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| My metal workbench keeps shocking me (and related ESD questions) |
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| electrozap:
I bought a house a few years ago, and it came with a very large workbench that has a flat metal top. I noticed more recently that the more I plug in, the more the AC voltage seems to be building up on the bench, and it's starting to get bad enough that it will lightly shock me while I'm working with an ESD strap. I'm somewheat concerned that the bench may potentially break my electronics projects and/or myself as I've been working from home with this setup. I have a large ESD mat that seems to cut down on the overall voltage as the metal connection goes through the mat to the bench, but without the mat plugged in, I get 11VAC (using a trueRMS multimeter). My projects stay on top of the ESD mat, but sometimes my wrists get shocked when they hit the metal surface while I'm soldering. I'd imagine that the esd mat is probably going to also have some ammount of AC fluctuation as the closest path to some sort of voltage would be straight to the metal workbench. If that is the case, then my ESD safe soldering iron would be 11V with respect to the piece that I'm working on, which wouldn't be good. I considered simply grounding the entire tabletop, but I'm a bit concerned that it might cause more issues down the line if there is a 0 Ohm connection to ground. Adding a second 1Mohm connection only cuts down on the voltage. It doesn't eliminate it. If grounding the benchtop is the better solution, please let me know. My concern is that if I'm at a voltage and I touch my grounded bench, then I would be the shortest path to ground. If that shouldn't be a concern, please let me know. Has anyone run into this issue in the past, and what would be the best practice for reducing or eliminating the induction issues with my metal workbench? I want it to be safe for me, and I'd also like it to be safe for my projects. I hope to have my kids work on projects with me in the future too, and I'd like to be able to not worry about it shocking them either. Thank you! |
| andy3055:
I think it is dangerous as you have noted. I would get rid of that surface and replace it with a plywood. Probably the earlier owner did not do the kind of work you are doing. |
| Benta:
No reason to replace the workbench. But of course the workbench and your mains connected instruments should be grounded, or more precisely: connected to "protective earth" (PE). The general rule is: all test equipment and whatever you can touch should be grounded/earthed (exception: hand-held instruments like multimeters). Things you're working on should be isolated, eg, through an isolation transformer, unless it's low voltage (<50 V). This will not protect you from live/neutral shocks on your (isolated) device under test! But that's intellectually manageable. |
| TimFox:
Your symptoms seem to indicate that the PE or 3rd-wire ground on the outlet near the bench is disconnected or high-impedance. Probably, each of your pieces of test equipment has a capacitor from line to ground (3rd pin) and from neutral to ground inside. If the ground connection is open, then the 3rd-wire is essentially driven by a capacitive voltage divider from the line-to-neutral voltage, which for US 120 V single-phase gives 60 V AC in series with several nF, and the series capacitance increases with the number of devices plugged in, as you reported in the first sentence. Easy to check: with an appropriate DMM, measure the voltages from the wider blade (neutral) and narrower blade (line) to the PE (round) at the outlet, with a piece of equipment plugged into the other outlet at that outlet box. As Benta said, any exposed metal you can touch must be connected directly to PE, but not to neutral. |
| coppercone2:
ehh you better make sure that ground is good if you want to trust a metal work bench.. i.e. check to see if it can trip a breaker with a ground short think about what happens if your protective earth is connected by a loose screw or in a rusty junction box. What happens if ground impedance is too high to allow enough current to flow to trip the breaker..? (voltage divider). I would say really you want to make sure it reliably trips the breaker if there is a fault. If you have a GFIC then its better of course, but being able to trip the GFIC is a benefit of ground, not the purpose. |
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