What Are the Applications of a Dual-Impedance Digital Multimeter?
Analog multimeters and solenoid testers, as classic troubleshooting tools, typically feature low-impedance input circuits of 10 kΩ or less. Although these tools are not fooled by ghost voltages, they can only be used for testing power circuits or other circuits where low impedance will not affect or alter circuit performance.
Optimizing the benefits of both functions through the use of dual-impedance meters allows technicians to accurately troublesensitive electronic or control circuits, as well as circuits that may contain ghost voltages, and to more reliably determine whether voltage is present on a circuit. On Fluke 11X series digital multimeters, the meter's Vac and Vdc switch positions are typically in the high-impedance mode. These switch positions can be used for most troubleshooting tasks, especially on sensitive electronic loads.
Ghost voltages originate from energized circuits and de-energized conductors running close together, such as in the same conduit or raceway. This arrangement forms a capacitor, creating capacitive coupling between the energized conductor and adjacent unused conductors.
When multimeter leads are placed between an open circuit and a neutral conductor, a complete circuit is effectively formed through the meter's input. The capacitance between the connected hot conductor and the floating conductor, combined with the multimeter's input impedance, forms a voltage divider. The multimeter then measures and displays the resulting voltage. Most modern digital multimeters have sufficiently high input impedance to display such capacitively coupled voltages, giving the false impression that the conductor is energized. What the multimeter actually measures is the voltage coupled into the open conductor. In some cases, these voltages can reach 80–85% of the hardwired line voltage. Failing to identify them as ghost voltages can result in wasted time, effort, and cost when troubleshooting circuit issues.
Ghost voltages are most commonly encountered at blown fuses in distribution panels, unused cables or conductors in existing conduits, open ground or neutral wires in 120V branch circuits, or in cartridges using 120V control circuits for assembly line or conveying functions. A significant ghost voltage can be coupled from the energized side of a blown fuse to the open side. When facilities or buildings are constructed and wired, electricians often pull extra conductors in conduits for future use. These conductors typically remain disconnected until service, but become capacitively coupled. For control circuits, the circuit is often routed adjacent to unused control wiring, resulting in a ghost voltage.
