What is a dual impedance digital multimeter good for?
Older troubleshooting tools such as analog multimeters and solenoid testers typically have low-impedance input circuits of 10 kohms 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 will not affect or change the performance of the circuit.
Ø With two functions By using the dual impedance measuring instrument, technicians can accurately troubleshoot sensitive electronic or control circuits and circuits that may contain false voltages, and can more reliably determine whether there is voltage on the circuit. On the Fluke 11X Series DMMs, the meter's Vac and Vdc switch positions are typically in the high impedance position. Use these switch positions for most troubleshooting, especially for sensitive electronic loads.
What is false voltage? Where do they appear?
Ø False voltages come from energized circuits and non-energized wires that are in close proximity to each other (for example, in the same conduit or wireway). This condition creates a capacitor that creates capacitive coupling between an energized wire and an adjacent unused wire.
Ø When the multimeter leads are placed between an open circuit and a neutral conductor, a complete circuit is effectively formed through the multimeter's input. 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 DMMs today have input impedances high enough to show this capacitively coupled voltage (thus giving the false impression that the conductors are live). 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 can be spent troubleshooting circuit problems
Ø Common places to encounter spurious voltages are blown fuses in switchboards, unused cables or wires in existing conduit, cassettes in 1 V branch circuits or where 1 V control circuits are used to control assembly lines or delivery functions disconnected ground or neutral. A spurious voltage of some magnitude can couple from the live side to the open side of a blown fuse. When constructing a facility or building and making electrical connections, electricians often run extra wire in conduit for future use. These wires are usually left unconnected until used, but capacitive coupling can occur. For control circuits, the location of the circuit is often adjacent to unused control lines, thus creating a spurious voltage.
