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We are designing a product which has to meet EMC directives. It consists of metal backplate folded into a chasis (the front is open). on the backcover there is a SMPS mounted. and on top of this we have our main PCB which has a signal ground plane on top of the PCB. the whole thing will be surrounded by a plastic cover with some space for user interface on the PCB. See image for clarification.

All SMD components will be on the back of the PCB, the front has the GND plane. The product will interface in a master-slave configuration via RS-485. because the units can be far away from eachother and be on different mains circuits, there can be big voltage differences between individual grounds so we also connect signal ground with the RS-485 connection.

My question is: (how) do we need to connect signal ground to safety ground (metal backcover)? I understand if we connect the signal ground to the safety ground, and also connect the individual signal grounds between units, we create a ground loop.

enter image description here

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OK you have multiple issues here.

Communication

EDIT: Despite being differential, RS-485 needs a "ground" return path to handle unbalanced signal currents. Since the distance is long and grounds can be severely different isolation is your best bet. Since you have not defined the requirement values, then it is impossible to tell you exactly how.

Isolated can mean the device Andy mentions, or opto-coupling the inputs.

Cabling

Again, you have not intimated what style of cabling you plan on using to carry the RS-485, specifically, whether said cable is shielded or not, and more importantly whether the connectors protruding from your board need to be earth grounded for safety reasons.

If the connectors are metal visible, or rather, metal touchable, they need to be safety grounded to the chassis.

If the cables are shielded, the shield should either be hard connected to safety ground, or loosely grounded through 10nF capacitors in parallel with 1Meg resistor.

The latter may not be possible if the connector in question has the shield attached to the connector shell which in turn is connected to the unit's connector which you already grounded earlier. That is unavoidable.

Internally

The system ground should indeed be connected to the safety ground, either through a hard connection or via a capacitor resistance combination. The grounding system designed such that no currents will flow through said grounds from the system itself. That is.. NO SIGNAL RETURN CURRENTS.

This grounding connection provides two functions, it prevents static build up in your electronics, and it provides an escape path for EMC and EMI currents.

All safety grounds should be star-connected individually to a single hook-up point.

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  • \$\begingroup\$ But in this application note link from TI it states that one should always connect the RS-485 with a ground wire? The cables which will be used depend on the equipment installer, so worst case it can be plain wire, not even twisted pair... Let alone be shielded. Although we can state this is necessary in the installation manual, we would like to design for worst case. Thank you for the advice on connecting signal GND with safety GND! \$\endgroup\$ – BertVano May 11 '17 at 16:54
  • \$\begingroup\$ @BertVano hmmm... Well, they do, kind of, have a point there... you'd be better isolating then. \$\endgroup\$ – Trevor_G May 11 '17 at 17:08
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The simple and sensible thing to do is use isolated RS485 transceivers. Connect grounds (for EMC and communication integrity reasons) via 10 nF capacitors that are sufficiently rated to accommodate power AC voltage differences.

enter image description here

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This is a very large topic which you have given no where near enough information to answer. If you want a real answer, you'd need to hire an engineer and give them at least a week to understand it all.

There are two options considered by most people: ground everything with large confident paths, or isolate everything (as mentioned in another answer) and have differential signalling.

My response is: design the system properly.

You describe a "safety ground" and a "signal ground". As I don't know the system, I don't know what these both are. You need to make sure you have a low inductance return path. That means keeping the distances as short as possible for the full path for every signal. I assume the safety ground is your ESD return path? Make sure there is a path from the ESD diodes to this safety ground is as short as possible, keeping the loop small. The signal ground will need to be connected to the signal source (make sure the route is as small a loop as possible, you may befit from using the safety ground, or you may not, depending on set up and the rest of the system). But really, there is way to much to go into, and too little information given.

The only quick answer I can do is agree with Andy aka and suggest going for differential signalling to remove the need a signal return path.

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  • \$\begingroup\$ Safety ground is connected to mains Earth (as it is a metal chassis this is obligated), signal ground is the zero volt reference shared over the PCB. Also the transceivers are differential already, but we need to connect GND between units to ommit possible 'signal ground' differences between units, as our transceivers have a maximum of -7/+12V common mode voltage difference. \$\endgroup\$ – BertVano May 11 '17 at 7:58

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