Using PE (earth) for signal cable shield? - Page 1
Forum section: electronics / general

author: Nominal Animal
author_rank: Super Contributor
timestamp: August 31, 2024, 04:15:45 pm
content: As I understand it, if the cable shielding is connected only to protective earth and metal enclosures, it should work well for shielding against EMI, regardless of the signal voltages, assuming insulation limits are not exceeded.In USB and similar relatively high-frequency signaling cables, the shield is connected to ground at the host or device connector, because there is no PE connection.Fixed Ethernet shielded twisted pair cables can have their shield connected to protective earth (PE, at the patch panel), however.If I ensure the cables I use do not internally connect the shield to signaling (not even to signal ground), and both ends similarly connect the shield only to PE, I see no reason why this should not work well and be safe. (I'm talking about USB, RS-232, RS-485, and similar serial communications cables.) The only risk or downside I can see is using or connecting to devices that do connect the shield to their internal ground, so I probably should use dedicated connectors to avoid confusion.Is my understanding correct? Are there issues or problems with this?

author: wraper
author_rank: Unknown
timestamp: August 31, 2024, 04:24:40 pm
content: PE has nothing to do with shielding, nor does it help to suppress EMI. USB shield must be connected to GND (either on PCB or to grounded metal enclosure) with low inductance path on both ends regardless if there is a PE or not.

author: JohanH
author_rank: Frequent Contributor
timestamp: August 31, 2024, 04:28:00 pm
content: Yes, it's irrelevant for shielding if the chassis is connected to circuit ground or PE.Here is a simple explanation:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2024, 05:16:39 pm
content: Consider a system like:Black: basic diagram. Suppose there's...Box 1 has earth connection, and a grounded, shielded output.Box 2 is ungrounded/floating, but don't worry, we've run a separate earth connection to the cable to "terminate" that end of the shield. (Or left it completely hanging. Homework: how bad is each, or which is worse?). It has a grounded shielded output.Box 3 has a grounded, shielded input, and earth connection.All earths collect via facility wiring back to the breaker panel and nearest ground rod.Red: suppose there's a magnetic field within the general enclosed area of this loop. What happens? Namely, what current flows in the loop, what are the loop characteristics (R, L, transmission line / antenna element length, etc.), what voltage drops are distributed where around it, etc.Box 2 inset, red highlighted: the "extra GND" does absolutely nothing at radio frequencies, the return path is spaghetti, at least several µH but 10s or even 100s might happen in a tall wood-frame building.Most of the loop voltage drops across the input,adding directly in series with the signal received from Box 1. Extreme noise has entered the system.Note that, even if we're using differential signaling, we still have to worry about the common-mode voltage. No differential receiver is truly perfect: even transformers have limited CMRR (varies with frequency) and breakdown voltage, and most (semiconductor) receivers have a limited range, for example 10-30V for RS-422/RS-485/CAN, between supplies for most others (instrumentation amps, general purpose diff amps, LVDS receivers, etc.). So the CMR (CM range) might be hardly a volt (e.g. 3.3V supplies, biased in the middle) in a very typical (low voltage analog or digital receiver) case.And, what we need to protect against, ranges from a couple volts CW RF (commercial immunity test levels), up to 10kV transients (ESD), give or take how much we need to withstand those transients -- maybe we don't mind if the signal gets momentarily corrupted, or even if the equipment gets disrupted (requires power cycle), but high-reliability applications might even mind the additive noise.Box 3, purple highlighted: whatever loop the input cable is a part of (actually none as shown, due to the above gap, but we can imagine an "extra GND" applied in the same way at the Box 2 output if needed), its current is shunted to ground around the circuit, and noise voltage is greatly reduced. How much it's reduced, depends on the impedance of that shield-to-ground link. Down at audio frequencies, we might be concerned that nothing is inductive, i.e. we don't obtain coupling value from the cables, they don't behave as transmission lines, or transformers*, and thus the resistance of those paths (including the cable shield itself) all have some voltage drop that manifests as a V_N in series with the input signal.*To a certain degree, transformers are transmission lines; certainly, they can be built directly using them. The space of possible (practical or otherwise) transformers is much wider than that of practical transmission lines, I would say, and so too the range of possible response or behavior; especially in terms of nonlinearity, when core materials are involved in various tricky ways. But we can restrict our consideration to the set of better-behaved transformers, that are transmission-line-ey, without much loss of generality for signal and EMC analysis purposes.The most important thing about earthing, for EMC purposes, is, if the lead length is long, its effect isnegligible, and for example, breaking the connection between cable shield and circuit reference plane or enclosure, introduces the full amplitude of whatever madness is going on outside that closed system.Put another way, the shield is an extension of the enclosure around the circuit, and presumably, the circuit is bonded to that enclosure to make best use of it (lest the circuit's internal current loops and voltage drops corrupt the signals exiting different points on the board). Topologically, it doesn't matter what the shape of the enclosure is, just as long as it's conductive and thick enough to enclose and contain fields within. This includes the act of opening holes into the enclosure, to a limited extent: we can treat an opening as a radiation port (port: a datum through which energy flows in and out; usually a pointlike node, such as a connector, or a cross-section on a cable), and rather than having total isolation as in the ideal case, we simply have a degraded but still largely satisfactory case (maybe the shielding effectiveness is min 40dB over frequencies of interest, but it's still fine given what the circuit does and what levels it's tested at). We can go further and open whole sides of the enclosure, exposing the PCB ground plane as its own enclosure wall; as long as signals and components are arranged at only short distances away from that surface, we get microstrip trace geometry with reasonable performance (for commercial levels). This is a transformational way to understand how a PCB can be useful by itself without additional shielding.Tim

author: RoGeorge
author_rank: Super Contributor
timestamp: August 31, 2024, 09:02:13 pm
content: Quote from: Nominal Animal on August 31, 2024, 04:15:45 pmNote these 3 are different in regards to the physical layer- USB uses a differential pair, 3.3V and voltage controlled. A differential pair means two single ended connections, just that when one wire is 0V the other wire is 3.3V and the other way around, so 2 signal wires (D+ and D-) and a GND.- RS-232 uses single ended signals +/- 5...15V, also voltage controlled, so a wire plus a GND for each Tx, Rx, RTS, CTS, etc. signal- RS-485 uses a differential pair which is in fact a current loop. That makes it much faster than RS-232. 485 is good for much longer distances, and is way more resilient to noises - often used in industrial environment (which is considered very noisy) to interconnect SCADA equipment over up to 1km+, or in the entertainment industry for DMX512 (to control light shows and alike scene equipment in open air concerts or in theaters), or inside PCs for the SCSI bus (SCSI disks), etc.By contrast with USB or 232, the 485 can work with only 2 wires, A and B (only a twisted pair of 2 wires, and apparently no GND or shielding), though the standard allows a 3rd wire, C, as a GND.In all 3 (USB or 232 or 485) the shielding (if there is any) is yet another connection, apart from the existing GND wire(s).The 5 minutes video chat with Rick Hartley is correct, it answers the question and gives some explanation, but even if you think you fully got it, you may want to watch this full talk (same guy): How to Achieve Proper Grounding - Rick Hartley - Expert Live Training (US)AltiumSeriously, don't skip that, and don't stop at the small mocking/rants moments at the beginning. Watch it in full, even if it's more than 2 hours (1 hour at 2x speed). Highly recommended.Keep in mind that the talk is mostly about designing good PCBs, with less EMI emissions, and with good immunity to noises, so the discussed issues are in this spirit, mostly against problems in the MHz to GHz range (and mostly for digital equipment), but the same principles applies to mains hum and low frequency, or for low analog signals, too. The topics are broader than only earthing, might be tempting to skip, but don't skip, watch it in full.If you prefer a book, the 2.5 hours live seminar by Rick Hartley is based on the book "Fast Circuit Boards - Energy Management" by Ralph Morrison (IIRC one of his mentors). Again, just like the live talk with a small rant, the book style is not very academic, yet (from an engineering perspective) it might bring to the reader more understanding than a 1k pages reference textbook about electromagnetism.

author: DiTBho
author_rank: Super Contributor
timestamp: August 31, 2024, 09:24:29 pm
content: book: Grounding and Shielding, 6th edition, Ralph Morrison

author: tooki
author_rank: Super Contributor
timestamp: August 31, 2024, 10:12:26 pm
content: Quote from: RoGeorge on August 31, 2024, 09:02:13 pmI havenever,everheard of RS-485 referred to as a current loop, and my understanding of it, having implemented and debugged RS-485 interfaces, is that it uses differential voltages just like USB: one is at 0V when the other is high, and vice versa.What are reasonably common are converters between RS-485 and 4-20mA current loops.Because of the differential signaling, the amount of current flowing in the cable is always the same. In current loop signaling, the amount of current itself is modulated to carry information.Furthermore, while RS-485 is often used without a ground, this is noncompliant and relies on some form of return path existing. As Texas Instrumentssuccinctly puts it:Quote from: RS-422 and RS-485 Standards Overview and System Configurations(Emphasis mine.)TheRS485-MODBUS standard(one extremely common use of RS-485) leaves no room for interpretation:Quote from: MODBUS over serial line specification and implementation guide V1.0(Emphasis in the original.)

author: Someone
author_rank: Super Contributor
timestamp: August 31, 2024, 10:16:49 pm
content: Quote from: Nominal Animal on August 31, 2024, 04:15:45 pmPretty much perfect. If you want the formal reference then it's AES48.Attached below is a long presentation on the ins and outs by a respected specialist Bill Whitlock, names to watch for in the literature are Neil A Muncy and Henry W Ott.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 31, 2024, 10:38:51 pm
content: Quote from: DiTBho on August 31, 2024, 09:24:29 pmNice one.Another ref. that I can recommend is "Compatibilité Electromagnétique", Alain Charoy, DUNOD. I have the original edition but it's now split into several volumes. Not sure it has been translated to English though.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 01, 2024, 08:41:28 am
content: Perhaps some background is in order.If we treat the enclosures and the outer shielding in the cables as a single continuous shield – which it would be, just oddly shaped –, the diagram simplifies to(click to embiggen; this is a tiny, 2944-byte SVG vector image)With class II supplies, there are no class Y capacitors, but no connection to protective earth either. The shield is connected to the DC GND. This is not optimal, because capacitively coupled noise (from e.g. parallel cables) has no easy path to ground.The red class Y capacitor exists if using class I supplies. Because our plugs are unpolarized, they often lead to tingles, or having 120V or 240V difference between PE and DC GND potentials. Rotating the plug 180° may or may not fix it, depending on the supply. And I'm not sure there is any actual guarantee of one of the mains pins being "neutral", or at the same potential as protective earth.The blue class Y capacitor is the "standard", assumed configuration in most references I've seen. My problem is that it is just not reliably available with double-insulated supplies (or "wall warts") here in Finland; they're all two-prong only, no connection to PE at all.My idea is to use a class II low-leakage IEC60601-1 supply likeMean Well RPS-30-7.5filtered and regulated down to 5V, and simply add an outer shield not connected to the DC circuits at all. Then, there are no class Y capacitors. (I do not know if it is safe and/or legal to add the blue class Y capacitor to such supplies here, or if one would help/be needed.)My projects are housed in cast aluminium enclosures. I use nylon M2.5 standoffs to hold the PCB off the lid, and reserve enough room inside the enclosure for the cable connectors. The cables go through minimum-sized U-shaped grooves with grommets between the lid and the enclosure. I know that if I were to do this, I'd also need to use a shielded DC power cable, or it would become an antenna.Right now, the situation is that with class II supplies there are no class Y capacitors, but any shield will be connected to the DC GND. Because there is no easy path to protective earth (or "neutral") for electromagnetically or electrostatically coupled currents, this is suboptimal, especially with low-leakage supplies. (I think the shielding works for EM noise, but capacitively coupled noise, and ESD, is a bit iffy. A resistor and capacitor in parallel between the shield and DC GND might be a good idea.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2024, 01:34:58 pm
content: AFAIK, class II supplies will still have Y caps in them, they're just to the DC bus (usually), or sometimes AC H/N as you note, which makes nonpolarized receptacles annoying. The leakage current is safe, but the voltage is real. A medical grade low-leakage one will be more pleasant to interact with, indeed.Why can't this be done?And if that's done, we collapse one connection:now the signals are always referenced to the ground plane / cable shield, and no ground loop flows so it's good at all frequencies.The earth connection isn't needed at all (probably) if a double-insulated supply is used, but the circuit-GND can still be connected to shield in the same way.Tim

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 01, 2024, 03:45:05 pm
content: Quote from: T3sl4co1l on September 01, 2024, 01:34:58 pmIt is the equivalent of just connecting one of the enclosures to a protective earth, so it can. I'm just not convinced it is worth it.Consider what happens when a statically charged person touches an enclosure or shield, causing an ESD event. Those are common during dry winter nights up north; heck, even snow is triboelectric. Add wool socks, maybe a warm pullover with blended threads, and you can easily get into the kilovolt range.While there is a path to ground/PE for the fault current, it is a long transmission line, and because of the shield-DC GND connection the sensitive electronics are close. Because of low voltages and currents in the DC circuits, a ground bounce is likely to result. It is unlikely to break the MCU, but it can reset or lock it up.This is why properly designed USB devices often have a snubber network between the DC GND and the USB shield: a resistor and a capacitor in parallel, and sometimes an additional resistor in series, between the shield and the DC GND. You see this in many different appnotes from various manufacturers, too. This will "low-pass filter" the ESD event, "delaying" or "spreading" it out, so it'll just flow to ground with less fuss, minimising any ground bounce. (Apologies for using wrong or silly terms. Me just a hobbyist here, using descriptive rather than technical terms for this, since I trust the USB chip manufacturers and haven't bothered to double-check this stuff from first principles. I have a physics background, so the theory isn't hard; but practical devices and situations are complex, and I've found that EE experience is more reliable than a theoretical investigation.)Specifically, I'm not convinced that using PE instead of supply ground makes much of a difference, when the supply ground and shield have such snubbers. Fast events are slowed enough that the path to ground is effective. It does not matter that much whether it is mains ground/neutral or PE, since there is very little current or energy in them.What I'd like to know, if I could get the same shielding effects by using a PE shield completely isolated from the low-voltage low-current circuitry. I cannot really do a meaningful practical tests with the equipment I have.I wouldn't mind a class Y capacitor between PE and DC GND, if I knew how to find out if it is safe or legal. In the diagram, this would be either no class Y capacitors at all, or the blue class Y capacitor only. Do note that I'm working with DC supply voltages in the 5V to 12V range, no more than 5W or so.

author: nctnico
author_rank: Super Contributor
timestamp: September 01, 2024, 05:07:36 pm
content: Quote from: Nominal Animal on September 01, 2024, 03:45:05 pmIn reality it works the other way round. Short story: connect all grounds together as T3sl4co1l pictured in the second picture for best EMC performance. Having seperate grounds is asking for trouble. This is what you'll learn in any EMC course.An ESD event has a large HF current. When you have seperated grounds, the ESD event can induce itself into your MCU ground. With everything connected to the same ground, there is nothing to induce because there is no loop area. The ESD energy will just dissipate through PE or whatever (or not at all).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2024, 05:38:57 pm
content: Right, but consider if one link or another is long or short. I should've labeled them:What then?Tim

author: selcuk
author_rank: Frequent Contributor
timestamp: September 01, 2024, 07:16:46 pm
content: Quote from: Nominal Animal on September 01, 2024, 03:45:05 pmI guess parallel RC connection is a measure for noises from low frequency magnetic fields. Separating the ground from the shield is not good for an ESD event as suggested before. You can connect the shield to ESD suppressor's ground first then to the circuit ground but that is a short between ground and shield as well. There is an application note from a manufacturer who recommends a solid connection between the ground and the shield against high frequency transients in this thread:https://www.eevblog.com/forum/projects/why-usb-c-gnd-is-being-connected-to-the-shield-of-the-cable-after-connecting/msg5269563/#msg5269563

author: wraper
author_rank: Unknown
timestamp: September 01, 2024, 07:27:23 pm
content: Quote from: Nominal Animal on September 01, 2024, 03:45:05 pmNope, those are random bullshit (capacitors, ferrite beads, resistors) and whoever put those components don't know what those components are supposed to do, especially considering they often have an opposite effect (capacitor vs ferrite bead). Unfortunately that bullshit found its way to some IC datasheets. Also USB-C specification explicitly forbids nonsense like that.https://www.unit3compliance.co.uk/hdmi-more-like-hdm-why-thoughts-on-cable-shield-grounding/

author: nctnico
author_rank: Super Contributor
timestamp: September 01, 2024, 07:31:27 pm
content: I think this stems from the old days where you'd find similar contructions around ethernet connectors. The shield of the connector is often connected to the case shield through a capacitor leaving a section of the circuit (behind the ethernet transformer) floating. The idea is that getting a lightning on the cable shield won't travel in the device (that much). However, it can be hell to get a contraption like this through modern day EMC testing. I've seen some interesting mods getting it done by adding a dedicated grounding strap to the ethernet shield to improve EMC behaviour.

author: madires
author_rank: Super Contributor
timestamp: September 01, 2024, 08:02:36 pm
content: AFAIK, in an ideal setup the ethernet cable's shield (in ethernet terminology it's called screen) should be directly connected to ground/earth at one side, and via a capacitor at the other, preventing DC ground loops while passing AC to ground/earth. And USB's shielding is a mess, broken by design (to fix issues you need USB isolators or transformer based PSUs).

author: wraper
author_rank: Unknown
timestamp: September 01, 2024, 08:09:38 pm
content: Quote from: madires on September 01, 2024, 08:02:36 pmThere is nothing wrong with USB shielding unless you break shield connection and introduce voodoo nonsense into it.

author: madires
author_rank: Super Contributor
timestamp: September 01, 2024, 08:14:27 pm
content: Connect two USB devices powered by SMPSU wall warts via an USB cable and have fun.

author: wraper
author_rank: Unknown
timestamp: September 01, 2024, 08:20:41 pm
content: Quote from: madires on September 01, 2024, 08:14:27 pmWhat fun? You basically described external HDD, printer, whatever self powered device connected to laptop/PC.

author: nctnico
author_rank: Super Contributor
timestamp: September 01, 2024, 08:24:20 pm
content: Quote from: wraper on September 01, 2024, 08:20:41 pmYep. And USB connectors are designed so ground / shield mates first.

author: madires
author_rank: Super Contributor
timestamp: September 01, 2024, 08:27:03 pm
content: Please think about where currents could flow.

author: wraper
author_rank: Unknown
timestamp: September 01, 2024, 09:34:54 pm
content: Quote from: madires on September 01, 2024, 08:27:03 pmIt does not matter whatsoever that some current will flow through the shield. Heck, with USB powered devices in most cases there will be more current flowing through the shield than that. USB-C standard demands that cables have GND pins/wires connected to the shield at both ends. The vast majority of micro USB cables also have GND wire and shield connected together although AFAIK it's not explicitly demanded by the standard.

author: SteveThackery
author_rank: Super Contributor
timestamp: September 01, 2024, 10:22:48 pm
content: Quote from: Nominal Animal on September 01, 2024, 08:41:28 amI've got a question about this diagram. As drawn, one side of the mains supply is connected directly to the protective earth. Isn't this a recipe for a big bang? Or at least, to trip the RCD.Another question: what is the red 'Y' capacitor for? I'm trying to imagine why you would couple the secondary back to the primary via a capacitor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 02, 2024, 05:36:28 am
content: Mains is normally neutral grounded at the panel, give or take local variants. Or legs of a three-phase circuit, in which case both are hot and we'd cure the error by showing neutral to earth and three voltage sources radiating from it, two of which go to the transformer/PSU.Either way, I would safely assume that connection is implied to be long -- wiring back to the panel, galvanic purpose only.Tim

author: madires
author_rank: Super Contributor
timestamp: September 02, 2024, 07:22:41 am
content: Quote from: wraper on September 01, 2024, 09:34:54 pmHave you considered the EMI suppression caps in the SMPSU wall warts?

author: Siwastaja
author_rank: Super Contributor
timestamp: September 02, 2024, 07:46:47 am
content: Quote from: SteveThackery on September 01, 2024, 10:22:48 pmI think the drawing depicts a complete equivalent circuit - one side of the mains supplyisconnected to the protective earth; where exactly depends on the power distribution system, for example here they are typically connected at the main circuit breaker box (in comes PEN wire, out go neutral and PE wires).

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 02, 2024, 10:48:36 am
content: Quote from: SteveThackery on September 01, 2024, 10:22:48 pmThe diagram describes the entire system, as drawn in various sources discussing earthing and shielding. The idea is to show the path to protective earth for any external sources of electricity, rather than allow it into the shielded circuitry. The supply is not fully drawn either, it's just a simplified diagram.Quote from: SteveThackery on September 01, 2024, 10:22:48 pmIt represents the class Y capacitor in class I isolated supplies, where it is supposed to be between the output and neutral. However, we have no neutral here; the sockets are not polarized at all.In a real switchmode supply, we have two full bridge rectifiers. The first one rectifies the mains AC. The class Y capacitor is typically connected here, between the output and the rectified mains voltage. This is what leads to the DC GND having such a big voltage difference compared to "neutral" or PE. The rectified voltage is then chopped up, transformed, rectified, and filtered. There is usually an optoisolator or similar to control the duty cycle, to keep the output voltage within the desired range.These are the four switchmode AC/DC supply types I know of, stolen from Mean Well datasheets, in block diagram form:(click to embiggen)First three are class I. The first one has a class Y capacitor on the output, between DC GND and PE/neutral. The second one has some kind of capacitor between the output and the mains side circuitry, as part of PWM voltage control. Third one has a class Y capacitor on the mains side only.Fourth is class II, and has no connection to PE, or even a functional ground ("neutral").My simplified diagram tried to describe all four in a simplified fashion, similar to how these are shown in the references on grounding and shielding.To draw the red class Y capacitor right, I'd have to add the first bridge rectifier at minimum, but I wanted to keep it as simple as possible.

author: SteveThackery
author_rank: Super Contributor
timestamp: September 02, 2024, 10:52:56 am
content: Thank you, @Nominal Animal - that's a fantastic explanation.I noticed a small difference in the symbols:

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 02, 2024, 11:03:43 am
content: Aw crap, I forgot: protective earth has that circle around the symbol. My fault, I apologise for the confusion!

author: SteveThackery
author_rank: Super Contributor
timestamp: September 02, 2024, 11:07:18 am
content: Don't apologise - it's me who messed up!So what do those two symbols mean? The one with three parallel lines, or the one with four slanting lines?

author: wraper
author_rank: Unknown
timestamp: September 02, 2024, 11:18:21 am
content: Quote from: madires on September 02, 2024, 07:22:41 amAgain, it's not some ultra precision measurement or audio equipment where you should be concerned about current in the shield. As of potential difference, solid shield connection that makes the contact first is important to prevent that voltage getting into data lines. Once you break that connection with voodoo nonsense, you remove that safety net during hot plugging and greatly increase the chance of damage.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 02, 2024, 11:19:13 am
content: This is the corrected block diagram, with the correct protective earth symbol:(click to embiggen)The symbol I erroneously drew into the diagram in the question is normally used for chassis or enclosure. I intended to draw the entire ground as a PE, as is common in grounding and shielding references, using a line with slanted hashes underneath to indicate solid ground, but decided it looked silly so simplified it –– and although I definitely know it, I just didn't notice I ended up using the confusing chassis symbol for the protective earth!My sincire apologies to anyone who looked at the original block diagram and considered the bottom left symbol as a chassis ground; it was supposed to be protective earth.(These symbols are standardized in IEC 60417. The standard ground symbol is #5018. Noiseless earth, #5019, has a downwards-facing half circle above the ground symbol, cupping the symbol, or acting like an umbrella above the ground symbol. The above protective ground, a ground symbol with a circle around it, is #5019. The wrong symbol in the initial post is #5020, indicating frame or chassis.)

author: m k
author_rank: Super Contributor
timestamp: September 02, 2024, 11:57:39 am
content: by TDK.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 02, 2024, 12:02:54 pm
content: Quote from: m k on September 02, 2024, 11:57:39 amYes: my use of class I and class II do not actually refer to the class requirements, but how those requirements are implemented in currently available low-voltage low-current switchmode power supplies. Apologies if that annoys or confuses, but I don't know a better way to identify the four typical ones I've seen as I described above. If there are better/more correct terms I should use, please do let me know; I'm just a hobbyist, after all!

author: m k
author_rank: Super Contributor
timestamp: September 02, 2024, 12:08:00 pm
content: Just a supplementary.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 02, 2024, 12:15:17 pm
content: Let me restate my thinking on this a bit further, in case it helps.Even if not connected to anything, the continuous shield – including cable shielding not electrically connected to the electronics – acts as a Faraday cage, as explained in thevideo JohanH linked to in reply #2.As the shield shape is complex, it is possible for external electromagnetic fields to generate a current in the continuous shielding; for example, if it has a loop in it. (Such loops typically involve a human directly or indirectly touching two different parts of the shielding, or putting two metal enclosures connected with a shielded cable on top of each other. So not by design, but by practical situation.)However, voltage can still electrostatically couple to the shielding, and in the case of AC sources, cause current flows in the shielding. ESD events can charge the shielding to rather high potentials. The shield having varying charge distribution in/on it will generate varying electromagnetic fields, including some inside the shielding, thus allow some unwanted noise "through" the shielding.Fixing the shielding to a specific potential, preferably close but not necessarily exactly local true earth (to minimize human discomfort from ESD zaps), keeps the charge distribution as flat and static as possible, minimising any kind of electromagnetic or electrostatic noise passing "through" the shielding.With respect to USB and others, I'm not interested in how standard cables are required to be constructed, I was only referring to the types of low-level, low-current signals used inside the enclosure. I could not use typical USB or Ethernet connectors anyway.Most importantly, I do not intend having multiple grounds in the electronics inside the shielding. The shielding is not connected to the innards at all, so the only ground within the shielding is the supply ground. (In that sense, it is similar to doing your electronics projects inside a shipping container, which happens to be grounded.) In the case of low-leakage class II supplies, the supply ground is not tied to any specific potential, so it might be sensible to tie it to PE or something somehow, perhaps class Y capacitor or some kind of safety circuit, I'm not sure. There may be electrostatic fields inside the shield (between the shield and the electronics inside), but that's fine, as long as it doesn't oscillate or vary too much. We already have one everywhere around us, typically on the order of 100 V/m vertically in open areas, because of atmospheric electricity.It seems that to verify whether it is safe or not, I'll have to make it first and then have a qualified electrician check it over.I don't like the blanket statements"you need to do it this way"without explaining why, because if I just wanted a solution I'd buy one and be done. I'm doing this at least as much to learn and advance my own understanding than achieving a specific outcome.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 02, 2024, 01:55:46 pm
content: Quote from: Nominal Animal on September 02, 2024, 11:19:13 amIsn't this fundamentally the same question that often comes up;"my DVD player only has a two pin power cord. Can/should I connect the metal case of the DVD player to protective earth?"

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 02, 2024, 02:04:19 pm
content: Quote from: Andy Chee on September 02, 2024, 01:55:46 pmI don't think so, because I'm asking about shielding against EM noise, both electromagnetic (radiated) and electrostatic (capacitively coupled), and mitigating the effect of ESD to the electronics inside. Situation is similar, but the question is different.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 02, 2024, 02:39:13 pm
content: I created a new block diagram of the various switchmode supplies, with (simplified) differences between typical implementations and my musings in color. Since the load within the shielding doesn't really matter, I simplified that too. The switcher and any power factor correction is hidden in the SW box. Although there usually is optoisolated feedback from the DC side to the mains side, and perhaps even regulation on the DC side, I omitted those because they're not relevant here.(click to embiggen)The four groups of differences and options, from left to right:Leftmost (green): Typical EMI suppression class Y capacitor on the mains side. I don't care whether this is used or not, as long as the supply is safe and legal.Second from left (red/pink): EMI suppression capacitor between DC GND and "neutral", except here in Finland (and in most of Europe using CEE 7/3 "Schuko" grounded sockets and CEE 7/4 plugs) mains sockets are unpolarized, and there is no guarantee one of them even is "neutral"; they're both considered live. This is the one that causes DC GND to not be anywhere near PE or true ground potential, and may cause "tingles" when touching e.g. USB cable shield.Third from left (brownish), below the second full bridge rectifier: Tying DC GND to PE potential. In this case, the shield is connected to DC GND and PE (short in the fourth group). Above, I've described my issues with this (ground bounce at ESD events), when the transmission line to PE is long.Rightmost (purplish blue): Connecting the shield to PE only, either directly (topmost; not my preferred option), or (my preference), via a capacitor and/or resistor for smoothing out ESD spikes. As I understand it, a snubber network shouldn't affect the operation of the shield much, but it would mean that in case of an electrostatically charged human touching the shield, they wouldn't feel a zap.Some of the members in this thread have stated that a snubber network (resistor and capacitor in parallel) between the shield and whatever potential it is tied to is categorically wrong and would not work. From a physics perspective, there is no reason why it should be wrong or why it would not work, so if there is such an engineering reason, I'd like to know. And I do mean other than"this is how these cables must be done to comply with the standards"or"everybody does it this way"or"because it is standard practice to do it this way", because I'm not making a product or installing this for others, just wanting to learn and find out if this would work (better than current solutions), and at most build and use it for myself for no real reason except this is a hobby. I do have a physics background, and I'm familiar with EM fields and related theory.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 02, 2024, 03:42:32 pm
content: Quote from: Nominal Animal on September 02, 2024, 02:04:19 pmWhat about Japan? By far the majority of household mains outlets are 2 pin only (over 95% of outlets).Does Japan have any increased issues or problems with EM & ESD, compared to countries with protective earth? I've never heard of any issues.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 02, 2024, 03:50:42 pm
content: Quote from: Nominal Animal on September 02, 2024, 02:39:13 pmOk, so you've established pretty well I think that these circuits are with respect to DC/mains/galvanic grounding connections.Thenfor EMC purposes, we are safe to assume all such connections outside the shield are spaghetti,and thus subject to unlimited external fields, and will offer potentially unlimited impedance (albeit only at resonant frequencies) between nodes in the circuit.Is that correct?When do we get to the EMC part then--? Or am I jumping the shark (well, "have I been"), and you (and/or others) have just been nailing down the galvanic equivalent circuit precisely here? (Which seems confident now, so, perhaps "next" is the answer?)Tim

author: David Hess
author_rank: Super Contributor
timestamp: September 02, 2024, 04:10:01 pm
content: Quote from: T3sl4co1l on September 01, 2024, 01:34:58 pmThe problem with that is that it makes a ground loop, and now the shield current couples into the signals that it is protecting. Differential signals in things like USB should be able to ignore this, but not all differential signals can.There are situations where EMC compliance requires the shield to be grounded at both ends, while signal integrity requires the shield to be connected at only one point.Does anybody remember Ethernet Thicknet and Thinnet? Grounding twisted pair Ethernet at both ends may lead to dangerous hilarity if there is a difference in ground potential, which is possible over Ethernet distances.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 02, 2024, 04:29:48 pm
content: I wonder how much the usage of the word "ground" has changed over time. Maybe we disparage old references (like "only ground one end") because they specifically meant safety earth, not local circuit ground; and we've reinterpreted it to mean local circuit ground, and much wailing and grinding of teeth ensues.Quote from: David Hess on September 02, 2024, 04:10:01 pmYou'll have to provide a reference for that, though. Signal integritydemandsthe reference plane be extended continuously from end to end. Which one is meant, is ambiguous: is the shield not actually the reference plane? is there another reference conductor in the cable? is the cable multi-shielded? is the signal differential? etc. That makes a big difference, whether this statement is simply acceptable, or utterly reckless...Tim

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 02, 2024, 05:23:45 pm
content: As I understand it, EMC (electromagnetic compatibility) is the desired outcome: not being affected by electromagnetic noise in the local environment. Sometimes EMC is meant only in the physical sense, sometimes it includes the local regulations. It is not clear to me when.EMI (electromagnetic interference) is the umbrella term for the unwanted effects of electromagnetic noise on an electrical circuit. There are four types of electromagnetic interference, or four types how noise can couple to the the electrical circuit:conductive, directly via wiresinductive, via varying magnetic fields to conductive surfacescapacitive, via varying electric fields to conductive surfacesradiative, via EM radiation to antenna-like structuresQuote from: T3sl4co1l on September 02, 2024, 03:50:42 pmWell, no, because that is kinda-sorta part of the question: where should the shieldingstart? Should the (entire?) supply side be covered by the same/continuous shield? Would a break in the shielding after DC filtering before the "sensitive" circuits help?You see, I can freely choose between different types of switchmode supplies, and decide whether they're within the shield or not, or perhaps separately shielded, but I don't want to build it from scratch, because I don't like dealing with voltages in excess of 20 volts or so. I want to add a filtering and post-regulation stage, too, dropping a couple of volts (to say 5V at say max. 1A load), getting a nice clean DC output.The secondary questions – after the initial shielding connection question – involve whether the SMPS should it be within the same shielding as the switchmode supply, or unshielded, or within the device shielding? I believe the answer is dictated by the practical behaviour of noise currents in SMPSes, depending on they're configured wrt. EMI suppression capacitors, so I wanted to show all my practical easy options above. It would be pretty funny to make an enclosure which generates a lot of EMI and traps it inside itself, blasting the circuits with more EMI inside than outside.. but not practical.We all know how standard and compliant circuits and wiring is done, I'm not looking for that. This is something I haven't seen before in practice, so I want to know why, specifically the reasons why it is not done in practice. The reasons could vary from "it's not how we do stuff" to highly specific engineering or manufacturing reasons to legal/code limitations.For now, we can leave the load as a simple black box. For wiring between "modules", I do intend to use connectors and cabling where the shield is continuous and not easily accessed inside the shielded environment. While this does mean transmission line behaviour in the shield, it is that equally for the DC supply rails, so I'm ignoring that part for now as unavoidable.Quote from: T3sl4co1l on September 02, 2024, 03:50:42 pmOther than the switchmode supply, although the load is digital circuits, it is low voltage and low current, and doesn't radiate much (although I have no equipment to measure that), as it is just your typical run of the mill microcontroller stuff. So, I'm not worried about emissions inside the shielding. I might even stick the MCU inside a secondary metal can, connecting the can and related shielding to the DC GND, if it is a problem.Outside the shielding is either a normal household environment, or a nasty workshop environment with brushed motors and sparky welding equipment within a hands reach. The latter is enough to occasionally glitch a microcontroller running in free air. This, because workshop shenanigans and DIY microcontroller gadgetry as soon as I have my own workshop.So, we can consider the shielding to exist solely for the outside nasty environment, plus an occasional ESD event due to a highly charged human touching the shielding, which I don't want to generate a ground bounce within the shielded circuitry.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 02, 2024, 05:42:15 pm
content: Quote from: David Hess on September 02, 2024, 04:10:01 pmIt is a problem here when connecting desktop computers via USB to devices powered by doubly-insulated supplies (wall warts), because the computer supplies do have a PE connection and the host USB GND is typically at (or close enough to) local ground potential, but the doubly-insulated wall warts' common voltage differs by a couple of hundred volts because of the unpolarized mains connectors and EMI caps between DC output and mains.I even started athread about USB isolators in hobby designsbecause it is such a common issue here and I deal with it often, but most members considered my solution wrong/improper/utterly silly. The question posed was why would I even use a wall wart and isolators, and not a proper lab supply which would be safer too.Sometimes it is difficult getting into the meat of a question about what is feasible and how and why, rather than how it is normally done.

author: m k
author_rank: Super Contributor
timestamp: September 02, 2024, 05:57:49 pm
content: I think we need at least 5 shields.1st is the one against a sword, so a general shield, like a casing that has nothing to do with a signal.3th shield is an inner shield of a waveguide, so pretty close to signal but still just pretty close.2rd is a shield not so close to a signal of a waveguide.4th is a 1st shield of a coaxial cable.5nd is a plate, a part of a signal that has also shielding properties.Hmm..the above is not defining the direction of shield.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 02, 2024, 06:09:22 pm
content: Quote from: T3sl4co1l on September 02, 2024, 04:29:48 pmWhile that was a rhetorical/descriptive question about the standard instructions related to this situation, I'll point out anyway that in my case, the shield is not the reference plane, there is a dedicated reference conductor within the shielding, I can use multi-shielded cabling if useful (with secondary shielding "grounded" to local reference plane according to common practice), and generally the signaling is single-ended. There will be some cables that are partially multi-shielded; I can choose between individual inner shielding, inner shielding for each twisted pair, and inner shielding for a set of non-differential but related signals, depending on what is needed.While I am the only one asking about this currently, this is a relatively common problem, so exploring the possible solutions ought to be useful for a wider audience, especially if it can be distilled into"you can dothisif you also dothatbut make sure you don't dothe other thing"for hobbyists to apply.

author: nctnico
author_rank: Super Contributor
timestamp: September 02, 2024, 06:20:43 pm
content: Quote from: David Hess on September 02, 2024, 04:10:01 pmYes and no. If you have HF coupling into your shield, then you'll likely won't pass EMC testing (radiated and/or conducted). So you'll need to fix your circuit so it doesn't couple HF currents into the shield. In most cases a power supply is the primary source of HF currents into a shield and / or ground. So typically I design the power supply to have filtering on at least plus but ideally on both plus / minus lines using common mode and differential mode filtering. Then the secondary side can be connected to shield + circuit ground.But to circle back to the question through a practical example: years ago I designed a device for use in HV labs. In order to protect it from the outside, I choose to leave the ground floating in respect to the chassis for some parts of the circuit which connect to the outside. Much like Nominal Animal sketched in reply #9. That went well until they placed the unit on top of a pole at 65kVAC and started drawing arcs. It would misbehave every now and then. Connecting the circuit ground firmly to the chassis ground fixed that problem. Somehow drawing the arcs produced enough energy to create a potential across to board. With the board connected to the chassis, everything is equalised (even though the entire unit isn't connected to ground obviously).

author: David Hess
author_rank: Super Contributor
timestamp: September 02, 2024, 08:01:00 pm
content: Quote from: T3sl4co1l on September 02, 2024, 04:29:48 pmIt means that a third wire inside the shield has to carry the return currents. This could be differential signaling like with twisted pair Ethernet, or a signal ground wire like with RS-232 or RS-422/485, or the inside shield of a coaxial transmission line.I included Analog Devices application note 347 as an attachment.

author: EPAIII
author_rank: Super Contributor
timestamp: September 03, 2024, 12:12:08 pm
content: Shield grounds/connections?I fought ground problems for over 45 years. Came to ONE conclusion: what works, works and what doesn't, doesn't. You can discuss theory all week long but when the shields hit the grounds, if there are problems, you have to work out what works. And if there are no problems, then theory be damned and LEAVE WELL ENOUGH ALONE.That being said, I have always used solid Earth ground networks in my installations. And I am not talking about the puny little conductors that electricians run to the outlets. Those are safety grounds intended to trip the breaker when an electrical power short occurs. They are not a real signal ground network.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 03, 2024, 01:12:57 pm
content: Quote from: EPAIII on September 03, 2024, 12:12:08 pmThis is precisely why I started the thread. I can work with the theory from a physics standpoint all I want, to any level of detail and complexity I want, but that's just the theoretical part. I need advice from experience. Including why certain things just won't work. (I'm just not swayed by authoritative arguments that say "do this, or it won't work". Ineedto understand the reasoning behind the advice, because that's why I'm asking; I'm not trying to find out about standard practices.)I also want it to be legal, but that's just because I am personally not experienced enough to determine when it is the rules and regulations that don't make sense. However, as I've read the Finnish electric safety laws and such, it seems that this kind of stuff is legal if a sparky with the correct credentials accepts it, and it works in practice. (Also, I'm working with very low voltages and currents, just a few watts at below 20VDC or so.)The snubber network between shield and ground is a good example of why experience matters. In theory, it works well. Many say it doesn't, but most haven't elaborated exactly why. I've looked for specifics, and read from others exactly what kind of ground bounce and EMC issues they can lead to in different circumstancesin practice, and from those, now have a much better understanding of the difference between theory and practice here. I cannot stress enough how useful it is to hear/read and understandwhy, the practical reasons and experience, rather than justwhatone should do.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 03, 2024, 01:19:49 pm
content: Quote from: Nominal Animal on September 03, 2024, 01:12:57 pmYou must obey the law no matter if the rules and regulations make sense or not. That's how the law works!You don't have the luxury to determine which laws you can ignore, you must comply with everything!

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 03, 2024, 01:47:52 pm
content: Quote from: Andy Chee on September 03, 2024, 01:19:49 pmMost legal systems have cruft and silliness because nothing is static and everything changes as time progresses. Just look at cable color coding rules. Not all laws and regulations are actually enforced, especially when they are changing because of such cruft.For example, in the state of Florida (and several others) in the USA, consensual sodomy (anal sex) is against the law. Nobody can or will be prosecuted, however, because of precedents (Lawrence v. Texas, US Supreme Court 2003). So, technically, it is against the law, it just isn't punishable, nor will any cops investigate anything related to breaking that (assuming all parties agree it was consensual, of course).Similar silliness might be regulations allowing non-electricians to connect say ceiling lights to mains using screw terminals (chocky blocks), but not to Wago connectors, because of the wording in the law/regulation hasn't caught up yet. (The latter are safer.) Thus, it is about understanding how the relevant laws and regulations are changing in the near to medium future, not about choosing what laws and regulations to obey and what to ignore. Stuff that capable EEs and electricians should be aware of, in my opinion.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 03, 2024, 01:59:48 pm
content: Quote from: EPAIII on September 03, 2024, 12:12:08 pmThe key is understanding the theory correctly. Then itwillmatch reality and save from endless problems.Problems:* Understanding EMC and SI is not trivial. It really is a complex topic;* This complex topic is made more difficult by a lot of poor quality pseudo "information", written in good intention but causing more harm than good.* Lot of unnecessary and wrong "rules of thumb""What works, works" is probably more useful thanincorrecttheories, butcorrecttheoretical knowledge is absolutely necessary because figuring out EMC/SI by experimentation only works in easiest cases and is too expensive. There is no way you could test out 1000 different combinations of wiring arrangements in an EMC lab where every test costs you say $1000. So you need expertise, you need theoretical knowledge. Testing is thenverificationof that theory. If theory is sound, failures in verification step are rare and small. If the gap between theory and practice becomes wide, you are listening to the wrong experts.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 03, 2024, 02:38:44 pm
content: Quote from: Andy Chee on September 03, 2024, 01:19:49 pmThis is just blatantly incorrect. It is completely normal business practice to even outright ignore laws, or apply interpretation which not everybody agrees with. Of course this requires care as it presents a risk to the business. Finally decisions are made in money, if a decision makes more money than fines paid, then it is doable.Individuals do the same, for example choose to drive faster than the speed limit and say "sorry" or pay fines if caught.Besides, in 90% of cases when someone on a forum claims that "law says exactly X", what the lawactuallysays is more complex than that.

author: KaneTW
author_rank: Frequent Contributor
timestamp: September 03, 2024, 03:18:25 pm
content: EMI is inherently a complex topic, so any blanket statement is questionable.That being said:* PE is a separate thing than shielding. It's intended to make sure that fault currents or short circuits do not cause dangerous situations.* PE is almost always tied to chassis ground, if the chassis is metal. Exceptions exist but are rare and not something you'd do without good reason.* The (metal) chassis functions as a shield for the device, and if feedthroughs/connectors are properly laid out the connector shields will be tied to chassis ground as to not result in gaps in the shield.* Shields should be tied at both ends unless there's exceptional circumstances. Any ground loop current should be handled by ensuring a sufficiently low-impedance connection between the grounds.* That low-impedance connection often ends up also functioning as PE in e.g. industrial automation

author: snarkysparky
author_rank: Frequent Contributor
timestamp: September 03, 2024, 07:52:15 pm
content: Yeah the only real way to approach the problem without toooo much guesswork is to draw a complete schematic of the system.OP has done this mostly but need to add noise sources and locations.Including* noise sources at all likely points.*coupling methods to target circuit points both inductive and capacitive*add caps and ind for the parasitic elements*proposed shields with parasitic coupling to them from the noise sources and coupling from the shields to the target circuit.* try to consider transmission line effects on cables that are long enough to worry about.what else ?? I suspect there is more.This is the hardest part, well maybe, gotta solve this nasty circuits problem now

author: Andy Chee
author_rank: Super Contributor
timestamp: September 03, 2024, 08:44:06 pm
content: Quote from: Siwastaja on September 03, 2024, 02:38:44 pmIgnoring rules & regulations is what Boeing did. Two crashed 737 MAXs and a loose door plug later. Money may temporarily solve the problem, but only if your revenue stream is greater than the cost of the mistakes you make.Then there is professional liability insurance for engineers. When a civil engineer designs a bridge, and the bridge collapses, the engineer's insurance will pay for the damage and deaths (unless the court proves that the engineer was untrained and incompetent).Finally you have entire countries breaking the law, such as China copying products.In summary, if you break the law, you are being unfair to others.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 03, 2024, 08:50:55 pm
content: Or financebros short selling various stocks dry, among other shady practices; but hey, if the FTC isn't going to police them, or at worst levies a modest $20M/infraction or whatever, while turning billions in profit -- that's basically saying go ahead, please take their money, give us the tiniest cut and be on your way, right?(This has been your daily reminder that law is not moral, and the two align only on accident, or in the specific few cases where we have fought for and demanded such.)Tim

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 03, 2024, 09:19:17 pm
content: I'm realizing now that if I ignore the fact that I use a switchmode supply, and instead treat it as a standard mains-powered exposed metal chassis device –– with the caveat that I do not have live and neutral, only two AC inputs ––, I can find literature describing how this is best done and why. Including when the device is split into multiple enclosing chassises via shielded cables. Quite standard stuff, really.(It's a bit humbling realizing how, uh, silly the question must sound to those who work with such things daily, and thoroughly understand these. All I can say, I've learned a lot from the discussion here. I truly appreciate the information, everyone.)There are some details I still need to understand much better, for example how to limit the noise from the switchmode supply to the load when inside the same enclosing chassis –– not just conducted, but radiated and maybe even magnetically coupled ––, but I'll try learning about that from the well-known sources (several mentioned in this thread) on my own first, before bothering others.One point about the DC GND: when working with USB hosts that have proper grounded power supplies and DC ground voltages at or very close to PE potential, and USB devices that use those wall warts that cause their DC GND to float at couple of hundred volts above or below that, I really like it when I canchoosethe ground potential for the kind of circuits I'm talking here, by connecting a ground before powering the circuit. It has been an useful tool. (Some of you might remember a thread about USB isolators before.) I hope that explains why I have been so reticient at tying the DC GND to PE here: didn't want to discard an useful tool too early/unnecessarily, that's all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 04, 2024, 01:28:16 am
content: Yep, isolation is a useful tool -- not just in galvanic terms (opto/logic isolator, transformer, etc.) but it works for EMI too, within reason.Suppose you have two boxes, joined by a shielded cable; if the shield is solid, that's all that matters, signal quality is maintained across them.Suppose the shield is cut in the middle, going for some length unshielded, with the cut joined by a wire; a voltage drops across this wire, depending on frequency relative to its length. Some of this voltage couples into other wires in the cable, but only partially; the coupling factor for wires in a multiconductor unshielded cable might be... 0.5 to 0.9 say, ballpark, but depends obviously on geometry. Noise on one reference plane, with respect to the other, couples only partially into the signals; as seen from the other, there's a noise source in series with the signals.If we put a ferrite bead around the cable, we improve coupling between shield wire link and the other signals. The shield currents likely also decrease (min(|Z_CM|) (as measured at EMI frequencies) has increased), further improving CMRR.Or, on a PCB, we might have a slot between two reference planes. It is acceptable to route signals between these planes if we have a CMC which couples the plane-to-plane voltage into the signals -- thus decoupling the signal currents between planes, and maintaining noise immunity. I showed an example of that here,https://electronics.stackexchange.com/questions/686009/what-is-the-best-practice-when-replacing-a-ribbon-cable-ferrite-clamp-with-chip/686238#686238Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: September 04, 2024, 07:07:26 am
content: Quote from: Nominal Animal on September 03, 2024, 09:19:17 pmA side note (because I know you are interested in correctness!), line and neutral are both wires that have attribute live attached to them. So the two AC inputs are two live wires, but you don't know which one is line and which one neutral; and even when you think you know (e.g. in countries with polarized plugs), you are still not allowed to assume. Therefore neutral is always live.The main distinction between PE and N is that with PE, you are kindaguaranteedit is not live. (There are exceptions to even this such as those super scaryneutral faults.)

author: m k
author_rank: Super Contributor
timestamp: September 04, 2024, 09:54:38 am
content: Take a machine of few tens on meters long or wide.It's big enough that it usually have different independent parts.Usually metal framed also.If you go circling around that machine you'll find that all around are thick Y/G wires connecting metal frames together.Then its many 3-phase supply lines are nothing special, just normal rubber cables here and there.The installation has made its own earth.Maybe there are also few different thick walled cabinets for different electronics.From there you may see leaving some thick plastic tubes.If those tubes have a flat surface it's likely that inside of that surface is a shielding, then single wires for normal logic connections.It's also possible that they are just cable collecting wrinkle tubes, but then there are no single wires, only shielded cables, except one, that thick Y/G wire.So own earth is maintained and external world is shielded out.Black magic starts if own earth is not present and distances are long enough, that's "earth long" not meters long, then the "long earth" must be cut.Some current here and there may not be a problem, maybe it's even disturbing all signals equally, but single ended driver may feel it differently, not the current, but the potential.Actually it's not even the potential that is a problem, its balancing route is.Flat ribbon cable with flat external shield, it's clearly shielding against external world.But it also has another function, it's damping noise between individual ribbon wires, that can also be its more important property.One more thing for that black magic.Nowadays we don't see open wire telephone lines anymore, so spark gaps may be out of the thought process.Back in the day when dumb terminals were the norm the rule was that wall side, or server side of the shield is grounded.Nowadays it can be different when both ends of the shield are floating.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 04, 2024, 01:19:21 pm
content: Quote from: m k on September 04, 2024, 09:54:38 amYep, exactly how I was thinking. Just because my voltages are lower and currents much smaller and in a sense I'm inverting inside and outside compared to say a CNC mill with 3-phase power, does not make the situation fundamentally different. Andthatis well documented as to what is necessary and/or useful and why.Quote from: m k on September 04, 2024, 09:54:38 amI'm old enough to remember the time when IDE (parallel ATA nowadays) became fast enough to switch from 40-wire to 80-wire cabling, with every second one a ground, to reduce crosstalk at the higher frequencies used. Soon, flat cables were bunched in the middle, and even shielding started to be used, even though the insides of a desktop computer enclosure aren't that noisy electromagnetically speaking.(I did use SCSI cabling too, especially with servers and Macs (pre-OS X), but I don't think I ever split an external one open to examine the construction; they were both too robust and expensive to waste that way, and I don't think I had a single one fail on me. On the servers prior to the turn of the millenium, I used flat ribbon SCSI cables.)Looking at history, and examining the reasons for the solutions they arrived at, isalwaysuseful in my opinion. The solutions are usually not the optimal ones, though, because we're human, and technical superiority has traditionally not been a main market driver.I find it helps predict what one doesn't yet know, like the use and purpose of guard traces for example.