But occasionally a digital multimeter will show voltage present, even on a circuit that should not be energized. Is it a real and potentially dangerous voltage, or merely “ghost voltage” caused by capacitive coupling between energized wiring and adjacent unused wiring?
“The other day I had a situation where I had 73 volts showing up on a circuit,” says Walstad, a second-generation electrician with more than 30 years experience. The circuit in question: a control circuit on a concrete transfer cart used to move material from the mixer to the molds in a factory making pre-cast concrete light poles. In the wet, dirty plant environment, the potential problems were many. “It should have been either 110 volts, or nothing.”
Hunting Down the Ghosts
In the past, tracking down such a voltage indication could send the technician off on a time wasting wild goose chase, looking for a problem connection that was never there. He could ferret out the ghost voltage with a low-impedance analog meter or a solenoid tester, or ‘wiggy’ - but that would require packing or fetching an extra tool.
But Walstad had extra help. The Fluke 117 Digital Multimeter he was using that day has dual impedance capability; incorporating both regular high impedance test capabilities and low impedance functions for detecting ghost voltages. By switching to the meter’s Auto-V/LoZ (low impedance) test setting, Walstad could see instantly that the 73 volts was only a ghost. “It saved me a trip back to the truck to get another meter,” Walstad says.
Electrician Eldon Walstad will occasionally run into "ghost voltages" on a circuit. Tracking these down can be very time-consuming.
Ghost Voltage Can Look Real
Ghost voltages can be caused when energized circuits and non energized wiring are located in close proximity to each other, such as in the same conduit or raceway. This condition forms a capacitor and allows capacitive coupling between the energized wiring and the adjacent unused wiring.
When you place your multimeter leads between the open circuit and the neutral conductor, you effectively complete the circuit through the input of the multimeter. The capacitance between the connected, hot conductor and the floating conductor forms a voltage divider in conjunction with the multimeter input impedance. The multimeter then measures and displays the resulting voltage value.
Most digital multimeters today have an input impedance that’s high enough to show this ghost voltage, giving a false impression of a live conductor. The meter is actually measuring voltage coupled into the disconnected conductor. But at times, these voltages can be 80-85 % of what the “hard” voltage should be. If not recognized as a ghost voltage, additional time, effort and money may be lost troubleshooting circuit problems.
How Impedance Affects Testing
Most digital multimeters used for testing industrial, electrical and electronic systems have high impedance input circuits greater than 1 megohm. This means that when the DMM is placed across a circuit for a measurement, it will have little impact on circuit performance during the test. This is the desired effect for most voltage measurement applications, and is especially important for sensitive electronics or control circuits.
Older troubleshooting tools such as analog multimeters and solenoid testers generally have low impedance input circuitry around 10 kilohms or less. While these tools aren’t fooled by ghost voltages, they should only be used for testing power circuits or other circuits where the low impedance will not negatively impact or alter circuit performance. They often don't comply with the current IEC 61010 safety standards and North American regulatory requirements.
The Best of Both Worlds
With dual impedance meters, technicians can safely troubleshoot sensitive electronic or control circuits, as well as circuits that may contain ghost voltages, and can more reliably determine whether voltage is present on a circuit.
“I like it,” Walstad says of the 117. “When you’re trying to prove something, you’ve got to know whether you’ve got real power there or not. If you think you’ve got power but don’t, then you can go off in a different direction trying to find problems when you really need to be back here working on something. You’re not wandering aimlessly, looking for it.” And there are no ghosts in sight.
Information courtesy of Fluke Corporation.