ben84

FX3U 0V interference

34 posts in this topic

Good evening all, I have an issue with an FX3U PLC that is driving me up the wall, but have a feeling it's something that may seem daft to an experienced user. I would be extremely grateful for any help. Basically, I have one PLC taking 24VDC inputs from 3 separate machines but I'm experiencing interference with what I believe to be the 0V. I have wired a bank of 3 similar machines to another PLC without issue, but this one seems to keep presenting problems. When one machine is totally switched off, the PLC LED indicators of the inputs from it dimly flicker and then pop on in time with a machine that is still operational. Could someone please enlighten me how I should be wiring these up? Each machine is sending out its own 24VDC inputs to the PLC, which I have connected directly to the relevant inputs. I have a 24VDC power supply that I have connected to two banks of DIN rail terminals (24VDC and 0v) with a common bar on each bank to give me several connection points. From these terminals I have connected the PLC for its power source, plus taken a 0V from each machine and connected these to the same 0V DIN rail terminals. The S/S terminal on the PLC is connected to the 0V of the PLC. I'm certain this is where I've gone wrong (even though I have a carbon copy of this that works with the other 3 machines). Do I need to take 0V from each machine to use as a reference for the inputs? If so am I creating problems by having them joined to the same DIN terminal as the PSU I'm using for the PLC? Again, I realise I may be making myself sound silly but I am learning as I go along. Thanks for any help in advance

Share this post


Link to post
Share on other sites
Did you also connect 0V to the S/S jumper?? If not, the internal power supply of the PLC will be used, which might not have the same reference point (0V) as your other power supplies... The PLC essentially has a small power supply on it's own, so you have four power supplies. You have two options: Disconnect the S/S from the 0V on the PLC, and then connect S/S to your common 0V from the other three powersupplies (which you obviously have already linked by referencing the same 0V) Keep the S/S connceted to the 0V on the PLC, but then also connect your S/S to your common 0V from the other three powersupplies (then all powersupplies will be linked by referencing the same 0V)

Share this post


Link to post
Share on other sites
Thanks for your reply. I see what you are saying, but I think in effect I have done what you are suggesting. The cabinet has a 0V rail and a 24V rail, and I've taken a wire from the 0V to the power input of the PLC, then taken a wire from this to the S/S terminal. So both the 0V and S/S are connected to the common 0V in the cabinet along with a 0V from each machine. I will double check they all lead to the common 0V, but thank you for answering that question. Now I know for sure they should all be connected to the same rail for referencing.

Share this post


Link to post
Share on other sites
I'd like to add a small, but serious remark to the recommendation, given by Kaare: all it is only an attempt to treat a "cancer tumor" by applying a plantain... The connection of zeros of 3 or 4 PSUs to one circuit in industrial automation is a bad idea, that may cause to damaging the equipment: all 3 or 4 units at one moment.

Share this post


Link to post
Share on other sites
Thanks for the advice. Only the 0V from each machine is connected to one another on the terminal blocks before being connected to the PLC, the 24V signals are wired directly from each machine to separate inputs, so will never technically cross over.

Share this post


Link to post
Share on other sites
I understand it. But the correct way is to connect the signals from different machines to separate input groups, if each machine generates potential signals. This way can be realized using a separate DI modules. Another way, which is correct too, is to use one power source at PLC side (not the internal AC/DC of PLC, but an external) and dry contacts at the side of each machine.If you choose own way, you have to keep a reserve money in your purse to replace all units in the automation system, in case if they will be damaged. Edited by Inntele

Share this post


Link to post
Share on other sites
Thank you, I understand. It is certainly food for thought. I will think of a way I can easily isolate each machine from each other. I am probably leaning towards the dry contacts/SSR's to provide that separation.

Share this post


Link to post
Share on other sites
Personally I would check that all the 0V's of each PSU in each cabinet were connected to ground (earth). I have had similar problems when on of them was left floating. This is standard practice on all out panels.

Share this post


Link to post
Share on other sites
And after that all AC/DC in all electric cabinets cease to be isolated... Usually the HV side of AC/DC is already connected to the ground.

Share this post


Link to post
Share on other sites
I'd agree with NIghtfly .... connecting the 0V to earth is our standard as well. And check that the 0V terminal on the PLC is connected to earth and that the S/S terminal (because of this) is also connected to earth. I once left the S/S terminal not connected to anything and the inputs had a mind of their own, going on and off at will. Edited by Colin Carpenter

Share this post


Link to post
Share on other sites
It make me sad

Share this post


Link to post
Share on other sites
And my standard to ground secondary 0 volts Rodney

Share this post


Link to post
Share on other sites
Even if the number of adherents of grounding zeros here will exceed 99,(9)%, it can not change me in the opinion: any control system must be designed, installed and connected properly !!! If an isolation protection, or IP protection, or other safety parameters is declared, they must be strictly observed... PSU manufacturers doesn't wire zero of PSU output with ground, and declares parameters of their PSU for wiring, that described in documentation to the device. Also, if you create a power grid with isolated neutral in an electric cabinet, don't connect the grounding terminals of devices, that get a power supply from this grid, otherwise the grid cease to be isolated. The last is one of typical blunders of automation engineers... For 16 years in close contact with automation I have designed, debugged and commissioned a number of automation system, have looked and approved a lot of schematic design, prepared by other engineers. Both I and these engineers never did connect zeros to the ground, never got problem with it, never "danced with tambourine" and never "pray to idols", that they help to overcome problem with noise in circuits ! All kind of problem, I met during the long practice, were due to other (external) causes. Those causes has been identified and eliminated operatively, and after that step all automation systems works perfectly. So, if you meet with noise in input or output circuits of PLC, try find a root of problem, instead of to apply a patch.

Share this post


Link to post
Share on other sites
I certainly agree with Inntele! Connecting 0V to ground in any cabinet is a bad idea. Especially when it comes to noise from other peripheral devices. E.g. A frequency inverter that generates noise through the earth wires may return to the control circuits in the cabinet and generate noise. So as Inntele states; connecting 0V to ground in the lab is fine when you don't have to care for other equipment, however in real life you may experience seroius problems. I must also point out that the initial question was how to get 2 power supplies to work together in parallell, NOT if one should connect 0V to ground/earth. If anyone here have ever tried to debug electrical systems in any car (new or old) they sure know what were talking about! You have a small leak current in a circuit at location A, and the lights suddenly start to light up in location B in the same car - completely crazy. And for all you experienced UK people out there; yes, this problem also occurs when you connect 12V to the chassis in the car and wire all the 0V directly... There is a reason why you have 24V, 0V and GND on the PLC. 24V = +24V, 0V = 0V and GND = Ground! If there was no need for ground connection then Mitsu (and all others) would have internally connected 0V and GND inside the PLC. And 0V is NOT the same as GND. If you take it one step further and look at the Q series, you have one grounding point for the cables and one for the shield. Both is essentially the same however shield handles the noise, while cable ground handles surges on the power side. NONE of these two grounding points should ever touch 0V because it may affect the more sensitive equipment. And by the way; if a surge happens on the ground/earth (which is why ground/earth is there in the first place) in the cabinet, all your 24V equipment (essentially the expensive equipment) will most likely burn. Or what happens when the local electrician accidently connects one phase of 230VAC to ground instead of the correct phase? Would you take a chance on anyone connecting 230VAC to the 24V terminal of the PLC? If you don't dare to connect 230VAC to the 24V terminal on the PLC, why do you want to risk connecting 230VAC to the 0V terminal?? I could continue with examples forever, and please prove me wrong if necessary. You make your own descision, just be sure you know what you're doing before you walk into possible problems.

Share this post


Link to post
Share on other sites
Dear Kaare, I am pleased that our views, on how to build an electrical part of automation system, coincide. I propose to keep this topic an open and, when we'll have a time, to draw samples of design, how to provide correctly the electrical connections and how to fight with noise, that then to share them in the topic. I suppose, it will be a right action by our side.

Share this post


Link to post
Share on other sites
I have great respect for the technical knowledge that you both have, but I have to say that in 30 years or so of connecting 0V to ground, I have never had a single problem as described. I have never had any digital input cards go bang or fail to work properly. The only noise issues I have ever had have been with an RS485/422 cable connecting an HMI to a PLC that actually passed though an inverter panel (solved with a braided "Farady Screen" , and once I had a PT100 temperature probe that would read wrongly whenever any one of the installed inverters started up. This was solved by fitting a probe with a 4-20 transmitter on the actual probe rather than in a remote panel. I have actually seen an electrician connect a phase to earth .... or rather he was making up earth tails and one of them flicked momentarily onto an exposed phase terminal. Big flash, lots of UV, breakers went out as you'd expect but no problems at all with any of the PSUs or digital inputs. The only "ghost" inputs I've ever experienced were on an FX2N without the S/S terminal tied down to earth, and these were exorcised when it was tied down. I've never known a Mitsubishi PLC get a corrupted programme even when welders have been striking up their TIG welders on the stainless panel that the PLC was installed in. Also, tying the 0V to earth makes it a lot easier to find live 24VDC signals out on the process floor as you can used an earthed chassis as the negative for your digital multi meter. It does seem strange that all us UK users say the same ... wonder what's different?

Share this post


Link to post
Share on other sites
Dear Colin, There are no key differences in hardware or power grids. The single difference, I see, each specialist passed own professional school and gained own professional experience. Some met with burned-out, some other are still lucky. A couple of damaged PLC might be sufficient to doubt what is not so in PLC connection. Besides Mitsubishi products slightly more resistant to stress than its european competitors: the safety margin for japanese is about 50% in contrast to 30% for european. When was working with Siemens, I tired to resolder the driver of RS485 port, although we comply with all precautions at connection. Later, I found out what was not so in the schematic of PLC 'feeding'.

Share this post


Link to post
Share on other sites
i admit having hard time determining correct meaning of some of your sentences. but there is little doubt in what was meant by the quoted above - this follows the same mantra of "tying something to GND is a bad idea, heck it may even fry something or - at the very least this will introduce noise into sensitive equipment and somehow bring the world down". i've been designing automation since late 80s and have yet to see any proof of that. and all of my designs have one of DC output terminals in PSU tied to GND (usually negative, sometimes positive). connecting things to GND is not meant to make design less safe, quite the opposite. designs using floating voltages can work, no doubt there. anything can work while nice and new. the issue is when something does go wrong (and risk of this is becomming greater as equipment is getting older - decaying insulation, layers of dust, moisture soaking collected dust etc.). the greatest chance of accidental shorts is having something shorted to ground. if the design is using floating supply, such failure will NOT be detected and, since undetected, this can lead to accumulation of faults (a really bad situation in my opinion). then you can have system running crazy since it does not know how to differentiate real and fake signals. and things like this would likely be a nightmare to troubleshoot. another thing i see in above quote: burning PLC hardware. as mentioned in some 30 years i have not seen this and many of my designs are still running - in different countries and very different climates. anyway, suppose PLC card does get burned and need to be replaced. why is this such a bad thing? i would much rather have entire PLC (or cabinet...) burn down than to have a human injured or months worth of production to be scrap because of cheap $200 card. not to mention that such issue would be very easy and quick to troubleshoot. if PSU is grounded, and you get short to GND for example on one of PLC outputs, the output card MIGHT be damaged (if one did not use card with protected outputs nor design proper circuit). and of course this may stop production as well. but this is still a safer design since it will not run amok undetected and cause who knows what down the road due single fault (or an accumulation of faults). finally there ARE things which are more sensitive... and i have seen and used them... for example PCs using DAQ and whatever ports (such as RS232 or USB for example) connected to various components powered from different sources. Sometimes ground potential between those is significant and PCs common is always internally grounded (so all PC ports like RS232, USB etc are referenced to GND). in this case you don't have much choice - use optoisolation. leaving other side floating only buys you some time but does not resolve issue.

Share this post


Link to post
Share on other sites
@panic mode: So to avoid misunderstandings, this is how you connect various components? I'm always interested in hearing about practical installation, especially if it works better than theory and/or my normal installation routine, so please explain in detail or provide drawings (which says more than a thousand words...) And I am very, very interested in hearing about how humans may get injured by my (and Inntele's) solutions. In Norway, where I'm normally located (and I've travelled some too in various parts of the world), we have strict laws against connecting equipment so that people may get injured so I'm very interested in learning how my (and Inntele's) way may cause injury to humans. Please provide a drawing of how current may cause injury to people in the case we are discussing now. At last you say that you sometimes connect positive to GND from the PSU. I'm also very interested in learning how you handle the AC GND in this case. Do you really connect AC GND to +24VDC, and how do you avoid circuit brakers to fail? Again, maybe this is a lot of misunderstandings, so drawings would be great to prove our stands!

Share this post


Link to post
Share on other sites
sorry for late reply, i am not dodging the discussion, simply was out of country on a business trip... the short answer is yes, although schematic representation does not quite illustrate how all gnd connections are really made. they need to be in one "point" (grounding block/bar), not scattered like shown in schematic. i live in Canada so yes, we too have regulations. also all new equipment must be certified before it is used. some companies have CSA certified shops and can apply CSA label themselves but most companies that build automation are simply inviting inspector to check and approve any new piece of equipment as needed (by CSA, ESA, QPS, Entela or similar). btw, i meant no disrespect to you or intelle. in fact i admire your attitude and interest . on another note i don't do electrical design in my current job but this is an interesting topic so i will gladly engage in a debate, that is what forum is for and i hope we all benefit from it. to me, not learning new things means falling behind... i know i like to talk to my peers and see things from different perspective (despite my opinions coming out strong sometimes ). one of the things that i see often neglected is how equipment will behave later on, after some wear and use/abuse. this is where failsafe design comes to mind (which we all seem to strive for). there re many strategies and ideas that can be used to reduce risks, one of them is use of grounding. i also mentioned this in the attachment. normally industrial power supplies are isolated so one can choose which side to ground (if that is what you intend to do). of course you only connect one of the DC terminals to GND. grounding +ve terminal is not common here since we use PNP devices most of the time. Japanese companies and their minions do what is common in Japan - use NPN. In this case positive terminal of the DC power supply is grounded and negative side is "live" (this is where fusing and switches are). getting NPN I/O modules is not hard but getting safety devices (light curtains etc.) that fit your application and use NPN OSSD is another story (apparently they are common in Japan). so, one uses safety devices which are PNP and all non/safety related control hardware is using NPN. then one must use more than one power supply. one is used for safety circuit and has negative terminal grounded. other power supplies have positive terminal grounded. all of them are in the same cabinet, usually side by side. Electrical safety.pdf

Share this post


Link to post
Share on other sites
and here is an example of multiple circuits (with different supply voltages) being referenced to GND. no short circuits...

Share this post


Link to post
Share on other sites
Dear gentlemen, Sorry for my silence also, I was very busy this week and still have not a time to prepare drawings. But I mark the issue as an important, so I'll back with design examples, that all could criticize. As for misunderstanding, I think we understand each other quite well. If a misunderstanding is present, it's only due to difficulties in expression of thought. Certainly, panic mode are right absolutely, the human life has the highest value. When I said "if you create a power grid with isolated neutral in an electric cabinet, don't connect the grounding terminals of devices, that get a power supply from this grid, otherwise the grid cease to be isolated. The last is one of typical blunders of automation engineers...", I have not meant that the circuit (for example a secondary circuits of an isolation transformer) never can be grounded. I meant another: if at least one device in the grid is grounded, then the grid is not isolated. While some "eagles" of automation forget this simple and obvious thing. Later I'll find and publish the schematic by one such "eagle". It will be an illustrative example how has not to be done. If the grid is isolated, then a protection for human of electric shock should be necessarily provided.

Share this post


Link to post
Share on other sites
Well, I guess we all get a bit excited when discussing interesting things, so sorry if I were a bit harsh too! This is definetively interesting, so let's keep it going - we might not even agree at the end, but learning different views is certainly great! @panic mode, I just have one follow-up question for now: What kind of 575/600V system do you use in Canada (IT/TN-S.....)?

Share this post


Link to post
Share on other sites
In the United States NEC (National Electrical Code) and Canadian Electrical Code (CEC) the feed from the distribution transformer uses a combined neutral and grounding conductor, but within the structure separate neutral and protective earth conductors are used (TN-C-S). The neutral must be connected to earth only on the supply side of the customer's disconnecting switch.

Share this post


Link to post
Share on other sites
So I've done some research (contacted a cabinet designer and a cabinet builder) since I started to doubt my own knowledge (thanks to panic mode ). And we're still discussing TN-C-S like we have in Norway. Here's what I've come up with: Everything below 48V is, in Norway, considered safe for humans, no problem in Norway (maybe in Canada/US?)New PS do not ever transfer primary side voltage to secondary side voltage (I have been informed by some "older people" that this actually could happen in "the old days", like 15-20+ years ago)IF you start connecting any equipment to gnd, then you must connect ALL equipment to gnd. So basically don't start to do it in the first place. That's because the potential between gnd in one part of the facility may have negative V and the potential between a different positive may increase to above 48VSome "cheaper" Siemens equipment actually internally connects 0V to gnd (I'm not a Siemens guy, but I verified this information with a 3. guy that works a lot with Siemens equipment). However the "real" automation equipment do not internally connect 0V to gndIn Norway there are rules for monitoring gnd currents on the secondary side too. That means that there is no rule that says that it should brake in case of a low voltage leak current, however it should be monitored and give an alarm so that it can be fixed during the first machine maintanance stop. However: In some parts of the world, connecting 0V to gnd is actually mandatory if you do not have a current monitoring circuit braker on the secondary side. So then we're back to the 0V to gnd issue. However, my opinion is still that one should not connect 0V to gnd, but instead purchase a monitoring circuit braker that handles any possible leak currents on the secondary side. This works exactly as a monitoring circuit breaker on the primary side, which trips in case of a gnd fault. Further, I'm not into this kind of plant, but let's say a nuclear power plant: Do they also connect 0V to gnd? Because one thing is for sure: If you start connecting 0V to gnd, the brakers will at some point in time break and the whole plant will definetively stop at once! And as panic mode states, in time after some use/abuse something will definetively happen with the machine/plant, so what does one do when that happens? Let the machine/plant stop at once when the circuit brakers blow? And again; searching for faults is a nightmare if you start connecting 0V to gnd. Again, I'm giving the car with chassis=0V as an example. Everyone that has been trying to debug a car's electrical system in case of a fault know this. @panic mode: I sent your schematics of the "Electrical safety" to both my contacts, and they both was wondering if you had a different example than the primary side short-circuiting to the secondary side. As mentioned above, this is not a real issue in modern power supplies. And at last, I do NOT mean to offend anyone at all, but I was informed by the cabinet designer that connecting 0V to gnd is a "good way" of solving possible issues in a "bad cabinet design". In other words; if you don't design the cabinet/machine/plant properly, then connecting 0V to gnd is actually a good thing, exactly to avoid possible large potentials between 0V and gnd. I'm not saying that anyone here is designing bad circuits, but one of my contacts simply said that this is an actual "solution" IF the design is not "perfect".

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now