The present invention relates to a monitoring system for detecting earth faults in an electrical power supply grid. The system may comprise a plurality of monitoring devices mounted in a number of monitoring locations, each of the monitoring devices may include a detector for detecting level of harmonics in the electrical power. The level of harmonics is preferably detected in a specific frequency range. The monitoring devices may further include a memory device for storing a harmonics reference value and may still further include a processor for comparing the level of harmonics with the reference level. The monitoring device may even still further include a communication device for transmitting an alarm provided the level of harmonics is above the reference level for a specific period of time.