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, due to normal current flow in the power system. Large voltages can appear on the enclosures of electrical equipment due to a fault in the electrical power system, such as a failure of insulation.
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Terminology
Stray voltage is any case of undesirable elevated electrical potential, but more precise terminology gives an indication of the source of the voltage. Neutral to earth voltage (NEV) specifically refers to a difference in potential between a locally grounded object and the grounded return conductor, or neutral, of an electrical system. The neutral is theoretically at 0 V potential, as any grounded object, but current flows on the neutral back to the source, somewhat elevating the neutral voltage. NEV is the product of current flowing on the neutral and the finite, non-zero impedance of the neutral conductor between a given point and its source, often a distant substation. NEV differs from accidentally energized objects because it is an unavoidable result of normal system operation, not an accident or a fault in materials or design.
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Definitions
Official definition (draft)
In 2005, the Institute of Electrical and Electronics Engineers (IEEE) convened Working Group 1695 in an attempt to lay down definitions and guidelines for mitigating the various phenomena referred to as stray voltage. The working group attempted to distinguish between the terms stray voltage and contact voltage as follows:
- Stray voltage is defined as "A voltage resulting from the normal delivery and/or use of electricity (usually smaller than 10 volts) that may be present between two conductive surfaces that can be simultaneously contacted by members of the general public and/or their animals. Stray voltage is caused by primary and/or secondary return current, and power system induced currents, as these currents flow through the impedance of the intended return pathway, its parallel conductive pathways, and conductive loops in close proximity to the power system. Stray voltage is not related to power system faults, and is generally not considered hazardous."
- Contact voltage is defined as "A voltage resulting from abnormal power system conditions that may be present between two conductive surfaces that can be simultaneously contacted by members of the general public and/or their animals. Contact voltage is caused by power system fault current as it flows through the impedance of available fault current pathways. Contact voltage is not related to normal system operation and can exist at levels that may be hazardous."
Working definition
In spite of the above definitions, the term stray voltage continues to be used by both utility workers and the general public for all occurrences of unwanted excess electricity. For example, at the annual "Jodie S. Lane Stray Voltage Detection, Mitigation & Prevention Conference", held at the Con Edison headquarters in New York City in April 2009, which attracted the presidents of most major utilities from throughout the United States and Canada, the utility leaders continued to use stray voltage for all occurrences of unwanted excess electricity. The term contact voltage was used only once, possibly because "contact voltage" is generally the fault of the supply, network or installation company. Few companies are willing to openly discuss faults of theirs, let alone ones as life-threatening. It would seem that stray voltage is now the common term for all unwanted voltage leakage as it categorises the fault as part of normal operation, therein limiting liability.
In New York City, a woman named Jodie S. Lane was electrocuted by a five-foot by eight-foot road utility vault plate energized by an "improperly insulated wire" in January 2004. In the coverage of the growing concern regarding the role of public utilities in electrical safety in the urban environment that her death triggered, both the media and the New York state regulatory agency used stray voltage was for neutral-to-earth voltage (NEV), but conceded that the notoriety of the Jodie S. Lane incident had caused stray voltage to be a term that is well recognized by the public. At that point, the regulator used stray voltage for 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." In the same document, the commission accepted NEV to be a naturally occurring condition.
Since that time, the term "stray voltage" has had at least two very different definitions. This situation is cause for confusion among utilities, regulators, and the public. 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 due to capacitive coupling, current induced by power lines, EMF, lightning, earth potential rise, and problems stemming from open (disconnected) neutrals.
Origins
Coupled voltages
Ungrounded metal objects close to electric field sources such as neon signs or conductors carrying alternating currents can have measurable voltage levels caused by capacitive coupling. Since voltages detected by high-impedance instruments disappear or become greatly reduced when a low impedance is substituted, the effect is sometimes called phantom voltage (or ghost voltage). The term is often used by electricians, and might be seen, for example, when measuring the voltage at a lighting fixture after removing the bulb. It is not unusual to measure phantom voltages of 50-90 volts when testing the wiring of ordinary 120 V circuits with a high-impedance instrument. While the voltage produced may read almost to the full supply voltage, the capacitance or mutual inductance between the wires of building wiring systems is typically quite low and incapable of supplying significant amounts of current.
However, in overhead transmission work on or near high-voltage lines, safety rules require connecting a conductor to earth ground during maintenance, since induced voltages and currents on a conductor may be sufficient to cause electrocution or serious injury.
Capacitive leakage through insulation
Alternating current is different from direct current in that the current can flow through what would ordinarily seem to be a physical barrier. In a series circuit, a capacitor blocks direct current but passes alternating current.
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. It is possible to get a shock by only touching the hot wire, due to the person's body being capacitively coupled to the ground upon which the person stands, even if the person is standing on an insulated surface.
Induced voltages
Classical electromagnetic induction can occur when long conductors form an open grounded loop under and parallel to transmission or distribution lines. In these cases, current is induced in the loop when a person makes contact with it and ground. Since this involves real current flow, it is potentially hazardous. This type of induced current occurs most often on long fences and distribution lines built under high-power transmission lines.
Degraded insulation on power conductors
Stray voltage may be caused by damaged or degraded insulation. Failing insulation is essentially a high impedance fault which will allow current to flow through any available path to ground, a condition which can cause shocks or fires if left unmitigated. This leakage can occur when there is damage caused by physical, thermal, or chemical stresses to insulation on power lines, especially but not limited to underground or underwater cables. Examples of this damage are swollen or cracked insulation from overheating, abrasions caused by digging or ground seizing, and corrosion damage from salt or oil exposure. Electrical leakage can also occur due to moisture, salt, dust, and dirt buildup on open air insulators in overhead power distribution. If the leakage in these cases is severe enough, it can lead to a pole fire.
Leakage from single-wire earth return
The term "stray voltage" is used for the gradient (rate of change with respect to distance) of electrical potential in the surface of the soil, associated with single-wire earth return electricity distribution systems used in some rural locations. This gradient is low at points far away from the earth return connections, but increases near the ground rods where the metallic circuit enters the earth.
Neutral return currents through the ground
In three phase four-wire ("wye") electrical power systems, when the load on the phases is not exactly equal, there is some current in the neutral conductor. Because both the primary and secondary of the distribution transformer are grounded, and the primary ground is grounded at more than one point, the earth forms a parallel return path for the neutral current, allowing part of the neutral current to continuously flow through the earth. This arrangement is partially responsible for stray voltage.
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, and prevention, 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.
Electrolysis and corrosion
Dissimilar buried metals such as copper and steel can function as the poles of a galvanic cell, using moist soil as the electrolyte. Stray direct currents in soil may counteract the anti-corrosion effect of a cathodic protection system. Design of high voltage direct current transmission systems must take care so that current flowing in the earth does not cause objectionable corrosion to buried objects such as pipelines.
Typically an electric railway will have at least one of the rails used as a return conductor for the traction current. This rail is in contact with the earth at many places throughout its length. Since current will follow every parallel path between source and load, some part of the traction current will also flow through the earth. Where the railway uses direct current, this stray current can cause damage to other buried metallic objects by electrolysis and accelerate corrosion of metal objects in touch with the soil.
Public concerns about stray voltage
In metropolitan areas, stray voltage issues have become a major concern. Many of these areas have large amounts of aging underground and aboveground electrical distribution equipment in crowded public spaces. Even a low rate of insulation failures or current leakage can result in hazardous exposure of the general public.
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 a substation near her house repeatedly shocked her and members of her family whenever they were in the shower.
Effects
Persons
Small stray voltages may never be noticed and may only be detected with a voltmeter. Larger voltages may have a range of effects, from barely perceptible to dangerous electric shocks, or unintended electrical heating resulting in fires. Normally, metal electrical equipment cases are bonded to ground to prevent a shock hazard if energized conductors accidentally contact the case. Where this bonding is not provided or has failed, a severe hazard of electric shock or electrocution is presented when circuit conductors contact the case.
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 for a wet or surgical situation. Potential differences between pool water and railings, or shower facilities and grounded drain pipes are not uncommon as a result of neutral to earth voltages (NEV), and can be a major nuisance, but are usually not life-threatening. However, contact voltage resulting from damaged insulation on a current carrying conductor can be very dangerous, and can lead to shock or electrocution. Such a 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 - fences, telephone booths, 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 systems have protective devices such as circuit breakers or Ground Fault Circuit Interrupters (GFCI), designed to isolate such a fault. However, in the absence of protective devices, if the devices fail, or if they are not installed correctly, a fault will go undetected until it either causes a failure of the circuit or until it is found by a person.
Farm animals
Stray voltage can have harmful effects on animal health and productivity. Some dairy farmers have claimed damage to yields or stock caused by it.
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.
Legal proceedings in Wisconsin
In 2003, the Wisconsin Supreme Court upheld a judgement of $1.2 million against the Wisconsin electrical utility WEPCO in Hoffman v. Wisconsin Electric Power Company. The Hoffman family, dairy farmers near New London, had sued WEPCO after several years of declining production. WEPCO had measured on the farm currents due to stray voltage below one milliampere, the "level of concern" set by the Wisconsin Public Service Commission, but the court ruled on procedural grounds that the utility could be found negligent under common law even though they met the state standard. The Hoffmans had presented, the court said, a viable alternative theory that stray voltage had caused them economic harm.
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 power lines hurt their 1,000 cow herd and lowered milk production. The jury found that Xcel subsidiary -- Northern States Power -- 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.
Stray/contact voltage detection
Stray voltage is generally discovered during routine electrical work, or as a result of a customer complaint or shock incident. A growing number of utilities in urban areas now conduct routine periodic and systematic active tests for stray voltage (or more specifically, contact voltage) for public safety reasons. Some incipient electrical faults may also be discovered during routine work or inspection programs which are not specifically focused on 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 due to this coupling, in effect providing a misleading reading. For this 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 can not 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.
Source of the article : Wikipedia
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