Pitor I see a couple of issues that you want to deal with. ******You need to pick a safety standard to follow. EN60950 sounds like it would apply. It is a deep specification and will take many reads to sink in. Big issue is that one needs redundancy in your barriers. If you use 230V source that produces 24 or 12 Volts then you will want a double basic (reinforced) barrier in this power supply to keep everything simple. If you do this then the power supply output would be considered safe or SELV (must also VA limit). As such you should not have to worry about the cascading failure from a standard point of view because you have redundancy built in.
****** You need to deal with the Common Mode (CM) exposure to the link So if you specify the safety construction of this supply you could ground the RS485 to ground. Remember that the reason you are doing this is to provide a return path for the RS485. You now have to be concerned about 2 big issues. 1) you have created a large loop antenna with the ground that is an EMI antenna. 2) How solid are the ground connections 1KM apart and what type of noise will be between these ground points? If the grounding bonding is not well thought or has deteriated (old building) out you can have significant events between these nodes that can destroy your transcievers or at least disrupt your communications significantly. You know that the RS485 transciever can handle some common mode exposure, +/- 12/7 I think is the standard. You can bump this up some but the result is still small relative to the exposure potential. If in a building there is a fault to ground there can be some rather large surges that run through the system. Your RS485 will become one of the paths and have to take the hit. I suspect you will have some issue trying to gurantee that you can do this without isolation. One approach might be to bus the power out to the remote point and let it float. Use the power supply as the return. However that has its own set of issues that are a concern. Good luck on your reading and climbing the learning curve. Chris Wells ----- Original Message ----- From: "Piotr" <[email protected]> To: "EMC-PSTC" <[email protected]> Sent: Saturday, February 07, 2004 10:16 AM Subject: Re: RS485 and CM choke > > > Richard, > > Once more to the same mail: > > My system is the access control system (not heavy industry). > If my device is 12V powered (installer selects the poker supply - > I think standard as for alarm systems with Accu in it). > I wrote in instruction that RS485 connection is allowed only > in one building (the 1km is because all must be in series). > > Shell I consider in any way the safety ? > If I will be surge protected is enough ? > I was worrying if I am allowed to connect my PCB ground > to protection ground with 100 Ohm. It can help reduce > the ground differences for RS485, but directly to my PCB > ground can be connected for example door lock coil powered > from the same poker supply. > I consider poker supply fail and 230V goes to DC output. > If -12 is well grounded - breaker should do its job. > But if not - the 230V can go to the door metal frame > (door lock coils are not well isolated I think). > 1 kOhm is nothing if somebody has no differential breaker. > Is it my problem or I should only select that the poker supply > must be certified to .... ? > > I have read a lot about EMC and nothing about safety. > I will read EN 60950-1 and may be there is the answer. > > In some industry controller instruction I have read: > "It is the extra low voltage device and can be installed > as isolated or as grounded". It was powered from 12 > or 24V (not sure). > What this sentence was about ? > > Regards > > Piotr > > > ----- Original Message ----- > From: "Richard Hughes" <[email protected]> > To: "Piotr" <[email protected]> > Cc: "EMC-PSTC" <[email protected]> > Sent: Friday, February 06, 2004 2:54 PM > Subject: Re: RS485 and CM choke > > > > Piotr, > > > > You seem most concerned about EMC and functionality, but there is also > > safety to consider. > > > > You say that the line may be up to 1 km long. With such a distance it > > seems likely that the equipment you will be interfacing with could be in > > another equipotential zone, and possible on a different phase of the ac > > mains supply. There is also the possibility of transients being induced > > on your data lines and perhaps even the possibility of power cross. > > > > Of course, you may be able to excercise good control over the > > installation and so the problems I mentioned above (which are not > > intended to be an exclusive list) are all covered. However, there are > > good engineering reasons why long lines are electrically isolated. > > > > Regards, > > > > Richard Hughes > > > > Safety Answers Limited > > www.safetyanswers.ltd.uk > > > > Piotr wrote: > > > > > > > > Hello, > > > > > > I posted my question at sci.engr.electrical.compliance and was > > > suggested to > > > post it here. > > > So here it is. > > > > > > I'm trying to find if it is possible to do RS485 without optoisolation. > > > EN61000-6-2 and EN50130-4 needs interface to work with common mode > > > disturbance in 150kHz to 100MHz range of 10V (80% AM modulation) and > > > 150 Ohm > > > source impedance. This gives me 18V top signal level. I've read that > > > it is > > > good practice to add extra 6dB (measurements inaccuracy and element > > > distortions). > > > This way I have 36V. > > > I wont to communicate with 115kb/s (I plan to use IC-s with limited > > > slew-rate), so even high impedance common mode choke should make no > > > problem. > > > I assume up to 32 devices and up to 1km line. > > > I see two solutions: > > > Solution 1. > > > CM choke 2x2mH + 2.2nF capacitor at each line to ground. This reduces > > > 36V to > > > acceptable values but gives me 1.1nF differential mode. > > > Can this 1n1 be a problem ? > > > I can increase L and decrease C. But the higher L the lower its > resonant > > > frequency and I'll have more problems with attenuating higher > > > frequencies. > > > Solution 2. > > > CM choke 3x5mH with no capacitors. RS485 with third wire connecting > > > grounds > > > of communicating devices. I've read somewhere that only 3 wire RS485 > > > is the > > > right solution. In this third wire I can place 100 Ohm to avoid ground > > > difference currents. This solution seems to be very good. One coil > senses > > > the CM voltage and subtracts it from signal lines. If it is a good > > > solution > > > and RS485 is popular than CM choke manufacturers should have a large > > > offer > > > with 3 coil chokes of several mH inductance but they don't. CM chokes > > > with 3 > > > (and more) wires are widely offered but for higher frequencies. From > > > that I > > > think that I am doing somewhere a mistake. But where ? > > > > > > Connected with that subject is the question: Can I connect my circuit > > > ground > > > with Earth ground via 100 Ohm, and than directly connect circuit > > > grounds of > > > communicating devices with this third wire. May be the choke will be > not > > > needed at all (probably if I reduce my needs from 36V to 18V). > > > My devices are powered from external 12V power supply selected by > > > installer > > > (not me). > > > I don't know where the problems can arise from in this situation ? > > > > > > Thanks in advance for any comments. > > > > > > Piotr Galka [email protected] > > > > > > > > > > > > > > > ------------------------------------------- > > > This message is from the IEEE EMC Society Product Safety > > > Technical Committee emc-pstc discussion list. > > > > > > Visit our web site at: http://www.ieee-pses.org/ > > > > > > To cancel your subscription, send mail to: > > > [email protected] > > > with the single line: > > > unsubscribe emc-pstc > > > > > > For help, send mail to the list administrators: > > > Ron Pickard: [email protected] > > > Dave Heald: [email protected] > > > > > > For policy questions, send mail to: > > > Richard Nute: [email protected] > > > Jim Bacher: [email protected] > > > > > > All emc-pstc postings are archived and searchable on the web at: > > > http://www.ieeecommunities.org/emc-pstc > > > > > > > > > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ieee-pses.org/ > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: [email protected] > Dave Heald: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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