To try and improve noise levels during measurements (and as general ESD precaution), I went “green”:
That’s a green ESD mat, covering almost the entire workbench. It’s hooked up to the radiator for grounding. Note that the mat only provides a “dissipative” connection to ground, the surface still has several MΩ’s of resistance. It’s just to get rid of static electricity and to offer a clean protective working surface. Got the mat from Farnell, BTW.
Here’s what I see when pushing a scope probe onto the mat:
A clear 50 Hz pattern of a couple of volts (the amplitude increases when the probe is pushed more firmly onto the mat). This is with a standard 10 MΩ high impedance probe. The big puzzle is: where does this come from?
My explanation for now, is that the scope ground is “floating” a bit, due to the different devices hooked up to the house wiring. Note that the mat is tied to the radiator, not to the ground of AC mains. Since there isn’t any current flowing through the heating system (I hope!), I’m inclined to trust it more as being the “real” ground potential.
It’s no doubt completely harmless. Measuring AC current between these two ground levels with my most sensitive multimeter (a Voltcraft VC940), I see a very occasional blip of up to 0.20 µA flowing. Well under a microwatt.
To give an idea how crazy things can get, here’s the mat with nothing connected – turning it into one big antenna:
Who knows, maybe one day we could harvest even this sort of energy, eh?