Edward, This phenomenom,could also be called an Induced Voltage, it is common with long cable runs and where cables are bunched together in an installation. I have seen up to 98V AC, induced into cables,by static induction. But it is at extremely low current, it is nothing that should be feared from a safety perspective.
Re: phantom voltage??#20103 01/09/0307:15 AM01/09/0307:15 AM
Any time a current is induced into a wire a magnetic field gets built up around the wire. If the magnetic field is in close proximty to another wire it can couple to that wire, thats how a transformer works. If you use a proximity tester (tick tracer) you can pick up these trace voltages. Sometimes ballasts and transformers can induce pretty high voltages on ungrounded metal conductors, enclosures, conduit etc. although the load carying capibilities is very small. You can read the voltage with a meter because of its high impedance. Just ground it and it disapates.
Re: phantom voltage??#20104 01/09/0308:43 AM01/09/0308:43 AM
Another thing you'll notice is that the voltage you read depends upon the type of meter. As noted above, the high input impedance (typically 10 meg) of a digital meter results in a high reading.
If you swap over and measure using a traditional analog meter with a lower input impedance, you'll find that the reading drops substantially. In fact, because the input impedance of the meter varies from range to range, you'll find that the apparent voltage drops each time you switch to a lower range.
Where the voltage is caused by capacitive coupling due to proximity to an energized conductor, you're effectively forming a potential divider with that capacitance and the resistance of your meter.
[This message has been edited by pauluk (edited 01-09-2003).]
Re: phantom voltage??#20105 01/09/0309:37 AM01/09/0309:37 AM
Trumpy; You mentioned static induction. I was at a manufacturing food processing plant about 10 years ago. All their machines had a static ground attached from the conduit to the machines. Do you know if that eliminates phantom voltages to zero or was this just because of a hazordous location with flying combustables in the air or both??
Re: phantom voltage??#20106 01/09/0310:36 AM01/09/0310:36 AM
Many a lineman has died from this "static" voltage on high-lines. Conductors that are run parallel ( for miles ) with energized circuits on the same poles or towers or even close-by structures build-up a lethal voltage. These conductors must be grounded, near-by the person working on those lines.
Grounding the conductors being 'worked on' a mile or so away from the work site can have lethal voltages.