What are ghost voltages and where are they encountered
Ghost voltages occur from having energized circuits and non-energized wiring located in close proximity to each other, such as in the same conduit or raceway. This condition forms a capacitor and allows capacitive coupling between the energized wiring and the adjacent unused wiring. Ghost voltage is not related to power system faults, and is generally not considered hazardous.
When you place your multimeter leads between the open circuit and the neutral conductor, you effectively complete the circuit through the input of the multimeter. The capacitance between the connected, hot conductor and the floating conductor forms a voltage divider in conjunction with the multimeter input impedance. The multimeter then measures and displays the resulting voltage value.
Most digital multimeters available today have an input impedance that’s high enough to show the capacitively coupled voltage, giving a false impression of a live conductor. The meter is actually measuring voltage coupled into the disconnected conductor. However, these voltages, at times, can be 80-85% of what the “hard” voltage should be. If not recognized as a ghost voltage, additional time, effort and money will be lost troubleshooting circuit problems.
Absence or presence of voltage testing
Most modern digital multimeters are a high impedance type which is very sensitive and will pick up these voltages. This can result in confusion when fault finding. There are multimeters available with a low impedance setting (LoZ) that will not show these ghost voltages.
Select the Auto-V/LoZ function. This function has a low input impedance on the order of three kilohms. When the leads are placed on an open circuit that contains a ghost voltage, the low input impedance will cause the ghost voltage to dissipate and the meter will display a reading near zero volts indicating no voltage present.
For making capacitance measurements on cables with ghost voltage using FLUKE 117
1. Hold (RANGE) as you turn on the Meter to enable the low-input impedance Capacitance mode LoZ.
2. Wait until (LCAP) shows on the display.
In this mode, capacitance measurements will have a lower accuracy and lower dynamic range.