ELCB is the old name and often refers to voltage operated 
devices that are no longer available and it is advised you 
replace them if you find one. 

RCCB or RCD is the new name that specifies current operated 
(hence the new name to distinguish from voltage operated).

The new RCCB is best because it will detect any earth 
fault. The voltage type only detects earth faults that flow 
back through the main earth wire so this is why they 
stopped being used.

The easy way to tell an old voltage operated trip is to 
look for the main earth wire connected through it. A RCCB 
will only have the line and neutral connections.

Elcb is working based on Earth leakage current. But Rccb is 
not having sensing or connectivity of Earth, because 
fundamentally Phase current is equal to the neutral current 
in single phase. That’s why RCCB can trip when the both 
currents are deferent and it withstand up to both the 
currents are same. Both the neutral and phase currents are 
different that means current is flowing through the Earth. 
Finally both are working for same, but the thing is 
connectivity is difference.