Current over ethernet?
Physically switching Ethernet is hard to do, for the following reasons: 


Ethernet is high frequency. In this area, you have to watch out for reflections of the electrical wave, which happens when the "characteristic impedance" of the transmission path changes. Ethernet cables have a char. imp. of 100 Ohms. You would have to design your switch so it has a characteristic impedance of 100 Ohm, but a specific (DC) impedance of 0 Ohm, like the cables. 
The whole point of structured cabling is to remove physical connections in the cabling between the endpoints for this very reason. You connect each unit to a switch with a separate cable, so there are no disruptions in the characteristic impedance. 
Ethernet is not meant to be physically switched. A network switch device does not switch the cabling, it does receive, manage and forward the packages. 


About voltages and currents: Ethernet voltages mostly use -1 to +1 Volts. Currents would be in the range of a few 10 milli-Ampere, depending on frequency and length.