# Reliable way of measuring mains voltage by capacitive coupling with oscilloscope

The question is not about any precise measurements, just for the lack of a better title.

Trying to model what one of those non-contact voltage detectors would see when brought closer to a live wire.

In my understanding it should see an increase in the amplitude of the capacitively coupled waveform of the mains frequency because the closer the 2 conductors are the greater the capacitance.

Now when touching a metal object (e.g. ruler) with an oscilloscope probe it registers a mains frequency waveform. I would expect that if I get the ruler closer to a mains wire then the amplitude will increase, but it doesn't happen it actually decreases and increases when I move the metal object further away.

What's happening?

I guess there may be other factors at play like the fact that the oscilloscope is powered from the same mains (tried isolating it with a transformer but this didn't change this behavior), the mains has no ground wire etc.

• Are you trying for an accurate measurement in any way? Or just a 'ballpark' value? Usually differential potential gives more predictable results. – Sparky256 Jan 13 '18 at 23:16
• Just a 'ballpark', to test my understanding of how capacitive coupling to mains works and how to measure it with an oscilloscope. – axk Jan 13 '18 at 23:28
• Are you coupling to Line or Line+ neutral pair? try line in a loop and compare probe loop as a current sense. Then Voltage sense with long probe wire near Line (not neutral) and then wave hand over gap. – Sunnyskyguy EE75 Jan 13 '18 at 23:49
• "I would expect that if I get the ruler closer to a mains wire then the amplitude will increase, but it ... actually decreases and increases when I move the metal object further away." - were you holding onto the ruler when you did this? I tried connecting my scope probe to a ruler lying on the bench, then moving a power cord up to it. Peak voltage increased from 0.2V at 1m to 10V with the power cord on top of the ruler. My body alone had negligible effect unless I touched the ruler (then the voltage went up to 5V, and did not change when the ruler was brought close to the power cord). – Bruce Abbott Jan 14 '18 at 5:54
• I didn't touch the ruler, aware of the body capacitance contribution to the coupling. I should probably try in a different envioronment than my desktop, there's probably nothing inherently wrong with this setup but all the devices on my desktop like the PC and UPS that all get capacitively coupled to mains and to the probe. – axk Jan 14 '18 at 10:52