Isolation for a long distance industrial CAN + audio box

Question

I am working on an industrial control board + node network system. This system needs to be very robust, both for signal integrity as well as protection from transients.

The basic setup is shown here:

Since the Ethernet connection going to the server is fully isolated, I don't think there really is much to discuss there. Going from the control board to the IO board is a CAN bus and balanced audio line. These will either be on a separate cable, or on the same Cat6e cable.

All cables will be shielded, with the shield directly terminated to the enclosure/chassis. I have left the PCB ground and chassis unconnected in the image, as I would like suggestions on that as well. The plan would be to dedicate a single PCB layer for chassis ground. (Of course, if chassis gnd = digital gnd, then there will be multiple layers for them.)

For the following discussion, I will only show the CAN bus setup, as the audio will be fully isolated thanks to the transformers. It will also not require a common ground wire to equalize the grounds.

Option 1:

No isolation. Neither the CAN nor audio lines will have any isolation. To ensure that the common mode voltage stays within a safe range, CAN_GND = CONTROL_BOARD_DIG_GND = IO_BOARD_DIG_GND.

Option 2:

Full isolation on one side (let’s say the control board side for discussions sake.)

Option 3:

Full isolation on both sides.

For the isolation, assume that for the CAN, some sort of powered, digital isolator is used. For the audio, as stated above, a 1:1 transformer will be used.

My assumptions/thoughts:

1. Option 2/3 is an improvement over option 1, as it eliminates most ground loops. (There will still be a potential loop from chassis 1 -> shield -> chassis 2 -> earth -> chassis 1. Either capacitively from the chassis to earth, or if both metal boxes are mounted to a metal connection that is earth grounded.)
2. Option 3 may be an improvement over 2 because any surges/transients that hit the cable will be limited to the CAN transceiver, since both sides are isolated.
3. If there was a 8kV strike on the cable, option one would potentially show that voltage on the digital ground for both boards. Unless it was fully clamped and protected, this would destroy pretty much everything on both boards. Option 2 would show the voltage on the IO board digital ground only. Option 3 should be fully protected. Note: The CAN transceivers should survive in option 3, since the relative voltage between tx/rx and isolated transceiver ground would stay low, since all conductor would (or should) show the same 8kV.
4. Whichever route I choose, I will still fully protect lines with TVS diodes.

Questions:

1. Are my assumptions correct for the most part?
2. If they are, then I will likely go with option 3. Should the chassis on both boards stay isolated from the digital ground?
3. If I go with option 3, should I use a safety cap to bridge the isolation gap? How to use isolation to improve ESD, EFT and surge immunity in industrial systems seems to think that I should.
4. Any other notes/thoughts that I am wrong or missing here?

Answer 1

Any other notes/thoughts that I am wrong or missing here?

Answer: All your design Specs.

Rule #1

1. All designs must have "SPECS" or a design specification before choosing an implementation.

2. All specs must be validated (DVT), documented, and revised/guaranteed for customers with process margins, including yours. This is part of a contract that you choose to warrant.

3. Choose from Industry standards before re-inventing the wheel, unless you know of specific exceptions. Then compare different experts' advice/ IC datasheets and follow the common guides to achieve this.

4. Under the general guideline of EMI, ESD is a subset with well-defined standards such as the human body model, HBM of the fingertip (100 pF), and the Cart model. There are also EM ingress standards for pulse, continuous (CW), and modulated carrier which may be coupled by L or C physical properties of all conductors and insulators in the network as well as a choice of isolation and ground (=0V). Crosstalk is a parameter of choice for suppression levels. Often these are symmetrical with some emission spectrums by a shared filter.

ref

added

Questions:

1. Are my assumptions correct for the most part?
2. If they are, then I will likely go with option 3. Should the chassis on both boards stay isolated from the digital ground?
3. If I go with option 3, should I use a safety cap to bridge the isolation gap? How to use isolation to improve ESD, EFT and surge immunity in industrial systems seems to think that I should.
4. Any other notes/thoughts that I am wrong or missing here?

Opinions:

1. yes
2. Optional C=1nF RF cap like CAT5 terminations. or PE bonded to suppress SMPS floating CM noise. Verify with tests.
3. if protection diodes are faster than the arc surge Yes, if not maybe. It depends on layout and surge current paths, crosstalk or ground shift and slew rate in kV/ns. Model using 10 nH/cm on wires.
4. See 1st answer.

If ESD is rated for an impulse <10 us @ 8kV and the device is rated for 4kV for the 60s and you want to improve choose a tougher spec like 8kV for 1s, then one must examine all sources of Breakdown voltage using a current limited low C tester, examine slew rate limiting filters to attenuate 8kV impulses with impedance matching and current limiting tradeoffs to clamp. however it is easier to limit current from crosstalk if floating but CM rejection of RF depends on cable quality for imbalance, kinks, and dB of isolation at RF spectrum from impulses is an additional task.

For example, a BIL-rated insulator for 200kV 1,10us impulse can easily break down at 50kV continuous AC or DC due to air contamination with %RH and surface creepage. This stress is unlikely for CANBUS environments but the ratios tell you about ionization delays and how greater impulses can be withstood. E.g. 8kV 100 pF ESD.