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My metal workbench keeps shocking me (and related ESD questions)
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bdunham7:
First, I would ground the bench.  What is your ESD strap connected to?? It seems bizarre to have that much leakage, but I suppose it is possible.  I have a bench with a metal riser and all sorts of equipment on it, but it is all pretty high quality stuff, so maybe that makes a difference.  I don't see how you would have significant leakage just by being in proximity. 

Second, as suggested, an electronics workbench really should have a non-conductive surface.  I'd recommend a laminate or similar.

https://www.grainger.com/category/material-handling/storage-workspace/workbenches-shop-furniture/workbenches-and-accessories/workbench-components-and-accessories/workbench-tops


--- Quote from: electrozap on May 31, 2022, 09:29:56 pm --- The voltage from neutral to ground doesn't change,

--- End quote ---

How much voltage is there between neutral and ground?
electrozap:

--- Quote from: Gyro on May 31, 2022, 09:38:54 pm ---Regardless of the result of your grounding investigation, you should cover the metal benchtop with an insulating (read, highly resistive) surface. As andy3055 suggested, plywood, MDF or other sheet wood material are ideal. Include lipping battens around the edges too. The reason for this is that conductive metalic surfaces are really bad from the ESD perspective - a device pin making direct metallic contact will maximise the current spike induced by any potential difference.

--- End quote ---

Would a large rubber/high-temp-style ESD mat meet this requirement, or would I also need to put something between it and the metal surface? It should be dissapative and high-resistance. I currently have a relatively large one on top of the surface, and i think that I'd get a bigger one after reading this thread in order to minimize this risk. Thank you!


--- Quote from: Benta on May 31, 2022, 10:02:49 pm ---
--- Quote from: electrozap on May 31, 2022, 09:29:56 pm ---ESD resistances are typically 1Mohm. Is that for safety of the operator or safety of the electronics?

--- End quote ---
Don't mix up component protection with personal safety. All replies here are until now about personal safety for you and your family.

Ideally, a PE connection to your table and equipment would have 0 ohms. But it does not seem so.
You now have two options:
1: get in touch with your utility to get the ground/PE thing cleared/fixed.
2: make your own PE. This is extremely soil dependent. In moist humus/clay, hsmmering a 5-foot copper rod into the ground works wonders. In the Mojave desert it would be useless.

Concerning your workbench itself: I'd lay out a rubber mat (non-conducting) over the surface. The ESD mats are completely secondary to getting PE right and can be placed afterwards.


--- End quote ---

As I mentioned above, we had the electrial box replaced when we moved in. At the time, we had two grounding rods installed per the electrician's reccommendation at the time. The PE connection to the equipment is fine. I think the reason why the bench is at a different potential is that the bench currently has no physical connection to anything at all, and the only voltages that are applied are inductive. If I unplug everything and ground it, the voltage stays at 0V even after removing the ground wire. It also stays at 0V if I connect a wire from ground to the metal workbench. I was mostly wondering if grounding the bench that is currently floating is a safe thing to do.

My grounding setup is currently:

two earth ground rods -> breaker box (where ground and neutral are typically joined) -> GFCI outlet -> equipment/grounded ESD mat on top of an isolated metal surface. The question is if I should ground the metal surface and use that ESD mat as the isolation between ground and the piece that I'm working on.


--- Quote from: bdunham7 on May 31, 2022, 10:12:30 pm ---First, I would ground the bench.  What is your ESD strap connected to?? It seems bizarre to have that much leakage, but I suppose it is possible.  I have a bench with a metal riser and all sorts of equipment on it, but it is all pretty high quality stuff, so maybe that makes a difference.  I don't see how you would have significant leakage just by being in proximity. 

Second, as suggested, an electronics workbench really should have a non-conductive surface.  I'd recommend a laminate or similar.

https://www.grainger.com/category/material-handling/storage-workspace/workbenches-shop-furniture/workbenches-and-accessories/workbench-components-and-accessories/workbench-tops


--- Quote from: electrozap on May 31, 2022, 09:29:56 pm --- The voltage from neutral to ground doesn't change,

--- End quote ---

How much voltage is there between neutral and ground?

--- End quote ---

My ESD strap is connected to ground at an outlet through a screw that I confirmed is connected to ground as it is using grounded metal conduit. https://www.amazon.com/gp/product/B01M7RULJN/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1 This is my current mat.

The voltage between neutral and ground is 0V. This outlet is in the same room as the breaker box.

My assumption that it's inductive leakage is from the fact that I can change the voltage by moving the wall warts closer to and further away from the metal surface.
Benta:
OK, last input from my side, this is getting tedious:
GROUND YOUR WORKTABLE TO PE! PERIOD!
Second, why are you so focused on ESD? The important thing is, that your working area does not produce any false results. An ESD mat is conductive, giving problems. Cross the t's and dot the i's, please.
An isolating rubber mat would make everything reliable.
IF you at some point would have to handle sensitive parts, laying out an ESD mat with a connection to your table would be easy. Same with a strap.
Just for info: I've never in 40 years "zapped" a CMOS part due to ESD.

Last comment: the voltages you're measuring are not inductively coupled, but capacitively.
bdunham7:

--- Quote from: electrozap on May 31, 2022, 10:29:25 pm ---The question is if I should ground the metal surface and use that ESD mat as the isolation between ground and the piece that I'm working on.

--- End quote ---

Yes, that would be the way to go.  You could even run the bench ground directly to one of your grounding rods.  Green-insulated 12AWG would be plenty good.  Other colors would work but would be considered bad form.


--- Quote ---The voltage between neutral and ground is 0V. This outlet is in the same room as the breaker box.
My assumption that it's inductive leakage is from the fact that I can change the voltage by moving the wall warts closer to and further away from the metal surface.
My ESD strap is connected to ground at an outlet through a screw that I confirmed is connected to ground as it is using grounded metal conduit.

--- End quote ---

If the potential is getting high enough to shock you, then either the resistor in your ESD strap is too low of a value or there's just too much leakage.  I personally would ground the bench directly and then use the bench as the ESD strap ground.  That leakage is typically called 'capacitive' but I understand what you are saying--I just think that it seems like a lot.  I mean a whole lot.  You'd need tens of µA to even get a tingle and I'd expect body leakage to be nA or less.  Are the secondaries of these wall warts connected to something on the bench?

Anyway, yes--ground that bench!  Just be aware that static electricity shocks will still happen.

james_s:
The metal benchtop and any exposed metal that could conceivably become electrically energized should be bonded solidly to earth ground. Next if you're going to be working on energized electrical devices the surface should be covered in something that will prevent a direct short to this grounded surface, but that is a separate issue. If the metal is there, it should be grounded.
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