a data communications system and associated method of data transmission for transmitting data over a three phase power system between a surface and a sub - surface location for an arrangement such as an oil field electrical submersible pump , on each of the three conductors of a three phase cabled connection with an isolation mechanism operable to isolate any given conductor when a fault associated with that conductor is detected . with the system providing a separate ac signal and data transfer on each conductor isolation can be achieved while enabling continued operation .