Ghost/stray voltage from high impedance DMM

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WAsparky37493

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Washington state
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Electrician
Hi all, new to this forum but have been reading for a long time. Want to say thank you to all you guys as you have educated and helped me with many things.

I was recently doing a small job. I moved a light to center it from the room. This is an older home I'm assuming around 60s due to the anaconda romex in the house. The power is at the light and just one 14-2 without ground down to the switch. The light had quite a few other wires in it supplying receptacles and power over to other bedroom lights. I junctioned these all in the attic with a 2 gang. When I came down to test the switchleg and make sure I got the right wire from the switch hooked up to the light my hot stick was going off with the switch off. I removed the switch completely to open the switchleg and my hot stick was still going off. I then used my DMM to check voltage. I was getting 50 volts from hot to neutral at the light. When I tied the switchleg back to power I got proper 120 volts. With the switch off I hooked the light up to the hot and neutral. This made the voltage reading go from 50 volts between hot and neutral down to millivolts. Also when using my wiggly I got no voltage reading with the switch off and 120 volts with switch on.

This is my first time running into this. From what I have read I believe this is capacitive coupling from the switchleg wire. The swirchleg wire is in the box with all the power junctioned. One of the wires is tied in with all power and all power going down to the switch. The other wire Is tied into the new wire I ran out of the box to hit the new light box location.

This voltage concerned me a little bit. I first wanted to make sure things were bonded correctly. They appear to be bonded properly in the panel. #6 ground out to a rod, neutral is bonded to the GEC at the main panel. However I went from the metal fridge casing to A phase and read 55 volts with my high impedance DMM. With my wiggly I measured 0 volts. Could these be false readings due to my high impedance multi meter? Another note that may be important is this home has no equipment grounds on the majority of circuits due to the anaconda romex without an EGC.
 
Your meter is giving you false readings. use a fluke 115 DVM. It has a low ohm setting. My go to meter is a T+Pro, but the 115 is more versitile
Many old timers prefer a wiggy, but it is not as safe as the 115 or newer electronic meters
 
tom baker said:
Your meter is giving you false readings. use a fluke 115 DVM. It has a low ohm setting. My go to meter is a T+Pro, but the 115 is more versitile
Many old timers prefer a wiggy, but it is not as safe as the 115 or newer electronic meters
Click to expand...
Thanks Tom, I will be picking one of those up on my next trip to the supply house. My wiggy and my very simple klein analog tester did not show voltage. My fluke t5 and my klein DMM both showed the 50 volts.
 
WAsparky37493 said:
Also when using my wiggly I got no voltage reading with the switch off and 120 volts with switch on.
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This is the only accurate reading. The meters would also have read zero if you had the light connected.

I use a solenoid tester for troubleshooting, and a voltmeter only when I need to know the exact voltage.

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Capacitive coupling yes, also sounds like maybe no EGC in the switch loop either or it would have lessened or even nearly eliminated the voltage you were reading. Back in those days an EGC in said switch loop wasn't necessarily a common thing.
 
Your reading is not giving a 'false' reading. That 50V is really there.

The 50V is a 'phantom' in the sense that it is being supplied through a very high impedance. If you add a gnats whisker of a load on it the voltage would drop to 0. But before you add that load, it is really there.

This voltage comes from capacitive coupling across the conductor insulation or from the minute leakage current through the insulation.

For something like a residential switch loop this impedance is so high (and the current so low) that this phantom voltage is nothing more than a measurement nuisance. But in commercial settings operating at higher voltage with longer runs, this phantom voltage can be enough to shock someone.

-Jon
 
kwired said:
Capacitive coupling yes, also sounds like maybe no EGC in the switch loop either or it would have lessened or even nearly eliminated the voltage you were reading. Back in those days an EGC in said switch loop wasn't necessarily a common thing.
Click to expand...
You are correct, no EGC on switch loop, or on any of the original wiring of the home as it was all done with anaconda romex with no EGC wire
 
winnie said:
Your reading is not giving a 'false' reading. That 50V is really there.

The 50V is a 'phantom' in the sense that it is being supplied through a very high impedance. If you add a gnats whisker of a load on it the voltage would drop to 0. But before you add that load, it is really there.

This voltage comes from capacitive coupling across the conductor insulation or from the minute leakage current through the insulation.

For something like a residential switch loop this impedance is so high (and the current so low) that this phantom voltage is nothing more than a measurement nuisance. But in commercial settings operating at higher voltage with longer runs, this phantom voltage can be enough to shock someone.

-Jon
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Thanks for the detailed reply Winnie. That explains why once I hook the light up to the load drops down to basically nothing.
 
Anytime I am on the load side of the service equipment, my preferred tester is a solenoid tester. They typically have a CAT III safety rating which permits their use up to 1000 volts on feeders or branch circuits.
 
WAsparky37493 said:
Thanks Tom, I will be picking one of those up on my next trip to the supply house. My wiggy and my very simple klein analog tester did not show voltage. My fluke t5 and my klein DMM both showed the 50 volts.
Click to expand...
It’s a fluke 117. This meter has a volt alert built in. I use the volt alert to check for presence of voltage and the low z to check for absense
 
WAsparky37493 said:
You are correct, no EGC on switch loop, or on any of the original wiring of the home as it was all done with anaconda romex with no EGC wire
Click to expand...
EGC doesn't eliminate the capacitance, it just completes the circuit which will lessen or eliminate the voltage you see between the conductors. current is in the milliamp or even microamp ranges though so you aren't measuring it without pretty sensitive equipment. An EGC that is present in the cable but not connected for whatever reason will still read voltage though.
 
ptonsparky said:
Distance between plates of capacitor.
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That is what makes it relatively weak capacitor in the first place. Not connecting the EGC will leave it in an open circuit condition and will allow us to read higher voltage between the plates. Closing the circuit lets current flow, but is such a weak capacitor that it can't supply any significant amount of current. Similar thing happens if you connect low impedance meter across it you made a current path but the capacitor is weak enough it won't let any significant amount of current flow, a milliamp would possibly be enough you would actually register a voltage reading but we almost never get any reading at all in this sort of situation when using low impedance meter.
 
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