A differential optical system for the remote identification of faults on a mixed electrical power transmission line delimited by a first electricity substation and a second electricity substation, wherein said mixed line comprises at least one section of overhead line and one section of cabled line, the differential optical system comprising: at least one first optical current sensor situated at a first point of said mixed line, comprising: a sensor fiber (41, 51) designed to surround an electrical power transmission cable whose current intensity is to be monitored, and one passive optical circuit (48, 58), at least one second current measurement device independent of said first optical current sensor, situated at a second point of said mixed line, configured to take measurements of the current intensity that circulates through an electrical power transmission cable, and a fault discrimination device (70, 80) configured to process the current measurements coming from said at least one first optical current sensor and from said at least one second current measurement device, and to detect faults on said mixed electrical power transmission line, the differential optical system being characterised in that said sensor fiber is made up of one section of sensitive optical fiber (451, 551) connected at one end to said passive optical circuit (48, 58) and one section of insensitive optical fiber connected at one end to said passive optical circuit (48, 58) and wherein the free end of said sensitive optical fiber (451, 551) is connected by means of a splice to the free end of said insensitive optical fiber, in such a way that said section of sensitive optical fiber (451, 551) acts as an element for measuring the current intensity to be monitored and said section of insensitive optical fiber acts as a means of transmission of the measurements taken towards said passive optical circuit (48, 58), and in that said device (70, 80) is connected to said optical current sensor by means of single-mode optical fiber, wherein said single-mode optical fiber is configured for the sending of an optical signal and the return of the measurements taken by the sensor fiber (41, 51) up to said fault discrimination device (70, 80). at least one first optical current sensor situated at a first point of said mixed line, comprising: a sensor fiber (41, 51) designed to surround an electrical power transmission cable whose current intensity is to be monitored, and one passive optical circuit (48, 58), at least one second current measurement device independent of said first optical current sensor, situated at a second point of said mixed line, configured to take measurements of the current intensity that circulates through an electrical power transmission cable, and a fault discrimination device (70, 80) configured to process the current measurements coming from said at least one first optical current sensor and from said at least one second current measurement device, and to detect faults on said mixed electrical power transmission line, the differential optical system being characterised in that said sensor fiber is made up of one section of sensitive optical fiber (451, 551) connected at one end to said passive optical circuit (48, 58) and one section of insensitive optical fiber connected at one end to said passive optical circuit (48, 58) and wherein the free end of said sensitive optical fiber (451, 551) is connected by means of a splice to the free end of said insensitive optical fiber, in such a way that said section of sensitive optical fiber (451, 551) acts as an element for measuring the current intensity to be monitored and said section of insensitive optical fiber acts as a means of transmission of the measurements taken towards said passive optical circuit (48, 58), and in that said device (70, 80) is connected to said optical current sensor by means of single-mode optical fiber, wherein said single-mode optical fiber is configured for the sending of an optical signal and the return of the measurements taken by the sensor fiber (41, 51) up to said fault discrimination device (70, 80).