Introduction to the Purpose of a Dual-Impedance Digital Multimeter
This means that when a digital multimeter is placed in a circuit for measurement, its impact on circuit performance is minimal. This is the effect required for most voltage measurement applications, especially for sensitive electronic or control circuits.
Older troubleshooting tools such as analog multimeters and solenoid testers typically have low impedance input circuits of 10 kiloohms or less. Although these tools will not be deceived by false voltages, they can only be used to test power circuits or other circuits with low impedance that will not affect or alter circuit performance.
By using dual impedance measuring instruments, technicians can efficiently troubleshoot sensitive electronic or control circuits, as well as circuits that may contain false voltages, and more reliably determine the presence of voltage on the circuit. On the Fluke 11X series digital multimeter, the Vac and Vdc switch positions of the instrument are generally in the high impedance position. These switch positions can be used for most troubleshooting work, especially for sensitive electronic loads.
What is false voltage? Where did they appear?
False voltage comes from electrified circuits and unpowered wires that are very close to each other (such as in the same conduit or trunking). This situation can form a capacitor, which will generate capacitive coupling between the energized wire and adjacent unused wires.
When the wire of a multimeter is placed between an open circuit and a neutral conductor, a complete circuit is effectively formed through the input of the multimeter. The capacitance between the connected thermal conductor and floating conductor is combined with the input impedance of the multimeter to form a voltage divider. The multimeter then measures and displays the obtained voltage value. Most digital multimeters today have a sufficiently high input impedance to display this capacitive coupling voltage (thus giving the false impression that the conductor is charged). The multimeter actually measures the voltage coupled to the disconnected conductor. But sometimes, these voltages can reach 8085% of the "hard wired" voltage. If they are not identified as false voltages, it will consume additional time, effort, and funds in troubleshooting circuit issues
The most common locations where false voltage is encountered are the blown fuses in the distribution panel, unused cables or wires in existing conduits, and ground or neutral wires in 1-V branch circuits or card boxes that use 1-V control circuits to control assembly lines or transport functions. A certain amount of false voltage can couple from the live side of the blown fuse to the open side. When constructing facilities or buildings and conducting electrical wiring, electricians often pass additional wires through conduit for future use. These wires usually remain unconnected before use, but capacitive coupling may occur. For control circuits, the location of the circuit is usually close to unused control lines, resulting in a false voltage.
