Stray voltage
Encyclopedia
Stray voltage describes the occurrence of electrical potential between two objects that ideally should not have any voltage difference between them. Small voltages are often measured between two grounded
objects in distant 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.
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 shock
s, 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.
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.
(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:
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. Her death made public electrical safety in the urban environment a major concern for utilities, and raised public awareness of the issue. The term “stray voltage” was used by the media and the NY State regulatory agency. In 2005, the state acknowledged that “stray voltage” properly refers to 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" to describe 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 to describe 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.
sources such as neon signs or conductors carrying alternating current
s 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's 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.
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.
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.
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.
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.
, 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.
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.
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 for 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.
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.
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.
Electrical power is typically not billable by the power utility company if it does not pass through a customer premises electrical meter. Power leakage in the electrical distribution system is real power that is wasted, and is a real economic loss to the utility. Therefore, the power utility should be motivated to find and repair power leakage, both for economic reasons and liability concerns.
Industrial, commercial, and residential users of electricity should be similarly motivated to identify and correct electrical faults on their property. In addition, environmental conservation groups
have become interested in this aspect of energy conservation
.
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. Accuracy can be affected if the user is at an elevated potential him/herself or if the user is not making firm contact with the reference terminal of the device using a bare hand.
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 voltmeters may measure elevated voltages from non-energized objects due to this coupling, in effect providing a misleading reading. For this reason, 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". If an object in question 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 positive indication that no objects are energized in a tested area. Electric field detectors respond to all field sources, and positive indications must be verified with a 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 a first step, but it is possible that a dangerous condition can occur without warning.
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
objects in distant locations, due to normal current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
flow in the power system. Large voltages can appear on the enclosures of electrical equipment due to a fault
Fault (power engineering)
In an electric power system, a fault is any abnormal flow of electric current. For example, a short circuit is a fault in which current flow bypasses the normal load. An open-circuit fault occurs if a circuit is interrupted by some failure. In three-phase systems, a fault may involve one or more...
in the electrical power system, such as a failure of insulation.
Small stray voltages may never be noticed and may only be detected with a voltmeter
Voltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...
. Larger voltages may have a range of effects, from barely perceptible to dangerous electric shock
Electric shock
Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....
s, or unintended electrical heating resulting in fires. Normally, metal electrical equipment cases are bonded to ground
Electrical bonding
Electrical bonding is the practice of intentionally electrically connecting all metallic non-current carrying items in a room or building as protection from electric shock. If a failure of electrical insulation occurs, all metal objects in the room will have the same electrical potential, so that...
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
Electrocution
Electrocution is a type of electric shock that, as determined by a stopped heart, can end life. Electrocution is frequently used to refer to any electric shock received but is technically incorrect; the choice of definition varies from dictionary to dictionary...
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
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
than wet skin or a wound, so voltages that would otherwise be unnoticed become significant for a wet or surgical situation.
Terminology
"Stray voltage" describes 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 impedanceElectrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
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.
Official definition (draft)
In 2005, the Institute of Electrical and Electronics EngineersInstitute of Electrical and Electronics Engineers
The Institute of Electrical and Electronics Engineers is a non-profit professional association headquartered in New York City that is dedicated to advancing technological innovation and excellence...
(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 to describe 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 the term "stray voltage" for all occurrences of unwanted excess electricity. The term "contact voltage" was used only one time. It would seem that the term "stray voltage" is now the common term used to describe all unwanted voltage leakage.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. Her death made public electrical safety in the urban environment a major concern for utilities, and raised public awareness of the issue. The term “stray voltage” was used by the media and the NY State regulatory agency. In 2005, the state acknowledged that “stray voltage” properly refers to 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" to describe 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 to describe 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
Capacitive coupling
In electronics, capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. This coupling can have an intentional or accidental effect...
, current induced by power lines, EMF
EMF
- Music :* EMF , a British band** "EMF", a bonus track on EMF's album Schubert Dip* E.M.F. , a 1983 album by GG Allin* English Music Festival, a British music festival- Organizations :...
, lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
, earth potential rise
Earth potential rise
In electrical engineering, earth potential rise also called ground potential rise occurs when a large current flows to earth through an earth grid impedance. The potential relative to a distant point on the Earth is highest at the point where current enters the ground, and declines with distance...
, and problems stemming from open (disconnected) neutrals.
Coupled voltages
Ungrounded metal objects close to electric fieldElectric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
sources such as neon signs or conductors carrying alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
s can have measurable voltage levels caused by capacitive coupling
Capacitive coupling
In electronics, capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. This coupling can have an intentional or accidental effect...
. 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's 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
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
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 capacitorCapacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
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
Static electricity
Static electricity refers to the build-up of electric charge on the surface of objects. The static charges remain on an object until they either bleed off to ground or are quickly neutralized by a discharge. Static electricity can be contrasted with current electricity, which can be delivered...
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 inductionElectromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
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 to describe 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 distributionElectricity distribution
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...
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 phaseThree-phase electric power
Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...
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 cellGalvanic cell
A Galvanic cell, or Voltaic cell, named after Luigi Galvani, or Alessandro Volta respectively, is an electrochemical cell that derives electrical energy from spontaneous redox reaction taking place within the cell...
, using moist soil as the electrolyte. Stray direct currents in soil may counteract the anti-corrosion effect of a cathodic protection
Cathodic protection
Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. The simplest method to apply CP is by connecting the metal to be protected with another more easily corroded "sacrificial metal" to act as the anode of the...
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
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
, this stray current can cause damage to other buried metallic objects by electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
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
Consolidated Edison
Consolidated Edison, Inc. is one of the largest investor-owned energy companies in the United States, with approximately $14 billion in annual revenues and $36 billion in assets...
in New York City
New York City
New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...
has had frequent incidents of stray voltage, including the electrocution death of Jodie S. Lane in 2004, while walking her dog in Manhattan
Manhattan
Manhattan is the oldest and the most densely populated of the five boroughs of New York City. Located primarily on the island of Manhattan at the mouth of the Hudson River, the boundaries of the borough are identical to those of New York County, an original county of the state of New York...
. 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
NSTAR
NSTAR is a utility company that provides retail electricity and natural gas to 1.4 million customers in eastern and central Massachusetts, including the Boston urban area....
(formerly Boston Edison
Boston Edison
Boston Edison can refer to:*Boston-Edison, a neighborhood in Detroit, Michigan.*Boston Edison Company, an operating unit of NSTAR...
) 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
Toronto Hydro
The Toronto Hydro-Electric System is the local distributor of electric power in the City of Toronto. In 2005 the utility served a peak load of over 5,000 MW and had nearly 600,000 residential and 70,000 commercial and industrial customers, and had around 1,600 employees. In 2005 the corporation...
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.
Effects on people
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 for 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.
Effects on farm animals
Stray voltage can have harmful effects on animal health and productivity. Some dairyDairy
A dairy is a business enterprise established for the harvesting of animal milk—mostly from cows or goats, but also from buffalo, sheep, horses or camels —for human consumption. A dairy is typically located on a dedicated dairy farm or section of a multi-purpose farm that is concerned...
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
University of Wisconsin–Madison
The University of Wisconsin–Madison is a public research university located in Madison, Wisconsin, United States. Founded in 1848, UW–Madison is the flagship campus of the University of Wisconsin System. It became a land-grant institution in 1866...
, 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 CourtWisconsin Supreme Court
The Wisconsin Supreme Court is the highest appellate court in the state of Wisconsin. The Supreme Court has jurisdiction over original actions, appeals from lower courts, and regulation or administration of the practice of law in Wisconsin.-Location:...
upheld a judgement
Judgement
Judgment is the evaluation of evidence in the making of a decision. The term has three distinct uses:* Informal - Opinions expressed as facts....
of $1.2 million against the Wisconsin electrical utility WEPCO
Wisconsin Energy Corporation
Wisconsin Energy Corporation , based in Milwaukee, Wisconsin serves more than 1.1 million electric customers in Wisconsin and Michigan's Upper Peninsula and more than 1 million natural gas customers in Wisconsin through its utility subsidiary, We Energies...
in Hoffman
v. Wisconsin Electric Power Company. The Hoffman family, dairy farmers near New London
New London, Wisconsin
New London is a city in Outagamie and Waupaca Counties in the U.S. state of Wisconsin. The population was 7,295 at the 2010 census. The city has a Saint Patrick's Day Parade, Irish Fest, and week-long festivities, when the city's name is changed to "New Dublin" for the week. The American Water...
, 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
Wisconsin Public Service Commission
The Public Service Commission of Wisconsin is an independent regulatory agency responsible for regulating public utilities in the energy, telecommunications, gas and water companies located in U.S. state of Wisconsin...
, but the court ruled on procedural grounds that the utility could be found negligent under common law
Common law
Common law is law developed by judges through decisions of courts and similar tribunals rather than through legislative statutes or executive branch action...
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.
Energy wastage
Stray electrical currents caused by stray voltages can dissipate significant amounts of electrical power on a continuous basis. This wasted power usually appears as heat, which can further degrade insulating materials, or even start electrical fires. In rainy or damp conditions, the emission of visible steam from an electrical utility box, lighting pole, or manhole could be due to an electrical fault, and should be reported to the power utility company for investigation.Electrical power is typically not billable by the power utility company if it does not pass through a customer premises electrical meter. Power leakage in the electrical distribution system is real power that is wasted, and is a real economic loss to the utility. Therefore, the power utility should be motivated to find and repair power leakage, both for economic reasons and liability concerns.
Industrial, commercial, and residential users of electricity should be similarly motivated to identify and correct electrical faults on their property. In addition, environmental conservation groups
Environmental movement
The environmental movement, a term that includes the conservation and green politics, is a diverse scientific, social, and political movement for addressing environmental issues....
have become interested in this aspect of energy conservation
Energy conservation
Energy conservation refers to efforts made to reduce energy consumption. Energy conservation can be achieved through increased efficient energy use, in conjunction with decreased energy consumption and/or reduced consumption from conventional energy sources...
.
Stray/contact voltage detection
Stray voltage is generally discovered during routine electrical work, or as a result of an 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
Electric field proximity sensing
Electric Field Proximity Sensing or EFPS is a sensory system that relies on the fact that an electric field can be perturbed by the existence of a nearby object, provided it is at least slightly conductive. One type of EFPS is The People Detector...
, 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. Accuracy can be affected if the user is at an elevated potential him/herself or if the user is not making firm contact with the reference terminal of the device using a bare hand.
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 voltmeters may measure elevated voltages from non-energized objects due to this coupling, in effect providing a misleading reading. For this reason, 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". If an object in question 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
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...
. 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
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
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 positive indication that no objects are energized in a tested area. Electric field detectors respond to all field sources, and positive indications must be verified with a voltmeter to eliminate false positives. Electric field proximity sensing
Electric field proximity sensing
Electric Field Proximity Sensing or EFPS is a sensory system that relies on the fact that an electric field can be perturbed by the existence of a nearby object, provided it is at least slightly conductive. One type of EFPS is The People Detector...
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 a first step, but it is possible that a dangerous condition can occur without warning.
See also
- Earth potential riseEarth potential riseIn electrical engineering, earth potential rise also called ground potential rise occurs when a large current flows to earth through an earth grid impedance. The potential relative to a distant point on the Earth is highest at the point where current enters the ground, and declines with distance...
- Earthing systemEarthing systemIn electricity supply systems, an earthing system defines the electrical potential of the conductors relative to the Earth's conductive surface. The choice of earthing system can affect the safety and electromagnetic compatibility of the power supply, and regulations can vary considerably among...
- Electrical bondingElectrical bondingElectrical bonding is the practice of intentionally electrically connecting all metallic non-current carrying items in a room or building as protection from electric shock. If a failure of electrical insulation occurs, all metal objects in the room will have the same electrical potential, so that...
- Disturbance voltageDisturbance voltageIn telecommunication, a disturbance voltage is an unwanted voltage induced in a system by natural or man-made sources.In telecommunications systems, the disturbance voltage creates currents that limit or interfere with the interchange of information...
- Neutral and ground
External links
- University of Wisconsin–Madison Report on Stray Voltage
- 'Electrified Cover Safeguard' website
- 'Stray voltage' website from the LaCrosse Tribune, including their award-winning coverage
- Self-Help Guide: Stray Voltage Detection, Wisconsin Farm Electric Council (2/1997), well-written, for farmer-consumers, at
- Midwest Rural Energy Council Stray Voltage portal
- Wisconsin Public Service Stray Voltage site
- Public Service Commission of Wisconsin Stray Voltage documents (technical)
- Pacific Gas and Electric Power Quality Bulletin No. 2, "Stray Voltage" (10/2004)
- First conference about "Stray currents in our environment" - November 29, 2007, Ester Technopole Limoges, France
- Stray voltage description and mitigation