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 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.
Terminology The terminology, stray voltage may be used in any case of undesirable elevated electrical potential. 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 electrical 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.
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, the presidents of most major utilities from throughout the United States and Canada 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 their faults, let alone those that are seen as life-threatening. It would seem that stray voltage is now the common term for all unwanted voltage leakage because it categorises the fault as part of normal operation and so limits liability.
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.
Causes Coupled voltages
Ungrounded metal objects close to electric field sources such as neon signs or conductors carrying alternating currents may 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).Phantom voltage is also a term used in audio engineering in which a voltage is intentionally applied between the ground wire and both wires of a balanced audio line to provide power for a microphone or other device. The concept is usually referred to as phantom power. 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 common to measure phantom voltages of 50–90 V in testing the wiring of ordinary 120 V circuits with a high-impedance instrument. The voltage produced may read almost to the full supply voltage, but 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. That is since induced voltages and currents on a conductor may 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.
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.
Induced voltages Classical electromagnetic induction can occur when long conductors form an open grounded loop under and parallel to transmission or distribution lines. In those 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.Jaffa, KC: "Magnetic Field Induction From Overhead Transmission and Distribution Power Lines on Parallel Fences", "IEEE Trans. on Power App. and Sys., 100(4):1624-36Patel, S, Lambert, FC: "Induced Stray Voltages from Transmission Lines", IEEE PES T&D Conf & Expo 2006, 21–24 May 2006, 254-9
Degraded insulation on power conductors Stray voltage may leak via 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 occur also from 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 utility 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,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.
Railway stray current