The amount of "phantom" voltage as measured depends on the impedance of the meter. Two meters used one at a time on the same circuit can well show different voltages with the higher impedance meter registering the higher voltage.
On average, digital meters have higher impedances than analog meters.
The "voltage leak" is right throuigh the insulation of the wires in the Romex cable. In other words this plastic insulation is probably not defective but it is not "perfect". In reality the impedance is lower at 60 Hz compared with DC (and lower still for higher frequencies).
(copied from another foru )
There are 3 measurement points, the hot wire, the disconnected (dead) wire, and ground.
Hot wire to ground is 120 volts period.
Impedance #1 is the "resistance" of the insulation all along the route the wires were juxtaposed.
Impedance #2 is the meter.
When you make the measurement we now have a circuit, hot wire through impedance #1 to the "dead" wire through impedance #2 to ground.
(Roughly due to the oversimplifying needed to achieve a short English sentence) The voltage as measured by the meter is impedance #2 divided by the total impedance (#1 plus #2), times 120.
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Last edited by AllanJ; 11-15-2009 at 06:25 PM.