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ISO3088: ESD, EFT and surge protection of RS-485 isolated node with no protective earth connection

Roberto Panico
Prodigy 30 points
Part Number: ISO3088
Other Parts Discussed in Thread: STRIKE

Hello,

this question refers to ESD, EFT and surge protection in an RS-485 isolated node which cannot be connected to protective earth neither on the bus side nor on the other side. Both GND1 and GND2/GND3 are floating wrt PE. No possibility of connection of the PCB to PE. The node connects to the master through a three-wire cable (data+, data-, data common) which can extend up to 100m.

I use 3 TVS diodes: 1 between A2 and GND3, 1 between B2 and GND3 and one between A2 and B2.

My question is: is still worth using the TVS diodes on the bus side port? Or are they useless?

Thank you for the support.

Roberto

  • 0
    TI__Expert 6195 points
    Hi Roberto,

    Thanks for asking your question on E2E!

    On the right side of the device I do not see any issue with GND2/3 floating with respect to protected earth. The left side of the device is isolated so there is no issue with GND1 floating with respect to protected earth.

    TVS diodes on the bus side are still important for protecting the device in ESD and EFT events, but I think you only need 2/3. The ones between A2 and GND3 and B2 and GND3 I think are essential. These will sink charge to local ground. I think the TVS diode between A2 and B2 will not buy you much as the chances of a differential strike are low, and even if there was one the other two TVS diodes would be able to sink the charge to local ground.

    Let me know if you need me to clarify anything or you have additional questions!

    Best regards,
    Dan