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." 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. 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
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