FX3U 0V interference

[panic mode 246]

modern PSU or not - this has nothing to do with internal construction of the device. if the DC end is floating and you throw pile of dirt on it, there is no guarantee that DIRT will not form conductive path from AC side to DC.. i used word dirt, but this could be anything else (melted wires,...). PSU itself may not be compromised but its environment can. what is going to ensure in this case that either of DC terminals is safe?

[kaare_t 155]

Hi, sorry for my late response. Busy with a project. Well, if you want to throw dirt (or soak it in water for that matter), I totally agree. In Norway we encapsulate equipment with an IP graded box or similar if possible contamination. I would strongly suggest to do that in all cases, since the equipment itself may not be very happy about contamination in general...

[Inntele 60]

Hi everybody, The points in the Kaare's post matches completely with what I'd like to say. I can not add anymore. Points of electrical design are my duty and I remember about it.

[panic mode 246]

i guess i my point is not getting across... and yes - enclosures are used here as well (who would have guessed that?). so that does not mean that work is complete when things are in the box. i have worked in different industries (chemical, nuclear, automotive...) and have seen a lot of different modes of failure. nobody is intentionally torching or wearing down cable insulation, dropping tool on equipment, letting chemicals drip on things or leaving cabinet doors open while running production etc. accidents happen when least expected. good design will minimize potential damage or risk to life. meeting code is just the bottom line to cover your behind. and in some places codes are not even enforced, following them is voluntary. there is a lot of equipment running in the industry. there is a lot of parts which may be difficult to access and check periodically. and how many maintenance teams are on top of things (many i have met are unable to keep up or have no clue where to start). many circuits spread across large area, including those that are hard to spot and check. leaving things ungrounded (among some other practices) is a sloppy design and an unfinished business in my book. i already explained reason - PSU (or whatever) may be internally safe but that is just one building block, if it is installed in a way where circuits are spreading to other. you as a designer are responsible to build entire system that uses many such building blocks. all proponents of leaving things floating, seem to have various beliefs, usually stating something like grounding will "ruin signal integrity", "cause short circuit", "something will burn" etc. i would like to ask for proofs or real examples. i have not seen anything yet and i did share my example circuits and reasoning why burning things may be desirable. anyone cares to convince me otherwise? please... need more examples? there is a pesky little thing called signal to noise ratio. say some PLC digital input rated for 24VDC will treat voltage above 14V as true and voltage below 9V as false. 5V transient could potentially falsify signal. if you use TTL logic, rail to rail supply is just 5V and margins are of course lower. digital devices of today have pushed limits of power consumption and as a result supply voltage is lower and lower. one common thing that everybody own (computer) is using CPUs with billions of circuits all operating from about 1V source. this means that difference between logic levels is very small. CPU, RAM etc are operating at very high frequencies and there is a lot or "random" pulses. guess what, PSU and motherboard are grounded in dozen or so places. all working without an issue. but those are digital signals, how about analogs? check your oscilloscope, it is designed to measure fast changing signals accurately. guess what, standard scope inputs are single ended and BNC ports for your probes are grounded (connected to AC ground). in late 90s i was part of the control team that commissioned Farmland 6 plant for Mosaic (near Lakeland and Bartow Florida). i was in the plant when hurricane was making cows fly (ok, i have not seen actual cows fly but just about everything else was flying around and if cow was there it would not be standing...). lightning hitting left and right without pause, it was so bright one could read newspaper without other source of light. btw Florida is seeing hurricanes every year sometimes multiple times and Europe has nothing similar to this sort of extreme weather. Btw. control system there is using thousands of analog inputs and there are hundreds of PID loops. sensor cables are literally hundreds of meter long. the plant run without a hitch (and still does). need i go on? your turn...

[kaare_t 155]

OK, to put it really simple: I guess that in your nuclear power plant: When you have a ground fault (as you say you experience a lot), all your equipment goes dark because you've got fuses braking when a ground fault occurs (the fuse breaks since you suddenly have a short circuit in your system). And by the way; yes we do have ground faults in Norway too. But we don't take the chance of a chemical plant (sorry, we don't have any nuclear power plants) completely stopping at once (without any shutdown routine) when the fault occurs. The computer equipment you are describing (or a small computer TTL) use such tiny currents that they are not even detected on a common earth-safe fuse (30mA in a household, 100mA in a household wet-room) which automatically breaks when detecting two different currents on the two phases. Hence not relevant at all in this case.

[panic mode 246]

?? where did i say that??? grounds need to be tied to common point (which prevents loops). if there is a fault that trips some branch circuit, this does not have to bring entire plant or country down. i was trying to present how things looks like for wide range of signals (amplitude and spectrum) but my point was missed. i provided circuits and explanations and got nothing in return. so i give up... proofs or examples? received none... all i see are deeply rooted beliefs how tripped branch circuit trips, will somehow snowball and stop entire plant, then country, then continent, "we are all doomed...!" or "god forbid engineered circuit protection works and stops something - don't ever let it trip" or "any time something is connected to ground bad things will happen...". peace... i'm out...

[kaare_t 155]

I tried to explain it simple, with my example, in the previous post but here it is again, with a sequence of how and when things happened. Hopefully that will ease the understanding! You said: Connect 0V to GND I said: Don't connect 0V to GND, in case of a GND leak current (gnd-fault), the fuse will trip You said: Divide the plant into branches (which I agree on), so that when the gnd-fault occurs, only a portion of the plant goes down (and yes, the branch will go down) I say (right now): One or more of the branches will be critical for system opreation and cannot go down And you start implying that my opinion is "...belief, and that this is doomsday and so on..." (not the first time you offend me...), well here's something right back at you: A PSU has got both 0V and GND connections, but I guess that your opinion is of a greater matter than all the major PSU suppliers in the world (and not to start with all the other component suppliers)... I know that this is "working" for you, but that does NOT mean it is correct. As a general reference to opinion-based "facts"; old relay control systems also worked but it doesn't mean that they are better than PLCs (it is a reason why we changed to PLCs)! Welcome to the new world! And NO: An automation system is NOT a personal computer!! Who exactly is basing their system design on opinion, and who is basing it on electrical, functional and proper design?

[Inntele 60]

Dear gentlemen, Please, let's discuss the subject without grievances and insults toward opponent. @panic_mode You are offended that opponents doesn't want to hear the arguments and doesn't want to discuss your thesises and conclusions that were explained in details in the "Electrical Safety" document. Also, in contrast to you, your opponents did not yet prepare a similar doc, which would reflect their points of view. Partly you are right, but partly are not. I've read the document, prepared by you, with great attention! It's written very lucidly, argumented and well-illustrated. The mistakes, I see, are contained in its thesises, but because I respect you, your opinion and, until can present the similar doc, consider that have not a right to criticize your. Why the doc is not yet ready? When I've proposed to leave the topic as open, I wrote: "when we'll have a time to prepare it". I still not have. As Kaare, I'm all-in-one. An engineering is only a part of our activity...

[Veganic 8]

I don't know if you have all finished with this thread but I'll add this. Both options are va5lid depending on other measures. You are arguing about the difference between SELV and PELV power supplies. The requirements for SELV are quite demanding in separation vof circuits. PELV poses risks of equipment damage. In ISO60204 I cannot find reference to SELV , only to PELV (possibly supplied by a SELV power supply). So does everyone using SELV used strict seperation of "high" voltage and control wiring? Everyone who doesn't is connecting one leg of the supply output to ground? Good, we can all be friends again. But please comments are welcome / abuse accepted. (The 60204 standard and use of SELV is the part I need to clarify.) Apologies if this has lots of typos. I'm on my phone as my antivirus is blocking this site.