Seems like an LC network, i.e defined impedance that is connected between neutral and PE ?
then it triggers an RCD if the neutral deviates from PE.
Works only of course if the defect is after the bonding point, or if the PE comes from a separate earth rod...
Could it be that the LC is tuned to 50Hz, so that normal sine does not pass, but HF/harmonics of DC will trigger the RCD ?
I would have expected more like a 25V VDR or so.
In any case, it's probably not nice, because injecting currents into the RCD can make it inefficient against it's original purpose....