Phantom voltage

Phantom voltage (sometimes referred to as "ghost voltage") is voltage that appears in an electrical conductor such as a wire, even though the wire is disconnected from an electrical circuit. The term is often used by electricians, and might be seen, eg, 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 120V circuits.

Causes

Causes of phantom voltage may include:

*Capacitive coupling in which there is capacitance (a build up of opposing electric charges) between the conductor in which the phantom voltage is detected and nearby circuitry.
*Inductive coupling in which the magnetic field produced by current flowing in a nearby circuit induces current flow in the conductor where phantom voltage is detected.

Capacitive phantom voltage

Two parallel wires, insulated from each other but running side-by-side, effectively form the plates of a capacitor. If an alternating current (AC) is applied to one of the wires while the other wire is not connected, then AC voltage will tend to be capacitively coupled to the disconnected wire. If there are no other wires nearby this coupling will be nearly complete, and the voltage measured on the disconnected wire will be nearly the same as on the "live" wire. However, if there are other wires nearby (such as a ground wire in the same cable) then in effect a capacitor voltage divider may be created, and the voltage on the disconnected wire will measure somewhere between the "live" wire and the other wires.

While the voltage produced by capactive coupling may be startlingly high the amount of capacitance between the wires is typically quite low and incapable of supplying significant amounts of current. [cite web
title = Phantom Voltages
url= http://www.nema.org/stds/eng-bulletins/upload/Bulletin-88.pdf
accessdate = 2008-01-08
] As this term is used by electricians and engineers, "Phantom Voltage" refers to inductive coupling.

Inductive phantom voltage

In these cases, magnetic fields from an adjacent wire or other voltage source (such as a high tension tower) induce current and therefore voltage on the tested wire. This inductive coupling is enough to cause a fluorescent tube to glow when placed within the magnetic field of a high tension tower.

Other meanings

"Phantom voltage" is also a term used in audio engineering where a voltage is applied between the ground wire and both wires of a balanced audio line, in order to provide power for a microphone or other device. This concept is more properly referred to as "phantom power".)

"Phantom voltage" is also sometimes used to describe a situation where a voltage develops between the grounding conductor of an electrical installation and the "true" earth ground. This phenomenon is more properly referred to as "stray voltage". [cite paper
author = Reinemann, Douglas J
title = What Do We Know About Stray Voltage?
publisher = University of Wisconsin—Madison
date = 28 March 2003
url = http://www.uwex.edu/uwmril/pdf/StrayVoltage/03_What_do_we_know_about_Stray_Voltage.pdf
format = PDF
accessdate = 2008-01-08
]

Finally, the term "stray voltage" is sometimes used to denote the controversial concept of high-voltage power lines causing injury to people and animals in the buildings nearby. This is a completely different topic from the "phantom voltage" discussed above.

References

Wikimedia Foundation. 2010.

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