What is the purpose of a dual impedance digital multimeter?
Ø Older troubleshooting tools such as analogue multimeters and solenoid testers typically have low impedance input circuits of 10 kilohms or less. While these tools will not be fooled by spurious voltages, they should only be used to test power supply circuits or other circuits where the low impedance will not affect or change the performance of the circuit.
Ø Best Use of Both Functions By using a dual impedance measurement instrument, technicians can** troubleshoot sensitive electronic or control circuits, as well as circuits that may contain spurious voltages, and are able to more reliably determine whether a voltage is present on a circuit. On Fluke 11X series digital multimeters, the Vac and Vdc switch positions of the meter are typically in the high impedance position. Most troubleshooting tasks can be performed using these switch positions, especially for sensitive electronic loads.
What are spurious voltages? Where do they occur?
Ø Spurious voltages come from energised circuits and unenergised conductors in close proximity to each other (e.g. in the same conduit or raceway). This situation can form a capacitor that will create capacitive coupling between the energised conductor and the adjacent unused conductor.
Ø With the multimeter lead 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 and floating conductors combines with the multimeter input impedance to form a voltage divider. The multimeter then measures and displays the resulting voltage value. Most of today's digital multimeters have a sufficiently high input impedance to display this capacitively coupled voltage (thus giving the false impression that the conductor is charged). What the multimeter actually measures is the voltage coupled into the disconnected conductor. Sometimes, however, these voltages can be as high as 8085% of the hard-wired voltage. Failure to recognise them as spurious voltages can cost extra time, effort and money when troubleshooting circuit problems.
Ø The most common locations where spurious voltages are encountered are blown fuses in distribution panels, unused cables or wires in existing conduit, and disconnected ground or centre wires in 1 V branch circuits or in card boxes using 1 V control circuits to control the assembly line or delivery function. A spurious voltage of a certain magnitude can be coupled from the live side of the burnout fuse to the open side. When constructing a facility or building and performing electrical wiring, electricians often run extra wires through the conduit for future use. These wires usually remain unconnected until they are used, but capacitive coupling can occur. In the case of control circuits, the circuit is often located close to unused control wiring, thus creating a spurious voltage.
