How is this ELCB connected to earth?
The Onesto KRC3 is a residual current circuit breaker and these types of devices operate by measuring a difference current flowing in live and neutral rather than specifically measuring a leakage current to ground.


  ONESTO KRC3 230/240V 25A 300mA KRC3 Residual Current Device Leakage
  protector Circuit breaker


The above description is taken from here

The difference between live and neutral currents IS earth current in effect: -



As you can see from the above, EARTH is included but isn't needed and some devices down provide a thru-going connection for earth at all. In north America they are called ground fault circuit interrupters: -