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A monitoring system for detecting earth faults in an electrical power supply grid providing a power signal includes a plurality of monitoring devices, each of the monitoring devices including a detector for detecting the level of harmonics in the power signal, wherein the level of harmonics is detec

A monitoring system for detecting earth faults in an electrical power supply grid providing a power signal includes a plurality of monitoring devices, each of the monitoring devices including a detector for detecting the level of harmonics in the power signal, wherein the level of harmonics is detected in a specific frequency range. Each of the monitoring devices further includes a memory for storing a harmonics reference value, a processor for comparing the detected level of harmonics with the reference level, and a communication device for transmitting an alarm if the detected level of harmonics is above the reference level for a specific period of time. Each of the detectors includes an optical sensor for detecting the harmonics by use of the Faraday effect.

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The invention claimed is: 1. A monitoring system that detects earth faults in an electrical power supply grid, the system comprising a plurality of monitoring devices mounted in a plurality of monitoring locations, each of the monitoring devices comprising: A Faraday effect detector that detects a

The invention claimed is: 1. A monitoring system that detects earth faults in an electrical power supply grid, the system comprising a plurality of monitoring devices mounted in a plurality of monitoring locations, each of the monitoring devices comprising: A Faraday effect detector that detects a detected level of harmonics in a power signal in a cable in said grid, said detected level of harmonics being detected in a specific frequency range; a memory device configured for storing a harmonics reference value; a processor that compares said detected level of harmonics with said reference value; and a communication device responsive to said processor to transmit an alarm signal indicating the detection of an earth fault in response to said detected level of harmonics being above said reference value for a specific period of time. 2. The system according to claim 1, wherein said specific period of time is one to two cycles. 3. The system according to claim 1, wherein said processor calculates at least one of absolute content of harmonics and relative increase in harmonics. 4. The system according to claim 1, wherein each of said monitoring devices further includes software that causes said processor to adapt said reference value to variations in harmonics caused by varying load conditions of said electrical power supply grid. 5. The system according to claim 1, wherein a multitude of monitoring devices are located at each of said plurality of monitoring locations for monitoring a multitude of phases in a multi-phase electrical power supply system. 6. The system according to claim 1, wherein said system further comprises a server receiving said alarm signal, said server including information relating to said monitoring locations, said server determining the direction of a fault from a monitoring location based on said alarm signal and said information relating to said monitoring locations. 7. The system according to claim 1, wherein said monitoring locations are transformer stations, said transformer stations including electrical power input cables electrically connected to a transformer for transforming electrical power in said electrical power input cables from a first voltage level to a second voltage level being lower than said first voltage level. 8. The system according to claim 1, wherein said monitoring locations are branch points where a line of said electrical power supply grid branches off of said grid. 9. A method for detecting earth faults in an electrical power supply grid, said method comprising: detecting, using a Faraday effect detector, a detected harmonics level in a power signal in an electrical power supply cable in said grid; providing a reference harmonics value; comparing said detected harmonics level to said reference harmonics value; detecting an earth fault in said grid if said detected harmonics level is higher than said reference harmonics value for a specific period of time; and using a communication device to transmit an alarm signal if an earth fault is detected. 10. The method according to claim 9, wherein said detector detects said detected harmonics level over a first period of time and a second period of time, said first period of time being shorter than said second period of time. 11. The method according to claim 10, wherein said first period of time is less than 1 second and said second period of time is greater than 1 minute. 12. The method according to claim 10, wherein said alarm signal is transmitted if a ratio of said detected harmonics level in said first period to said detected harmonics level in said second period is above said reference value. 13. The method according to claim 9, wherein said detecting and comparing steps are performed a specific number of times in a specified period.