A fault current detection device comprising: a housing configured to be mounted to a support structure of an overhead power line; a magnetic field sensor located in or on the housing and configured to be positioned in proximity to, but not in direct contact with, an electrical conductor of the overhead power line, the magnetic field sensor being configured to obtain magnetic field measurements; a processing component located in or on the housing and configured to analyse the measurements obtained by the magnetic field sensor and to apply a fault detection algorithm upon detecting a change in a magnetic field value which exceeds a predefined threshold, the fault detection algorithm including a duration test which is passed if the change occurred for more than a predefined period of time and a return to normal test which is passed if the magnetic field value substantially returns to a normal or prior level following the change, the processing component being configured to classify the change as a fault current only if the change passes both the duration test and the return to normal test; a communication module located in or on the housing and configured to transmit a fault alert to a remote master station in response to the processing component classifying the change as a fault current, wherein the master station is configured to be communicatively coupled to a plurality of fault current detection devices and having a processor configured to apply an alert ranking algorithm to determine which fault current detection device is closest to a fault in the event that a subset of the fault current detection devices each transmits a fault alert to the master station, wherein the alert ranking algorithm includes identifying or establishing locations of each of the subset of fault current detection devices, the subset of fault current detection devices defining a fault current path, identifying or establishing a location of an upstream source associated with the subset of fault current detection devices, wherein the upstream source is a closest upstream generating station or substation associated with the subset of fault current detection devices, the alert ranking algorithm further including ranking the subset of fault current detection devices according to their respective distances from the upstream source along the fault current path, and classifying the fault current detection device in the subset which is furthest downstream from the upstream source along the fault current path, being the fault current detection device with the highest or lowest ranking, as closest to the fault; and a power source for powering the magnetic field sensor, the processing component and the communication module. a housing configured to be mounted to a support structure of an overhead power line; a magnetic field sensor located in or on the housing and configured to be positioned in proximity to, but not in direct contact with, an electrical conductor of the overhead power line, the magnetic field sensor being configured to obtain magnetic field measurements; a processing component located in or on the housing and configured to analyse the measurements obtained by the magnetic field sensor and to apply a fault detection algorithm upon detecting a change in a magnetic field value which exceeds a predefined threshold, the fault detection algorithm including a duration test which is passed if the change occurred for more than a predefined period of time and a return to normal test which is passed if the magnetic field value substantially returns to a normal or prior level following the change, the processing component being configured to classify the change as a fault current only if the change passes both the duration test and the return to normal test; a communication module located in or on the housing and configured to transmit a fault alert to a remote master station in response to the processing component classifying the change as a fault current, wherein the master station is configured to be communicatively coupled to a plurality of fault current detection devices and having a processor configured to apply an alert ranking algorithm to determine which fault current detection device is closest to a fault in the event that a subset of the fault current detection devices each transmits a fault alert to the master station, wherein the alert ranking algorithm includes identifying or establishing locations of each of the subset of fault current detection devices, the subset of fault current detection devices defining a fault current path, identifying or establishing a location of an upstream source associated with the subset of fault current detection devices, wherein the upstream source is a closest upstream generating station or substation associated with the subset of fault current detection devices, the alert ranking algorithm further including ranking the subset of fault current detection devices according to their respective distances from the upstream source along the fault current path, and classifying the fault current detection device in the subset which is furthest downstream from the upstream source along the fault current path, being the fault current detection device with the highest or lowest ranking, as closest to the fault; and a power source for powering the magnetic field sensor, the processing component and the communication module.