The way that the ELU works, is that the Live and Neutral wire go through an iron core. The core then has a secondary winding connected to a trigger mechanism. When the amps going through the Live is returned through the Neutral, the current difference is zero. When there is an Earth leakage on either the live and Neutral, then the current in the Live and the Neutral is not the same and does not cancel out, and the difference then if greater than 20mA for a 20mA ELU, triggers the mechanism, and it trips.
This only works if the Earth connection to Neutral is done before the ELU and has good mechanical connection. However if the Earth to Neutral connection is lost/broken, then the ELU will not detect a leakage to Earth, as there is none being the missing link from Neutral to Earth. This is the same scenario when connecting a Genset to the ELU with out a Neutral/Earth bridge, and the reason that the SANAS states that alternative supplies require their own Earth/Neutral bond, else the ELU does not pick up an Earth leakage fault..