What are some uses for a dual impedance digital multimeter?
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.
Best of Both Worlds By using dual impedance measuring instruments, technicians can accurately troubleshoot sensitive electronic or control circuits, as well as circuits that may contain spurious voltages, and more reliably determine the presence or absence of voltage on a 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?
False voltages come from energized circuits and non-energized wires that are in close proximity to each other (such as in the same conduit or wireway). This condition creates a capacitor that creates capacitive coupling between an energized wire and an adjacent unused wire.
When you place the multimeter leads between the open circuit and the neutral conductor, you effectively form a complete circuit 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
The most 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.
