[grounding] Grounding (PE / GND) of a system containing many OEM electronics - Page 1
Forum section: electronics / projects

author: Limhes
author_rank: Contributor
timestamp: July 29, 2024, 06:54:40 am
content: Our company's flagship product is a Machine (according to CE terminology) albeit a very high-tech one, containing lasers, optics, translation stages, and many electrical devices, including a PC and multiple screens. It is a combination of OEM electronics but also consumer grade electronics.To guarantee performance, I need tosever the link between Protective Earth (PE) and all other grounds (DC power, analog signals, digital communication). This is necessary to be able to create a grounding scheme where I can control all the return currents, such that there is additional noise on the analog signals (ie. their grounds). Severing this link is also considered best practice according to EN 60204-1:2018 Annex H, to which our product conforms. Obviously, enclosures and cable shields should stay connected to PE (for safety and EMC).Now, there are a handful devices that connect PE with internal DC power ground:Some OEM devices (for safety): their manufacturers are willing to make modifications for us, so that we can make the PE connection to our internal PE bus barPC, screens, USB hub: it seems that it is common for these devices to have DC ground and PE internally connected? WTF? Why???I see two options:Get PC, screens and USB hub that do not have this internal connection: might prove difficult, if at all possibleIsolate the "computer" GND from the GND of the rest of the system. This might be possible, but might prove difficult with e.g. the USB3 connections or low latency digital signals (we are talking sub-ns).What are your thoughts?

author: jonpaul
author_rank: Super Contributor
timestamp: July 29, 2024, 09:28:13 am
content: Hallo:Your firm is in EU, you raise serious safety and compoliance issues,I would NOT dfepend on this or any free forum for serious advise.My suggestion:1/ Ask a paid compliance consukltant spoecalied in electrical safety, EMI and so on, for the specific stand requirements and tests.2/ Contact the testing and approval agencies, eg TUV, VDE, for review and assistane.3/ Be sure your firm has very good product liability and employee accident insurance policies.WARNING I ACCEPT NO LIABILITY FOR THIS NOTE.Have an absolutely fantastic day!Jon

author: PGPG
author_rank: Frequent Contributor
timestamp: July 29, 2024, 09:34:42 am
content: Quote from: Limhes on July 29, 2024, 06:54:40 amI think that one of the reason can be they have to pass ESD tests and their USB connectors are enough accessible so spark from ESD gun can reach their GND and will be searching the way to PE. Probably it will make less problems in device if it just had ready way for it then when it will first test insulation inside device to find its own way.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 29, 2024, 09:42:37 am
content: Quote from: Limhes on July 29, 2024, 06:54:40 amThe description is too vague, but anyway, it doesn't sound like a good idea.Not sure what you plan to do, and why. Also, the return path is important, but more importantly is the area enclosed. The energy is in the fields, not in the wires. Then, at high frequency, or when dealing with sharp edge signals, the energy couples into anything around: pipes, wires, metal structures, walls, etc.Do you have a few examples, examples showing in what way the performance is affected, and by how much?These books might help:Grounding and Shielding Circuits and Interference 2016 - Ralph MorrisonGrounding and shielding techniques in instrumentation 1977 - Morrison, Ralph

author: jonpaul
author_rank: Super Contributor
timestamp: July 29, 2024, 09:52:33 am
content: RoGeorge....Bravo for the Morrison books, I also reccomend our old friend Henry OTT:Noise Reduction Techniques in Electronic SystemsElectromagnetic Compatibility EngineeringAmicalment!Jon

author: selcuk
author_rank: Frequent Contributor
timestamp: July 29, 2024, 10:47:08 am
content: Is your product a medical device? If not and you don't know what to do, at least do the same as your desktop PC. PC manufacturers comply with many standards. Metal parts of the PC (main board, case, power supply) are connected to the DC ground (return path). This is for EMC compliance and signal integrity issues. Metal case is also connected to the PE since this is mandatory for human protection. I cannot see an easy way to separate these.There is an isolation on the ethernet port of the PC but that is a different case since the cable can go other buildings (with different ground potentials).This is an open access book. Grounding chapter:https://link.springer.com/chapter/10.1007/978-3-031-14186-7_14

author: Limhes
author_rank: Contributor
timestamp: July 29, 2024, 04:30:16 pm
content: Thanks for your replies. Let me clarify, because I seemed to have created some heavy emotionsOur product is CE certified and safe, since everything that needs to be connected to PE is connected to PE and thoroughly so. We follow the relevant standards in all ways applicable. Wedowhat we are doing. I did not meant to give you the idea that we would, ourselves, start tampering with the internals of PC or screens, if I gave that impression. I meant severing the connection by design, by choosing different components, by talking to manufacturers, etc.Imagine the following situation: We have an isolated AC/DC power supply in a metal case powering a DC lamp. Incoming PE (with mains) is connected to the metal case. All good. The DC side of things is floating, since it is its own separate circuit. Now imagine this at a much bigger scale. A complete system with many AC/DC power supplies, all floating inside a big metal case, fully protected (externally!) by a good functional bonding to PE. Then imagine some single ended analog signals inside that internal circuitry, where the return current is not fully going through the negative of that signal cable, but takes an alternate path. This issue we can tackle by laying out a good grounding strategy, like star grounding as much as possible and including galvanic isolation where possible/reasonable.Then we have situations where, for reasons unknown to me, PE is made part of that grounding scheme as well, and part of the shunted RF signals find their way onto the single ended analog wires. That is more or less the situation we have.And there is no safety reason why PE should be part of this circuit.It also only happens in consumer goods, it seems, and not in our OEM electronics. In the latter cases, everything is nicely floating inside metal cases that are bonded to PE.Now, let me try to rephrase my question: The internal connection of PE and DC GND in PC and screens, messes with the analog signals in the rest of the system. I see two options: (A) find different (industrial?) PCs and screens where this connection is not made, or (B) isolate those circuits. Isolation is necessary, as far as I can see right now, mainly in USB where the DC GND is connected to the GND of the rest of the system. Ethernet is obviously no problem, because isolated, and we have no other connections. However, USB3 is the real problem I see.Any serious input is appreciated, not the rants I got before. More questions I happily answer, but rants are a bit too much...

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 29, 2024, 04:59:20 pm
content: Avoiding ground loops in major measurement setups with PE-earthed PCs and peripherals is practically the same problem - but it´s solution is not impossible and not even very difficult.You need to identify the parts that receive analouge signals and parts that purely work digital. The parts that work purely digital like the PC itself, a HDMI or DP screen, a larger ethernet switch etc. have no problems with "little" ground loops. The crucial signals of USB3.0, DP, Ethernet etc. are differential if not even isolated, so that they easily cope with some common mode disturbances.More complicated are sensor units or analouge converters. The best option is if you can do a star-like ground connection from the AD-converter to your setups PE and to the conected devices. As long as you do not span a loop over the analouge parts, e.g. by connecting one device to two different grounding stars, you won't get any closed circuit for unwanted current.It really helps to sketch up the hole system and to separate it into purely digital areas and into different analouge grounds that are - if possible - each in a star-like shape and not interconnected.

author: Limhes
author_rank: Contributor
timestamp: July 29, 2024, 05:24:01 pm
content: Thanks Phil! You're spot on, and your description of the problematic system is exactly ours. Also the difficult case is exactly the one we have issues with: a device with USB, DC power, and single ended analog outputs. USB and analog outputs go into the PC, and there is your first ground loop. We can isolate the USB2 connection and then there is still a ground loop through DC ground via PE (because internally connected), back to the PCIe DAQ card.These issues are years old, so we are used to working with them, but we are trying to work them out of the system. Ideally we'd want to move to ADCs located at the devices themselves, effectively creating an "all digital" system. That introduces the issue of synchronizing all the ADCs, not to mention the NRE costs.Out of interest mainly: why would some manufacturers choose to connect internal GND with PE? That seems like a hack to me, and you can create connection schemes with return currents finding their way through the PE node, even through the mains plugs or back to the RCD trip switches.

author: wraper
author_rank: Unknown
timestamp: July 29, 2024, 05:37:58 pm
content: Quote from: Limhes on July 29, 2024, 06:54:40 amBecause it must be either like that or double insulated.

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 29, 2024, 05:51:23 pm
content: Can't you try to make the PC digital only? If I have sensitive analogue signals I always use external ADCs that do not have an internal PE-GND connection so that you can do the star-like grounding.Even if you have special PCI-Express hardware then it may be possible to put it into an external housing with dedicated DC-power supply. I´d always try to keep valuable signals far away from the PC.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 29, 2024, 07:02:10 pm
content: Quote from: Limhes on July 29, 2024, 04:30:16 pmDon't be an ungrateful. Don't despise the good will of others trying to help and put you on the right track. The replies you got were not rants. You are the one ranting here, particularly ranting against how the consumer goods are wired, because you don't fully understand why it is like that. Then you rant against the received replies, and despise them because they didn't validate your wrong ideas.You think you know, but you don't. You tell you will answer questions, but you didn't answer the question already asked. We don't have a crystal globe to guess your layout and problems. This reply is a rant but don't worry, I won't interact with you from now on. You don't deserve any help with that attitude of yours.

author: Limhes
author_rank: Contributor
timestamp: July 30, 2024, 06:40:18 am
content: I am very sorry if I cam across ranting. I am seriously just interested in why stuff is made the way it is.What I felt from the first replies, was a very judgemental view on my question, even though we know very well what we are doing. The first few replies, mainly by Jonpaul, come across like I am a kid in a candy store, instead of the senior engineer I am with a PhD and many years of working in the instrumentation field with a lot of knowledge on CE. Maybe I phrased the initial question in the wrong way, that tends to be a shortcoming of mine.Again, let me try to answer the previous posts:PGPG: That makes sense, and I can see why that would happen. The mystery of why PCs and their peripherals have PE and GND connected is more understandable this way.selcuk: Nope, not a medical device, but a scientific measurement setup. A rather large one. All very sensitive and accurate stuff.Talking with professionals from other companies, they are often similarly confused about the topic. I have heard opinions/strategies ranging from: (1) PE is for safety and should not be connected to any signal carrying circuit (except RF for EMC) to (2) if we cannot handle our noise issues effectively we tie all GND connections to PE.The reason why I opened this topic is to hear more opinions/strategies on this topic, and I am afraid that I failed in getting my point across on that. Apologies again.Still, I wonder what you think about this. There are many more people here with experience in this field that might want to share their opinion on this topic.Phil1977 summarized the issue well: How to avoid ground loops in a system where you don't have the opportunity to break all ground loops? Keeping the PC all-digital is the one I like the most so far, and is also our preferred option. The issue lies with a large install base where this is not the case, and with a rather big investment in making all ADCs external to the PC.I hope I've clarified some things here.

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 30, 2024, 06:58:54 am
content: Can you try to modify the PC to use a 12V/24V DC Power supply?In that case you could use a floating 12V supply to power the PC and connect your own measurement setup ground potential to the PC case and its analogue components?I personally know these 12V-ATX-power supply modules only for very small, maximum ITX systems. I frankly don't know how it works for larger setups. But if you really need to get rid of the PCs PE then it could be a safe and low-noise option.PS: Why am I writing of low noise: You could use an isolation transformer to power just the PC power supply, nothing else, without PE connection. It may work, may be safe enough for laboratory use (if another, trustworthy ground connection to the PC housing exists), but you can get into EMI-hell if you use a large SMPS like in the PC without its designed PE-connection. You could try it to just split up a ground loop that is actually disturbing you. But if it needs to be permanent, I´d definitely go for a well filtered 24V-DC-intermediate supply.

author: Limhes
author_rank: Contributor
timestamp: July 30, 2024, 07:14:05 am
content: I do indeed want to talk to our PC supplier about possibilities like the ones you state. The PC is a custom build with CE, so we cannot start changing its internals on our behalf.I am also looking into suppliers of industrial PCs, to see what the possibilities are there..edit: I do foresee that going in this direction is an uphill battle. Screens, USB hubs, etc. will all work against this.

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 30, 2024, 07:28:17 am
content: USB hubs can be galvanically isolated, though full-spec USB3.0 isolators of European sources are >€200,-Aliexpress sells one for around €50,- but I have no experience with it.Many screens can also be DC-powered.But at some point you do not only have 50Hz-ground loops ala Kirchhof, but the bigger the system gets the more capacitive and radiative effects arise. I don't know your measurement PC, but before I would isolate screens and USB-hubs, I´d definitely try to use external analogue hardware.

author: gf
author_rank: Super Contributor
timestamp: July 30, 2024, 07:54:05 am
content: What also helps to address these kind of problems are balanced connections for the analog signals.

author: uer166
author_rank: Super Contributor
timestamp: July 30, 2024, 08:05:17 am
content: Quote from: Limhes on July 30, 2024, 06:40:18 amIn non double isolated systems (and for other various reasons), you likely can't avoid ground loops due to all the PE ties to secondary. That is okay and obviously a solved problem: differential signals (how USB, HDMI, DP, etc work in a PC).Just do a good design with differential drivers/receivers if accuracy matters with lots of CMRR headroom. (This is the actual good engineering solution).If you can't do above then:Improve the single ended performance a bit by also tying all the secondary grounds of the various boards together where the SE analog signals live with a low impedance: this creates even more ground loops but actually improves ground loop noise where it matters.

author: Haenk
author_rank: Super Contributor
timestamp: July 30, 2024, 08:10:01 am
content: +1 for isolating the connections (there are USB isolators readily available). For networking, you could go fibre.For the computers/monitors go "industrial", with external power supply running on a single voltage supply - which could be replaced by a linear supply. Which in turn would be easier to implement into a grounding scheme.

author: uer166
author_rank: Super Contributor
timestamp: July 30, 2024, 08:14:49 am
content: Quote from: Haenk on July 30, 2024, 08:10:01 amHow does this help with a single ended analog signal going between 2 systems where the secondaries are tied to PE?

author: Jeroen3
author_rank: Super Contributor
timestamp: July 30, 2024, 08:52:43 am
content: Quote from: Phil1977 on July 30, 2024, 07:28:17 amYes, I have a few here. They connect GND to PE on the primay AND SECONDARY. As in, they won't fit the proposed solution.Floating the secondary side of pc's and peripherals is an emi nightmare, and you shouldn't.There is this Y-cap for a reason.Isolate the signals affected by ground noise.It's is typical in industry that the secondary of supplies is tied to ground such thatit faults when there is a short to ground.Floating it is allowed, but you need insulation monitoring. For consumer tech it is not mandatory. But often still done, be it AC coupled.I'm sorry, but you're trying to solve this problem from the wrong side. It will only make things more complicated.X-Y problem this is.

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 30, 2024, 09:23:34 am
content: Quote from: Jeroen3 on July 30, 2024, 08:52:43 amWhat do you mean? The mentioned isolators are not mains powered, they have no PE. They have a primary GND and a secondary GND, and if these are connected then the isolator is defect because it´s not fulfilling it´s one and only functionAnd yes, floating a whole PC is an EMI-nightmare. But powering it by floating DC while the PC GND is connected to your instrument GND can solve the problem of PE-loops.There´s not the one and only countermeasure for noise floor. PE ground loops are often seen guilty if you have a poor analogue signal design, and isolating PE is neither an effective nor a safe countermeasure for bad instrument design. But anyhow, it can be an issue and sometimes isolators are a solution.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 30, 2024, 09:31:23 am
content: The usb isolator hubs separate primary from secondary, but they don'tfloatthe secondary. You need the single usbstick likeones for that.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 30, 2024, 09:34:36 am
content: Accept the fact that many devices connect PE to their DC GND because it's so much easier to go that way, than to do the opposite. I.e., you can always take a part of equipment thatdoes notdo this, and tie them together, but doing the opposite is difficult.This means, also accept the fact that single-ended signals referenced to said GND are inaccurate due to ground loops that cannot be avoided. Therefore use single-ended signalling only for where this analog accuracy does not matter, and differential signaling when it matters. Do not try to make single-ended measurements better by forcing them to act like pseudo-differential by esoteric star grounding practices which then require you to cut those GND-PE ties: in a complex mixed signal system that is a recipe for EMI disaster.Isolated signals are kind of an alternative for differential signalling and in some cases it might be easier.

author: Limhes
author_rank: Contributor
timestamp: July 30, 2024, 09:40:21 am
content: Perfect, all very helpful information! Thanks a lot. Let me summarize:Keep the PC and peripherals as they are, because they need to be mostly for EMI reasonsKeep the sensitive analog stuff away from the PC where possible and isolate/control them separatelyTo clarify a bit more: it is an integration problem more than a design problem. If we could design from scratch, we know how to handle it. We are "just" tying together commercial electronics.@uer166: "tying all the secondary grounds of the various boards together where the SE analog signals live with a low impedance" -> That we have never considered but I remember seeing something similar being mentioned in EN 60204-1:Is that similar to what you mentioned?@Siwastaja: That's what we've been doing for the past 10 years. Unfortunately we (wrongly) rely on those single-ended analog signals being very accurate. I guess we have some more fundamental things to consider here as well, and maybe reconsider some of the choices and assumptions from the past that have left us with an unmanageable situation, basically.

author: m k
author_rank: Super Contributor
timestamp: July 30, 2024, 12:38:25 pm
content: Have you been around so long that old ISA was the first analog card bus?If so then much have changed since then.Mini-PC is one of the newer things.Some have an add-on card slot, but it's probably pretty short.How much data is needed?Unshielded 100M Ethernet would isolate, standard allows a distance up to 100m.Industry seems to try it so that higher frequencies are in a single cage.Cabinet walls are also pretty thick.

author: uer166
author_rank: Super Contributor
timestamp: July 30, 2024, 04:23:43 pm
content: Quote from: Limhes on July 30, 2024, 09:40:21 amYeah this is exactly what I meant. Though the image shows a cable, I meant more generally wide/low impedance ties that can be a cable, a big flat grounding braid, chassis, etc. IMO the issue is not "screen current" that it solves, but rather reducing the voltage drop between the boards' ground references to which your "accurate single ended" signals are referenced to.

author: Limhes
author_rank: Contributor
timestamp: August 01, 2024, 07:34:12 am
content: Thanks for the clarification! This goes on the list of strategies to be tested.Thanks all once again, very much appreciate the input.