Ethernet over servo motor control lines

Hello All.
I have two distant locations connected with 3-wire cable - "+", "-"
and "common". Voltage 24V DC can occasionally appear between "+" or
"-" and "common". I have to install Ethernet communication through
this cable. Modification of  special Netgear  "Ethernet over Power
line" switch solves the problem but I have to disassemble the switch,
cut and reconnect some wires.
 It's too difficult to rebuilt about 200 Netgear switches. Maybe
somebody knows the device, which does it 'as is'?
Thanks.