What is the purpose of a dual-impedance digital multimeter?
Analog multimeters, solenoid testers, and other old-fashioned fault tools typically feature low-impedance input circuits of 10 kilohms or less. While these tools are not deceived by false voltages, they can only be used to test power circuits or other circuits where low impedance does not affect or alter circuit performance.
By using a dual-impedance measuring instrument, technicians can safely and accurately troubleshoot sensitive electronic or control circuits, as well as circuits that may contain false voltages, and can more reliably determine whether a voltage is present in the circuit. On Fluke 11X series digital multimeters, the meter's Vac and Vdc switch positions are generally in the high-impedance position. These switch positions can be used for most troubleshooting tasks, especially for sensitive electronic loads.
What is False Voltage?
False voltages arise from energized circuits and unpowered wires in close proximity (e.g., within the same conduit or trunking). In such cases, a capacitor-like effect forms, creating capacitive coupling between the energized wire and adjacent unused wires. This coupling induces a voltage in the unpowered even though they are not connected to a power source, leading to misleading readings on measurement tools, especially high-impedance multimeters.
When placing a multimeter's leads 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, combined with the multimeter's input impedance, creates a voltage divider. The multimeter then measures and displays the resulting voltage value.
Most modern digital multimeters have sufficiently high input impedance to reveal such capacitively coupled voltages-thereby giving a false impression that the conductor is live. What the multimeter actually measures is the voltage coupled into the disconnected conductor. In some cases, these voltages can reach 80–85% of the hard-wired voltage. Failing to identify them as false voltages can lead to wasted time, effort, and money during circuit troubleshooting.
