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We are developing a wearable device that is battery powered which is enclosed with a plastic enclosure. As a requirement for safety, we submitted the device for EMC testing, which includes Indirect ESD (Air Discharge of 15kV) and Direct ESD of 8kV. We managed to pass the Direct 8kV (Grade B) but failed for 15kV air discharge. The ESD strike and the general diagram of the device are shown below.

The general electrical diagram and ESD strike position for Air Discharge

I would some of your suggestions on if it is necessary to have a connection (directly or through a TVS diode) to the GND. If yes, which GND should it be?

There are also some suggestions from another post regarding the placement of an additional Unidirectional TVS diode from the enclosure to the GNDbattery and between the GNDbattery and GNDisolated. This is also included in the diagram (red box).

Things to note:

  1. We got grade C for the Air Discharge where the device powers OFF when the ESD strike happens and operates again when switched ON again, manually. We need a minimum grade B to pass this condition.
  2. The ESD strike is near and close to the push button switch, which is a membrane switch.
  3. Currently there is no connection between any GND to the plastic enclosure.
  4. We are trying to coat the plastic enclosure with permanent ESD Coating which will eventually turn the surface from a complete insulator to a surface resistance of ~10^8 Ω.
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  • \$\begingroup\$ Why do you have a primary and secondary side to begin with? How do you ground the "PCB & MCU" part of it? What are the green lines? \$\endgroup\$
    – Lundin
    May 15, 2023 at 10:32
  • \$\begingroup\$ Primary and secondary isolation is required for mains isolation as required by the IEC60601-1 standard. So we used an Isolated DC-DC converter to achieve that. The PCB & MCU is grounded to the ISOLATED part of the DC-DC Converter. The green lines are analog input lines that have exposed leads/electrodes. These lines are protected by a TVS diode array as shown in the diagram. \$\endgroup\$ May 15, 2023 at 10:44
  • \$\begingroup\$ Does "mains" isolation really include batteries? I don't know this standard, but it will be very hard to build a rugged system EMC-wise without connecting to the battery ground. \$\endgroup\$
    – Lundin
    May 15, 2023 at 10:52
  • \$\begingroup\$ Unfortunately yes, in the case of battery-powered devices, the "mains" is the battery power. So the isolation should be between battery ground and PCB&MCU ground. \$\endgroup\$ May 15, 2023 at 11:18

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