Why non zero voltage with one probe on wall outlet and other floating (NC)

[perkabrod]

I recognize this phenomina when trying to find a hum in a Marshall guitar amplifier.
Even with the unit switch off, the chassis had a few volts potential if I let one of the probe not touching anything but air, of course, as long as a power cord was plugged in to the unit.

Today I made a little test video but this time I used an AC-AC transformer because it's easy to access the pins and take measurements
IN VIDEO, the black probe is just touching the air, it's laying on the floor.
https://youtu.be/H6eFCG_4900

Note, most swedish households don't have ground connections except for kitchens or if the house was built in the 90's or later

[Dago]

The circuit still has capacitive coupling with the probe not connected and this is enough for a current to pass through and your meter to measure a voltage.

[perkabrod]

Thanks Kalle!

[alm]

The term to search for is stray voltage or ghost voltage. For example this Fluke page, with, unsurprisingly, a piece of equipment they want to sell you .

[jeroen79]

a piece of equipment they want to sell you .

Which basically is a 3k resistor priced around $60.

[alm]

Hey, they also give you a nicely insulated case. And possibly some overvoltage protection (PTC). While I wouldn't recommend any hobbyist to spend $60 on this, I wouldn't suggest putting a bare axial 3 kOhm resistor with some clip leads across a possibly live mains circuit either.

[David Hess]

The circuit still has capacitive coupling with the probe not connected and this is enough for a current to pass through and your meter to measure a voltage.

You can take advantage of this to make some difficult measurements.  One unfortunately common wall socket wiring mistake is to swap hot and neutral.  One lead of a high impedance voltmeter like any common multimeter will show a low voltage (close to zero) for neutral and ground and 120/240 volts for hot without relying on any other wiring to be correct.  It will also reveal an isolated 120/240 volt output by reading about 60 or 120 volts for hot and neutral; this could happen if the ground bonding wire is open.

Note that you get better accuracy by holding the common multimeter lead in your hand to increase the capacitive coupling but I do not really recommend this to others even though I do it myself.

[jeroen79]

Hey, they also give you a nicely insulated case. And possibly some overvoltage protection (PTC).

Not to mention a Fluke logo.

While I wouldn't recommend any hobbyist to spend $60 on this, I wouldn't suggest putting a bare axial 3 kOhm resistor with some clip leads across a possibly live mains circuit either.

Something similar could be constructed from a low power lightbulb and some suitable bananaplugs.