What is the purpose of a digital multimeter with a dual impedance meter?
Ø Older troubleshooting tools such as analog multimeters and solenoid testers typically have low impedance input circuits of 10 kilohms or less. While these tools cannot be fooled by false voltages, they should only be used to test power circuits or other circuits where low impedance does not affect or alter the performance of the circuit.
Ø Best use of two functions By using dual impedance measuring meters, technicians can accurately troubleshoot sensitive electronic or control circuits and circuits that may contain false voltages, and can more reliably determine whether voltage is present on the circuit. On the Fluke 11X series digital multimeter, the meter's Vac and Vdc switch positions are generally in the high impedance position. Use these switch positions for most troubleshooting tasks, especially with sensitive electronic loads.
What is false voltage? Where do they appear?
Ø Spurious voltages arise from energized circuits and unenergized conductors that are in close proximity to each other (such as in the same conduit or raceway). This condition can create a capacitor that creates capacitive coupling between the energized wire and the adjacent unused wire.
Ø When the multimeter lead is placed between the open circuit and the neutral conductor, a complete circuit is effectively formed through the input of the multimeter. The capacitance between the connected hot conductor and the floating conductor combines with the multimeter input impedance to form a voltage divider. The multimeter then measures and displays the resulting voltage value. Most digital multimeters today have a high enough input impedance to show this capacitively coupled voltage (thereby giving the false impression that the conductor is charged). What the multimeter actually measures is the voltage coupled into the disconnected conductor. But sometimes, these voltages can reach 8085% of the "hardwired" voltage. If they are not identified as spurious voltages, additional time, effort, and money will be spent troubleshooting circuit problems.
Ø The most common places to encounter false voltages are from blown fuses in electrical panels, unused cables or wires in existing conduits, in 1 V branch circuits, or on cards that use 1 V control circuits to control assembly lines or conveyor functions. A disconnected ground or neutral wire in the box. A certain amount of false voltage can couple from the live side of a blown fuse to the open side. When constructing a facility or building and performing electrical wiring, electricians often thread additional wires through conduits for future use. These wires are usually left unconnected until use, but capacitive coupling can occur. For control circuits, the location of the circuit is often adjacent to unused control lines, thus creating a false voltage.
