How to avoid ghost voltages
Ghost voltages can occur when an energised conductor is running in the vicinity of a de-energised conductor. This results in a capacitive coupling and a voltage is measurable in the de-energised conductor.
This commonly occurs in two way switching strappers with voltages between 60v and 120v being measured in the de-energised switch wire.
Most modern digital multimeters are a high impedance type which are 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.
NECA Technical highly recommends that this type of meter is used for any fault finding type of work.