Stray voltage

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. 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. 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. 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: 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. 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. 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.