in some cases it may only worsen the problem. According to Jim Burke, a fellow of the IEEE and an executive consultant with Synergetic Design in Raleigh, N.C., and Keary Dossier, an engineer with the same company, in “Stray Voltage: Know Your Terms,” there are several ways to ground the primary distribution system, such as the 4-wire multi-grounded, 5-wire system, 4-wire uni-grounded, 3-wire uni-grounded, and the ungrounded wye and delta systems. Each of these systems has its pros and cons. “Sometimes good grounds are beneficial, sometimes they are a problem source, and sometimes they make little difference — it all depends,” Burke and Dossier write.
For instance, good grounding has little effect on the magnitudes of fault currents, isn't required for line protection (using arresters), yet is required for shield wire protection. Therefore, better grounding may not resolve stray voltage issues.
Before any changes are made, particularly at the utility level, they must first be simulated to determine their effectiveness and cost. “Just like any other utility power system issue, you can use computer models to simulate the mitigation techniques, and if you're going to come up with a solution that requires some significant investment or changes on the utility side, you've got to be able to model it, simulate it, and determine whether or not it's going to fix the problem,” Dorr says.
Information dissemination. Mitigation procedures are useless if no one has access to them. The main audience for the IEEE guidelines are the electric utility personnel, electrical contractors, and electricians. “If it's related to electricity, the utility gets the first call, so that's the primary audience,” Dorr says. “The secondary audience is electricians who have to know whether or not the way the wiring has been done is the problem, and in many cases it is. Also, the third audience is the contractors who install the pools and the spas and those kinds of things. If they don't do it correctly, there can be a problem that is caused by the way they have installed the hot tub or the swimming pool. Any of these results, whether it's new mitigation techniques or survey results of neutral-to-earth voltage levels all over the country, are all going to be publicly accessible so that everybody has that information available to them.”
However, both the IEEE Working Group and EPRI are in the very early stages of compiling the information for publication. “This subject is just something where there's just too much information right now,” Dorr says.
The first step in educating the public is to find a common language. “The committee thinks we can't just continue to call everything that we can possibly be exposed to in the course of our daily lives one thing,” DeNardo says. “There are different sources and different levels of concern. The stray voltage issue, as it's been historically defined in the Midwest, is kind of a nuisance shock, so we're not worried about anything occurring in a sort of lethal sort of situation. But what is now being called ‘stray voltage’ in some East Coast states is much more hazardous. So we're trying to undo the confusion. Because it's going to require a national debate to figure out what these definitions are and how best to apply them, it's going to take a little while. This isn't going to happen overnight. We want to have the discussion before we create any long-lasting definitions.”
Regulatory support. There are few standards and regulations that specifically address stray voltage. The IEEE C62.92.4-1991, “Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part IV — Distribution,” covers the classification of distribution system grounding as it relates to temporary overvoltages. The guide discusses techniques for mitigating stray voltage but does not specifically address the concepts behind stray voltage.
Only a few states address stray voltage in their statutes. “The state of Wisconsin has some good guidelines in terms of doing the measurements and what needs to be done at what voltage level for cows,” Dorr says. “Some of the other states have the same thing, but it's all for dairy farms. When somebody's got a swimming pool with an elevated-voltage concern, there aren't any states that say ‘Ok, if it's above this level, some kind of remedial action needs to be taken. If it's below this level, nothing needs to be done.’ ”
DeNardo expects the IEEE guidelines to cover more than the rural stray voltage concerns. “We're hoping that the product here that we end up with will be useful to people that set the rules,” he says. “It will be based on science, so hopefully, it will be something that will be useful in discussions of regulatory agencies or legislative bodies or whoever is making rules with respect to these type of exposures. That's the idea. Get together to compile a science-based guide that will be useful to these organizations so that when they do make decisions, they have some basis in science, that these aren't just political decisions.”
Unite and conquer. Once the guidelines are published, both the IEEE and EPRI hope there will be a lot less confusion regarding identification, testing, and mitigation. Dorr says that a tentative date for publication of guidelines is 2010. “It's not going to stop it,” Dorr says. “Whether you're in a house or outside, there're always places where people can get shocked. So the problem is never going to go away. Ideally, if the problem exists, there should be a very easy-to-understand and useful way to diagnose the problem, determine the source, and then take care of it.”
At this point in time, IEEE states that it's too early to have a timeline for a release of the work. “Because of the confusion that surrounds this issue, because of the controversy that surrounds it, the IEEE sees it as something that's important, and we'll take our time to get it right,” DeNardo says. “We have a large group of people from various parts of the country and various industries and associations looking at this, and it will take a little while but we'll get something together that will be useful in the long run. Then we can move forward with the work EPRI has done already and put together something of a guide. Hopefully, it will be user-friendly so that it won't be just something electrical engineers can read but something that the general public can hopefully understand.”