first of all… decrease the impedance of your voltage meter by adding a 10k resistor
most likely the measured voltage will drop to a few cents then. parasitic voltages…
With an impedance of several mega-ohms which is common for multimeters, you will measure the emc caused by a passing mosquito…
if the voltage does not dissappear →
next thing: your system ‘itself’ should be grounded at one single point only:
Certainly there is the ground relay issue - this relay connects the chassis groud of the inverter to PE BUT only in case the AC connection gets opend. So no problem, too.
‘Problems’ meant by this are loops that ‘receive’ electro-magnetic fields. In electical design it is alway a goal to avoid loops like that.
Without doing a propper ‘single-point star-wise grounding’ you will always have the RCD tripping since filter currents are submitted the device_gnd and not get lead to ground.
