Question about Steffen FI adapter

RCDs measure the differential current. So if the current is discharged incorrectly somewhere, it trips and thus protects. If you have your own/autonomous power supply, you will hardly have a real earth and the current is not discharged anywhere, which means that in many cases the RCD will not notice anything. It would protect a little if there were continuously earthed cables and faulty equipment insulation somewhere. If, on the other hand, a single bare live cable (without contact to the earth conductor) is touched somewhere, nothing happens with or without a residual current circuit breaker. In this case, the RCD is of somewhat less use than usual. However, it is not a bad idea to use it, especially if it is also used in a stationary power supply (where not all installations have an RCD).

In fact, this is possible but you have to make a ground loop (this allows you to blow the circuit breaker without a "real" ground (it all depends on the power station/inverter) some power stations have a nut that allows you to make a ground by attaching a cable. If there is no cable, the device works in floating neutral. If you connect a class 2 device (double insulation) an earth connection is not necessary.