I have around 12 purely DC PCBs inside a metal case. Each has a MCU which connects to the main controller PCB with SPI or I2C. Each is powered by one shared ATX PSU which also sits inside of the case.

Obviously the case and any exposed metal needs to be connected to Earth. But every PCB is screwed directly to the case and around 4 PCBs have metal connectors or Pots that also touch or are screwed to the case.

I am struggling to get my head around what to do in regards to DC Ground and Earth, do I connect them if so how or do I isolate them?

I can't find any info on how this is done in a commercial application which would comply with the relevant EMC and safety standards.

What should I do with the shells of the Pots and Connectors on the PCBs? They are going to be connected to earth through the case, so should I isolate them from the DC ground on the pcb?

What should I do with the mounting holes? Should I have them plated and connect the DC ground to earth or should I not plate them and leave plenty of clearance around them for the screw heads?

It seems like I have the following options:

  1. Connect DC Ground to Earth, it then wouldn't matter about the Pot and Connector shells.

  2. Connect each shell to to DC Ground via an 1M/4.7Ω Parallel RC Tag. This would only be ok if each shell made a solid connection to earth.

  3. Have a separate Earth plane on each PCB connecting the mounting holes to the shells.

  4. Leave the shells and mounting holes completely isolated from the DC Ground on the PCBs


Not a real answer, but it got too long for a comment.

There is not really enough information to give a really good answer. It may work OK with everything grounded to the case. EMC regulations are performance based. The FCC doesn't tell you how to ground things. It just tells you how much RF you are allowed to radiate into space. If you have a bunch of stuff inside a metal chassis, and grounded to it, you have a good chance of passing radiated emissions just because your metal chassis will act as a Faraday cage.

But, the most important thing is that you MUST run a GND wire from board to board with every serial connection. So if the main board has an SPI interface connected to a peripheral mini-board, you must run a dedicated GND wire along with the other SPI wires. Preferably, the GND should be twisted with the SPI clock wire. If possible, it would be even better to run a GND wire along with every signal, and twist it with its signal wire. It is almost like you are wiring a differential pair, with the signal being one element of the pair, and the GND being the other element of the pair. This will greatly improve (reduce) radiated emissions, and also preserve signal integrity.

It doesn't sound like there is any major safety concern (as far as shock hazard goes). You just need to give some consideration to wires and fuses and short-circuits. You just don't want a wire to ignite a fire if it carries a large fault current as a result of a short-circuit somewhere. A few strategic fuses can probably take care of this.

The only shock hazard would be associated with how the AC power enters the box and how the chassis is grounded to the protective earth ground. I am not an expert on that stuff. I think you just need one solid, low-resistance connection from protective earth to the chassis. Sometimes a chassis will have a metal stud for this purpose.


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