My metal workbench keeps shocking me (and related ESD questions) - Page 1
Forum section: electronics / general

author: electrozap
author_rank: Contributor
timestamp: May 31, 2022, 08:38:46 pm
content: 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!

author: andy3055
author_rank: Super Contributor
timestamp: May 31, 2022, 08:44:36 pm
content: 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.

author: Benta
author_rank: Super Contributor
timestamp: May 31, 2022, 08:58:56 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2022, 09:10:46 pm
content: 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.

author: coppercone2
author_rank: Super Contributor
timestamp: May 31, 2022, 09:11:31 pm
content: 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 shortthink 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.

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2022, 09:15:30 pm
content: When installing equipment in foreign countries, with suspicious electrical connections, we discovered the "Dan Effect", as in "It's hard to shock Dan, but...", when connecting tower computers with normal line filters to single-phase supplies, and connecting the computers to grounded equipment.

author: BrokenYugo
author_rank: Super Contributor
timestamp: May 31, 2022, 09:19:06 pm
content: Are you sure the outlets you are using are actually grounded? Most test gear is 3 prong with the earth wire connected directly to the case. You only really get leakage like that, assuming no equipment failures, if the ground is disconnected* or you have a lot of 2 prong equipment.Also, in any event, outlets in the bench area should be GFCI protected.*Quite common in older houses with 2 wire electrical, people will just slap in a 3 prong outlet and leave the ground floating or worse yet strap it to neutral.

author: electrozap
author_rank: Contributor
timestamp: May 31, 2022, 09:29:56 pm
content: Quote from: Benta on May 31, 2022, 08:58:56 pmThat makes sense. I'm not sure how I would go about replacing this surface or even go about getting rid of it. It's a huge thick metal slab that is 279.5cm/114" x 71cm/28" x 4.5mm/.18" thick meaning that it could be up to 163lbs/74kg of steel. I'd need to probably isolate it with a covering before considering getting rid of it. If I were to ground it though, is that consiered somewhat safe? ESD resistances are typically 1Mohm. Is that for safety of the operator or safety of the electronics? My understanding is that the chassis of large appliances are typically earth grounded. Would this be like that? The maximum voltages that I tend to deal with are 60VDC, but those are isolated power supplies.I personally installed and tested the GFCI protection circuit going to this particular set of outlets. That wouldn't help with the inductive load, but it would help if I were to ground out a voltage coming from mains to the workbench.Quote from: coppercone2 on May 31, 2022, 09:11:31 pmThe breaker box and the ground rods were replaced when we moved in. There was a grounding issue, but it isn't there anymore.Quote from: TimFox on May 31, 2022, 09:10:46 pmI checked this earlier. The voltage from neutral to ground doesn't change, and the voltage from live to ground and neutral stay the same, but the voltage from ground to the benchtop changes with the number of loads sitting on the bench. The worst offenders are wall warts sitting close to the bench.

author: Gyro
author_rank: Super Contributor
timestamp: May 31, 2022, 09:38:54 pm
content: 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.EDIT: For more detail, see the 'Charged Device Model' in the following reference...https://www.vlsifacts.com/esd-models-and-their-comparison-esd-part-2/

author: Benta
author_rank: Super Contributor
timestamp: May 31, 2022, 10:02:49 pm
content: Quote from: electrozap on May 31, 2022, 09:29:56 pmDon'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 to avoid short-circuits on the parts you're working on. The ESD mats are completely secondary to getting PE right and can be placed afterwards.

author: bdunham7
author_rank: Super Contributor
timestamp: May 31, 2022, 10:12:30 pm
content: 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-topsQuote from: electrozap on May 31, 2022, 09:29:56 pmHow much voltage is there between neutral and ground?

author: electrozap
author_rank: Contributor
timestamp: May 31, 2022, 10:29:25 pm
content: Quote from: Gyro on May 31, 2022, 09:38:54 pmWould 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 pmAs 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 pmMy 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=1This 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.

author: Benta
author_rank: Super Contributor
timestamp: May 31, 2022, 10:48:09 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: May 31, 2022, 10:49:23 pm
content: Quote from: electrozap on May 31, 2022, 10:29:25 pmYes, 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.QuoteIf 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.

author: james_s
author_rank: Super Contributor
timestamp: May 31, 2022, 10:59:25 pm
content: 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.

author: electrozap
author_rank: Contributor
timestamp: June 01, 2022, 12:08:24 am
content: Quote from: Benta on May 31, 2022, 10:48:09 pmThe ESD mat is dissipative, not conductive. If I take an ohm meter between the surface of the mat and the bench, it will show infinite resistance. I'm so focused on ESD because I work with people who wear polyester jackets and touch my boards after I finish working on them (I'm an embedded hardware engineer by trade), and we've been trying to work on better ESD practices company-wide. We've had broken microcontrollers, MOSFETs and other components. At this point, I don't want to leave anything up to chance on my end if it's at all possible. Apologies for being tedious.Quote from: bdunham7 on May 31, 2022, 10:49:23 pmWhen I short out the bench to the wall through the multimeter, it briefly shows a milliamp and then goes back down to close to zero. The capacitive part makes sense given that it will give a short burst of energy and slowly go down to zero when I short the bench through my multimeter. When I connect the 1Mohm resistor, the voltage goes up and down relatively slowly too. Next to the metal bench at the time, I had a fan, a laptop power supply, a 48V wall wart, a second 12V wall wart, a tool battery charger (It is in my garage), a soldering iron, a second fan for solder fumes, a 3D printer, a 60V benchtop power supply, and a phone charger. All that together, the phone charger and 48V wall wart were the worst offenders, and then it went down from there. I think the fact that I had a power strip sitting on the metal bench probably also made it worse. I was previously looking for physical connections until I saw that there weren't any and moving things effected the voltage.

author: bdunham7
author_rank: Super Contributor
timestamp: June 01, 2022, 01:17:33 am
content: Quote from: electrozap on June 01, 2022, 12:08:24 amThat's not really a valid distinction--ESD mats should have some resistance, but yours probably has more than your meter can measure.https://elimstat.com/how-to-use-esd-meter-to-test-mats/QuoteThat seems weird. If it were a capacitively coupled AC leakage, I'd expect it to be more or less continuous as you can't build up an static AC charge. What meter and what setting are you using to measure this? In any case, the solution will still be the same--ground bonding!

author: antenna
author_rank: Frequent Contributor
timestamp: June 01, 2022, 01:37:16 am
content: This reminds me of something that happened to me... I have a Paco G-30 signal generator (one of those tube era kits). There was a 3-lead split capacitor (like two capacitors in series) between the two transformer connections to ground. Unfortunately, the schematic showed a two-prong AC cord and the previous owner clearly followed the schematic right down to a lack of grounding. Anyhow, what happened was the set of capacitors there acted as a voltage divider putting 60v on the case of the signal generator, and if I would set it on my metal desk, that 60v coupled into the desk where I would be consistently reminded of the problem. For that reason, I would suspect any metal-cased equipment that is running on a 2-prong cable!

author: m98
author_rank: Frequent Contributor
timestamp: June 01, 2022, 07:22:52 pm
content: Just to chime in on the ESD setup, at work in the lab we use solidly grounded (direct connection to PE) metal sheets as the table surface, and just lay the ESD mats on top of that. Not to say that's ideal, but it works well enough. Don't see why you'd want a separate insulator below the ESD mat, just to then again connect it to a grounding point.

author: electrozap
author_rank: Contributor
timestamp: June 02, 2022, 08:03:46 pm
content: Quote from: m98 on June 01, 2022, 07:22:52 pmThat makes sense. I understand that putting another barrier may help with avoiding dead shorts and static with the board, but I didn't think that the mat itself would need to be separated from ground and then connected to it again.As an update to the surface, I used a drill and stepped up a small hole through the benchtop until I could drill in a flat self-tapping screw with a ring terminal through to the wood underneath, and I attached the other end to another ring terminal that I attached to PE/Earth Ground. I'm not seeing any AC voltage on the multimeter anymore, and the entire surface is grounded now.Thank you so much for the help everyone!

author: coppercone2
author_rank: Super Contributor
timestamp: June 03, 2022, 01:07:24 am
content: don't do a wood to metal safety ground. Put a insert into it and then screw into that or something along those lines. You want the spring that attaches the metal to the wire to be all metal and not have a wooden part. If it dislodges from the wood, you should still have a reliable safe electrical connection. A wooden washer is precisely what can get you into trouble with wires. How much do you trust a piece of wood to hold screw pressure? Maybe there are exceptions with certain kinds of wood or whatever (iron wood comes to mind), but it seems unreliable.A epoxied to wood hole brass insert comes to mind, but make sure the threads are clean. I recommend that you put other screws that are mechanical to hold it down though, and that method is fine, so its protected from heavy equipment sliding around incase you need to repair a boat anchor.If its all one piece go in from the side with a router or drill to make a cavity/cave that you expose flat sheet from on the top of the hole (use a chisel and sand paper files to clean it up) and bolt it down in that location with a bolt-sheet-nut contact, if you have a metal top thats glued to wood and not suitable for easy grounding. Keep deformable material out of screw tension path for electrical connection. If you do this, add good strain relief to the table side and some bumpers to prevent it being smashed on wall or get creative and go in from bottom etc (clever carpentry).