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Stray voltage is the occurrence of electrical potential between two objects that ideally should not have any voltage difference between them. Small voltages often exist between two grounded objects in separate locations by the normal Electric current flow in the power system. Contact voltage is a better defined term when large voltage appear as a result of a fault. Contact voltage on the enclosure of electrical equipment can appear from a fault in the electrical power system, such as a failure of insulation.
In New York City, a woman, Jodie S. Lane, was electrocuted in January 2004 by a five-foot by eight-foot metal road utility vault plate that was energized by an "improperly insulated wire." In the coverage of her death and the growing concern regarding the role of public utilities in electrical safety in the urban environment, both the media and the New York State regulatory agency used stray voltage for neutral-to-earth voltage (NEV), but conceded that the notoriety of the Lane incident had caused stray voltage to be a term that is well recognized by the public.
The regulator then used stray voltage to refer to any "voltage conditions on electric facilities that should not ordinarily exist. These conditions may be due to one or more factors, including, but not limited to, damaged cables, deteriorated, frayed or missing insulation, improper maintenance, or improper installation."NYS Public Service Commission, Case 04-M-0159, “Order Instituting Safety Standards” (issued Jan 5, 2005) In the same document, the commission accepted NEV to be a naturally occurring condition.
Since then, the term had at least two very different definitions, which confused utilities, regulators, and the public.Burke, J: "The Confusion Surrounding ‘Stray Voltage’", IEEE Rural Electric Power Conference, 6–8 May 2007, C1-C5. The term "stray voltage" is commonly used for all unwanted electrical leakage, by both the general public and many electrical utility professionals. Other more esoteric phenomenon that also result in elevated voltages on normally non-energized surfaces, are also referred to as “stray voltage.” Examples are voltage from capacitive coupling, current induced by power lines, EMF, lightning, earth potential rise, and problems stemming from open (disconnected) neutrals.
However, in overhead transmission work on or near high-voltage lines, safety rules require connecting a conductor to earth ground during maintenance. That is since induced voltages and currents on a conductor may cause electrocution or serious injury.
In power transmission systems, one side of the circuit, known as the neutral, is grounded to dissipate static electricity and to reduce hazardous voltages caused by insulation failure and other electrical faults.
Even a person standing on an insulated surface may get a shock only by touching the hot wire because of the person's body being capacitively coupled to the ground upon which the person stands.
Stray voltage is a result of the design of a 4 wire distribution system and as such has existed as long as such systems have been used. Stray voltage became a problem for the dairy industry some time after electric milking machines were introduced, and large numbers of animals were simultaneously in contact with metal objects grounded to the electric distribution system and the earth. Numerous studies document the causes, physiological effects,Norrell, RJ et al.: “Behavioral studies of dairy cattle sensitivity to electrical currents”, Trans. ASAE, 26(5) 1506-1511. and prevention,Donald, J, Hertz, CM, Winsett, I: “Results of initial field installations of magnetic saturation blockers for minimization of stray voltage on dairy farms” ASAE paper 156-170, 1984.Surbrook, TC, et al.: “Designing facilities to prevent stray voltage”, Proc. 2nd Nat. Dairy Housing Conf., 1983 of stray voltage in the farm environment. Today, stray voltage on farms is regulated by state governments and controlled by the design of equipotential planes in areas where livestock eat, drink or give milk. Commercially available neutral isolators also prevent elevated potentials on the utility system neutral from raising the voltage of farm neutral or ground wires.
The stray currents from railways create or accelerate the electrolytic corrosion of metallic structures located in the proximity of the transit system. Metal pipes, cables and earthing grids laid in the ground near tracks may have a much shorter usable and safety functional life.
In any situation where energized equipment is in intimate electrical contact with a person or animal (such as swimming pools, surgery, electric milking machines, car washes, laundries, and many others), particular attention must be paid to elimination of stray voltages. Dry intact skin has a higher resistance than wet skin or a wound, so voltages that would otherwise be unnoticed become significant in a wet or surgical situation. Potential differences between pool water and railings, or shower facilities and grounded drain pipes are common as a result of neutral to earth voltages (NEV). Potential differences can be a major nuisance, but are usually not life-threatening. However, a current carrying conductor with damaged insulation can result in contact voltage in unexpected places. Contact voltage energized metal parts can be very dangerous, and can lead to shock or electrocution. A contact voltage condition can arise spontaneously from mechanical, thermal, or chemical stress on insulation materials, or from unintentional damage from digging activity, freeze-frost seizing, corrosion and collapse of conduit, or even workmanship issues.
Contact voltage energizes objects which are normally safe – metal fences, metal telephone booths, metal street signs, etc. Anywhere buried electric wiring exists, a failure can occur in that wiring and create conditions that allow electricity to flow into the immediate surroundings. Some circuitry systems have protective devices such as circuit breakers or GFCI (GFCI), designed to isolate such a fault. However, in the absence of protective devices, a fault will go undetected until it either causes a failure or an energy discharge incident.
Dr. Douglas J. Reinemann, Professor of Biological Systems Engineering at University of Wisconsin–Madison, reported on stray voltages on dairy farms in 2003. Investigation of stray voltage claims must also consider other animal health concerns.
In 2017, a jury sided with farmers Paul and Lyn Halderson for a $4.5 million settlement against Xcel Energy. The Haldersons claimed stray voltage from high voltage power lines hurt their 1,000 cow herd and lowered milk production. The jury found that Xcel subsidiary – Northern States Power Company – was "negligent with respect to the delivery of electrical service." The jury awarded $4.09 million for economic damages and another $409,000 for "inconvenience, annoyance and loss of use and enjoyment" of property.
Consolidated Edison in New York City has had frequent incidents of stray voltage, including the electrocution death of Jodie S. Lane in 2004, while walking her dog in Manhattan. In 2009, the Jodie S. Lane Public Safety Foundation announced a publicly accessible website with maps showing thousands of reported stray voltage locations in New York City. In addition, the Foundation sponsors the "Jodie S. Lane Stray Voltage Detection, Mitigation & Prevention Conference", an annual meeting attended by power utilities and regulators from around the country to discuss stray voltage detection programs. The Foundation also initiated and advocates regular mobile scanning by utility companies for stray voltage hazards.
In Boston, NSTAR Electric (formerly Boston Edison) has also had problems with hazardous stray voltages, which have killed several dogs during the 1990s. As a result, the City of Boston government started a program to detect, report on, and repair stray voltage hazards.
Toronto Hydro pulled all employees off regular duty on the weekend of January 30, 2009 to deal with ongoing stray voltage problems in the city. This came after as many as five children were shocked though none suffered serious injury. The stray voltage problem had claimed the lives of two dogs in the previous few months.
In March 2013, Californian Simona Wilson won a $4 million lawsuit against her power company after stray voltage from an electrical substation near her house repeatedly shocked her and members of her family whenever they were in the shower. Woman Shocked in Shower by Stray Voltage Wins $4 Million Lawsuit Yahoo, 21 Mar 2013. Retrieved 22 Mar 2013.
The United States Social Security Administration, Administrative Law Judge, Edward Bergtholdt, in an August 17, 2000 decision awarded Michael Gunner permanent disability from exposure to stray voltage.
Equipment used to detect stray voltage varies, but common devices are electrical tester pens or electric field detectors, with follow-up testing using a low-impedance voltmeter. Electrical tester pens are hand-held devices which detect a potential difference between the user's hand and the object being tested. They generally indicate on contact with an energized object, if the potential difference is above the sensitivity threshold of the device. Reliability of the test can be affected if the user is at an elevated potential him/herself, or if the user is not making firm contact with a bare hand on the reference terminal of the tester.
Capacitive coupling is the mechanism used by electrical tester pen devices. Because the capacitance between an object and a current source is typically small, only very small currents can flow from the energized source to the coupled object. High-impedance digital or analog voltmeters may measure elevated voltages from non-energized objects from the coupling and in effect provide a misleading reading. For that reason, high-impedance voltage measurements of normally non-energized objects must be verified.
Verification of a voltage reading is performed using a low-impedance voltmeter, which usually has a shunt resistor load bridging the voltmeter terminals. Since very little current can flow from a coupled surface through the small shunt or meter resistance, capacitively coupled voltages will collapse to zero, indicating a harmless "false alarm". By contrast, if an object being tested is in contact with a current source, or coupled by a very large capacitance (possible but unlikely in this context), the voltage will drop only slightly as dictated by Ohm's law. In this latter case, real power is being delivered, indicating a potentially hazardous situation.
Electric field detectors detect the electric field strength relative to the user's body or mounting platform. By sensing electric field gradients at a distance, they can detect energized objects without making direct contact, making these instruments useful for scanning or screening large areas for potential electrical hazards. A low electric field reading also provides a definitive indication that no objects are energized within a tested area. Electric field detectors respond to all field sources, and any positive indications must be verified with a low-impedance voltmeter to eliminate false positives. Electric field proximity sensing also has other industrial applications from manufacturing to building security.
Since stray voltage cannot be seen, smelled, or heard, there is no easy way for the public to know when a dangerous condition exists. Periodic testing is an important precaution, but it is possible that a dangerous condition can develop without warning.
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