Can we consider that ethernet or RS-232 (RS-485) networks have no ground connection between the two ends of each cable?
Ethernet uses galvanic isolation with transformer coupling at both ends of the link.  RS-232 and RS-485 do not.  You have to use line coding in order to use AC coupling through either transformers or capacitors.  This is part of the Ethernet spec.  However, no low speed protocol supports this.  However, it should be possible to get a box that can provide isolation for RS-232 or RS-485.