What is an earth link detector & how does it work ?

Answer 1

I believe you are referring to a Ground Fault Interrupter (GFI). This device is intended to protect people whereas most circuit breakers protect equipment. The circuit breaker will trip when the current exceeds some threshold value which will prevent further current from flowing into the circuit until reset manually. In the GFI, an imbalance of current in the hot and neutral indicates current flowing through the earth ground which causes the GFI to trip. A current flowing through the earth ground means that potentially high voltages may exist on exposed metal elements that eventually tie to ground so a shock hazard may exist. This is why the GFI trips and requires a manual reset...it prevents people from touching something that should be a ground that is actually at an elevated voltage.