I would keep a sensible guard area around it though in case it did adopt a high voltage.
This could actually be the reason why some units work when first powered up, but then don’t work on the second and thereafter powerings up.
Maybe at the first power up, the floating copper area is not charged up to high voltage…but by the time the second power up comes, it is charged up to high voltage and interferes with the microcontrollers start up routine etc?
The other thing is it could be interfering with the eeprom in the micro. The micro does store stuff in eeprom throughout the operation.
Thanks, our board is 4 layer...we do have a ground plane directly under the micro, on the sub-top layer.
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Years ago I worked in a radar power supply place in London. One day the 14kV PSU output terminal was 1 metre away from a metal nut which was not quite earthed to a metal enclosure because the screw it was round was badly rusted and the nut was not contacting the metal enclosure, but was 1mm away from it. There was a loud snapping sound every second as the metal screw got induced up to high voltage and repeatedly flashed over to the earthed enclosure through the air.
There was no changing voltage there…just a high voltage terminal, and it surprised me how the nut that was 1 metre away could get induced up to such high voltage.
So I think we can assume that our floating bit of copper is going to be induced up to mains peak voltage.
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I am thinking we can do a test with some copper tape to ground the floating PCB copper....just to see what effect it has.