New ESD bench causing 50 Hz noise on the scope - Page 1
Forum section: electronics / testgear

author: manytoolsmike
author_rank: Contributor
timestamp: July 08, 2021, 08:05:06 am
content: Hello everyone,I have a new Viking Classic ESD workbench.https://viking-esd.com/production/workbenches_series/classic_workbenches/I now have a persistent 50 Hz noise on my scope. When I touch a probe to any part of the bench or ESD matt the 50 Hz noise gets stronger. Im guessing this is a normal thing with ESD rated benches because of the conductivity of all surfaces. What do I need to do in order to kill the noise? I will check the building wiring to see if the Grounding is correct. I was using the same set up (outlet and power strip) on a wooden work bench last week with no problem.Here is a youtube video showing the issue.https://youtu.be/xBGMzveROLQAll surfaces, drawers, panels and frame are ESD safe. There are copper ground straps connecting everything. The power distribution panel has all grounds connected to the chassis as well.Any suggestions?Mike

author: Kleinstein
author_rank: Super Contributor
timestamp: July 08, 2021, 02:52:57 pm
content: Normally a correct ground at the ESD environment should keep the mains hum small.The ESD grounded parts are grounded via a protective resistor of some 1 M. So some leakage current from a SMPs in a non grounded instrument could shift that ground level around. PCs are usually grounded and the scope and soldering iron should also have a direct PE connection. The culprit could be the F-gen and maybe to a lesser degree the power supply.How much hum voltage is there actually on the ESD mat, and how much does it drop of one adds some 1 M or 100 K to the scope ground.

author: esepecesito
author_rank: Regular Contributor
timestamp: July 08, 2021, 02:59:52 pm
content: Maybe you have a line filter in the power distribution? Because of the 2 capacitors to the live and neutral you have some voltage at ground.

author: manytoolsmike
author_rank: Contributor
timestamp: July 08, 2021, 03:19:28 pm
content: Thanks for the reply,In the video I am showing 4 Vac (8 Peak to Peak) measured on the ESD matt. The signal is equally strong where ever I check, and the signal is very noticeable when I am working.So If I follow your logic correctly, you are thinking it is coming from one of the instruments on my bench. Ill check them all one by one.The confusing thing is that turning off the switch on the built-in power bar reduces the signal to millivolt levels even when nothing is plugged into it. If fact when I unplugged the power bar the signal still remained high when the switch was on.I will try plugging all my power strips into the main bench power-bar. It is possible I have created too many ground loops by using different sockets for each power bar? Again this was not a problem on my old bench.I really don't think I need the ESD protection, the bench was only available in the ESD version locally. As a last resort I will try removing all the ground wires connecting the various surfaces on the bench. Essentially eliminate the ESD protection.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 08, 2021, 04:23:52 pm
content: The resistive coupled ground for ESD protection should more help that cause extra hum, so removing it would not really help. The effect of the build in power bar is odd: maybe some EMI filtering inside and a missing / wrong ground connection there.

author: Cymaphore
author_rank: Regular Contributor
timestamp: July 08, 2021, 04:27:30 pm
content: What kind of protective system / PE does your site have?If it is TT-System, that kind of phenomenon could point to a corroded earth electrode or a bad PE terminal in the installation.In that case, your electrician could perform a loop measurement and tell you about the quality of the PE (and also install a new electrode).

author: esepecesito
author_rank: Regular Contributor
timestamp: July 09, 2021, 11:36:08 am
content: Quote from: Cymaphore on July 08, 2021, 04:27:30 pmBut even in a TT-System, if all has the same PE, it should not be that noise... or I'm missing smthn?

author: Cymaphore
author_rank: Regular Contributor
timestamp: July 09, 2021, 02:05:48 pm
content: Quote from: esepecesito on July 09, 2021, 11:36:08 amIf it is TT-System with bad or broken PE, things will be a total mess. That's why radio amateurs in areas with TT-Systems usually have the very best earthing electrodes humanly possible, sometimes even with special plate earthers optimized for HF usage.Just think about it: If the PE is not (or only loosely) related to the powergrid but connected among many different devices (including some with switching supplies), you'll have all kinds of capacitive special effects and side effects of stray currents. Where does the leakage current of all those devices go and how does it swing? :-)But it's just a hint, let's see what manytoolsmike comments about his power grid situation and if the power connection (Zs and also Zi) at his site is confirmed to be good.

author: tszaboo
author_rank: Super Contributor
timestamp: July 09, 2021, 02:18:57 pm
content: You should connect the mat directly to an ESD grounding point. Even if your desk surface is dissipative.Something like this, or your local equivalent.ESD means both conductive and dissipative materials, or directly grounded (like a soldering iron) and weird things happen when you start mixing them.

author: manytoolsmike
author_rank: Contributor
timestamp: July 09, 2021, 02:22:33 pm
content: Quote from: Cymaphore on July 09, 2021, 02:05:48 pmIm thinking Cymaphore is correct (or at least partially so). I will check what is going on with the wiring when Im back in the lab. I have two power strips (one for the instruments, one for the computer stuff) plus the built in power bar which I use for devices under test and the soldering station. All three power strips are plugged into three different outlets located in the same wall (see the picture in my original post). I would not be surprised if the electricians left out the ground when they finished out our warehouse. The fact that this problem started when I switched to the ESD bench could be a red herring. Still, it is crazy that I can literally probe the bench work top and get a 4 volt signal!I will update when I try your suggestions.

author: esepecesito
author_rank: Regular Contributor
timestamp: July 09, 2021, 02:34:31 pm
content: Quote from: Cymaphore on July 09, 2021, 02:05:48 pmTo be honest, I do not understandThe oscilloscope measureses against ground. As long as ground is equipotential, all the same voltage, I would measure 0.There is no leakage current if there is some voltage, but no resistance. Anyway I do not want to hijack the thread. Thanks for the explanation, I will think about it.

author: manytoolsmike
author_rank: Contributor
timestamp: July 09, 2021, 05:36:35 pm
content: Quote from: esepecesito on July 09, 2021, 02:34:31 pmRead about "ground loops" I have encountered some pretty large earth ground variations working in the Oil Industry where we have control systems spanning kilometres of cable. Im not sure if that is exactly what is going on here but it is likely similar.Ultimately I will have a small distribution panel installed in my Lab to provide more isolation and keep my power outlets on separate breakers.

author: manytoolsmike
author_rank: Contributor
timestamp: July 12, 2021, 11:54:50 am
content: Hey EEVers,I figured it out. The Bench's built in power strip is the round Euro type plug and outlets.These plugs are not "polarized", you can plug them in either way. My country uses the UK style polarized plugs and the Ground is bonded to the Neutral in the Distribution panel. When I measured the Ground to Line I get the nominal 230 VAC;between Ground and Neutral I Get 4 VAC. Which was exactly what I was seeing on the scope.Turns out the Euro Plug was "upside down" in the UK plug adapter. This swapped the ground to Neutral relationship and caused the problem. Plugging the Euro plug in the "right way" has reduced the 50 Hz noise to a couple of mV above the noise threshold.As for the 4 VAC floating ground..... Ill not worry about it for now. My lab is located in an industrial business park and we have 3 phase power so the floating ground can be coming from several sources.The lesson is..... Pay attention to the "polarity" of unpolarised plugs that also connect the ground.Im guessing Europe doesn't bond their Ground? or how do they maintain consistency of the Ground to Neutral relationship with plugs that can connect "upside down"?Mike

author: Cymaphore
author_rank: Regular Contributor
timestamp: July 12, 2021, 01:48:07 pm
content: What concerns me:Why does it work now?PE is connected twice in CEE7/3 plugs, regardless of the polarity. And it should not matter witch way the live wire is going, unless there is something I don't know about your ESD-workplace.Does your socket outlet give you the same kind of readings between PE and the live / neutral?Very interesting.About the PE-Question, bit off topic:Technically, it should not matter. In the EU, there are EN-standards putting some strict basic requirements on the powergrid and how consumers are connected to it. But the power sockets differ among the countries.Most widely used are CEE 7/3 (like the one on your picture) and 7/5, or for the plugs 7/7 (works with schuko and the french standard) as well as the flat euro plugs. But the polarity doesn't matter much, unless you want to explicitly switch the live wire.In germany the most widespread solution is TN-C distribution grids where PE and neutral are connected to the PEN at the entrance of the building. PEN is grounded in regular intervals in the distribution network. Good grounding is mandatory for new buildings as well. Even in the industrial zone where my company is located I rarely see more than a couple of millivolts between PE and neutral for normal socket outlets, as it should be. But it can be different in locations with TT-System, where all of this entirely depends on the quality of the earth electrode.

author: esepecesito
author_rank: Regular Contributor
timestamp: July 12, 2021, 02:56:57 pm
content: Not having polarized plugs is a nightmare... I had problems with LED lights that remain "slightly on" depending on the polarity you plug them. I wish we would have the Australian standard plugs!!!

author: Cymaphore
author_rank: Regular Contributor
timestamp: July 12, 2021, 04:26:23 pm
content: Quote from: esepecesito on July 12, 2021, 02:56:57 pmNow completely OT, but I can't hold back: Personally I would prefer the IEC 60906-1 standard to become more widespread. Or the swiss SN 441011 based system.

author: SteveyG
author_rank: Unknown
timestamp: July 12, 2021, 05:19:23 pm
content: Sounds like originally there was no earth, so it was floating along with having any leakage currents from other equipment present on it.Generally avoid adaptors etc wherever possible.

author: Fungus
author_rank: Super Contributor
timestamp: July 12, 2021, 05:47:25 pm
content: Quote from: Cymaphore on July 12, 2021, 01:48:07 pmI'm not really sure it's working, just pretending to.In theory the oscilloscope BNC is hard-wired to the mains ground wire. I'd check out the ground wire, make sure it's really connected to something. Just because there's a green wire coming from somewhere doesn't meen it's connected down the line.Quote from: manytoolsmike on July 12, 2021, 11:54:50 amCorrect.(and it's the right thing to do - everybody's ground is slightly different so tying the electric company's "neutral" to your own personal ground isn't good for the wires)

author: manytoolsmike
author_rank: Contributor
timestamp: July 13, 2021, 11:11:08 am
content: Hey guys,I've lived and worked in more countries than I can keep up with. I have seen every plug setup and ground configuration imaginable. I also work the Marine, Military and Off-shore Oil industries; so I have given up arguing about what the best grounding and multi phase power distribution is. I do like the UK style plug; they are safe, robust, durable and properly protected. The argument of bonding or not bonding the ground goes a lot deeper than any of the comments I have seen on this thread. There are good reasons for doing it either way. I think the majority of the world does bond Earth to "Neutral", it sort of defines Neutral really.As for why the problem showed up at my bench. It has to do with the 4 Volt floating ground and swapping the chassis ground to the Line when the Euro Plug is "upside down". I already explained the mechanics of it, if you dont understand the electrical phenomena..... Hell neither do I really. Hahaha.Anyway Im just happy be bench is sorted out and I can get beck to work.