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I accidentally stuck my oscope probe into my ESD mat ontop of my work bench. I happened to look at my oscope. Surprise! There's a 20Vpp 60Hz voltage on the mat! (see picture) (note, the ground clip of the probe is not connected to anything)

The ESD mat is not currently grounded (before anyone jumps on me, I had it disconnected to move it, and haven't put it back yet because I haven't been dealing with anything that is ESD sensitive.) When I connect the mat directly to earth the voltage flat lines and all is well. When I connect the ESD wrist strap to the mat and then to earth, the voltage reduces to about 3 or 4Vpp. The wooden bench top itself measures nothing at all. There is a power strip screwed into the top of the bench, but its casing is ground to ?earth.

My question is how is that 20Vpp on the mat itself? If it isn't actually there, what am I witnessing? Should I directly earth my mat or is that dangerous? I have stopped working on the bench until I figure out what is going on.

NOTE:I will be happy to edit this question, or delete it if it is off topic or doesnt make sense. But I'm not even sure how to ask this question properly.enter image description here

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  • \$\begingroup\$ Your question is on topic but has been asked before. Someone will dig up a link soon. \$\endgroup\$
    – winny
    Jan 31, 2017 at 20:20
  • \$\begingroup\$ During our required industry training on ESD protection, it was pointed out that almost every electronic component is ESD sensitive, obviously some more than others. I recommend that you connect your mat ASAP; don't assume any components are immune. \$\endgroup\$
    – Barry
    Jan 31, 2017 at 20:22
  • \$\begingroup\$ here I think this is close, but mine was a bit more specific. \$\endgroup\$
    – Rayray
    Jan 31, 2017 at 21:13

2 Answers 2

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You are seeing stray voltage being picked up by an isolated (floating) surface. You will see something similar if you touch the tip of the probe while wearing insulated shoes, or attach a long cliplead to the probe. The mat, your body, the cliplead, all act as antennas. There are 60Hz electromagnetic fields in your lab, created by the wiring in the walls, the power lines, etc. Since the scope probe has a very high impedance, it needs only microamps of current to display a signal.

This is normal, and not dangerous. I would go ahead and ground the pad.

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  • \$\begingroup\$ found this that backs up your answer as well. \$\endgroup\$
    – Rayray
    Jan 31, 2017 at 21:14
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Your ESD mat is one plate of a 3-plate capacitor, that is, of 2 capacitors in series. The mat has HUGE area, with power-strip and incandescent lights and power cords all coupling ---- just a bit ---- to the mat. The mat also couples to Ground, just a bit. That you see 20 volts across 10Meg ohm means you have 0.1 microAmps of "displacement current" coupling thru the air.

To experiment, place aluminum foil under the mat, separate the foil from the mat with sheet of newspaper, and monitor the mat with scope probe.

Then ground the foil. I'd expect to see the probe waveform collapse to zer.

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