ELCB Trips when connected
why it consume power when all the MCBs are down?


The short answer is: It can't. The only way power can be consumed is if you have a complete circuit.

Someone has miswired something if you are getting current flow with all the circuit breakers turned off. 

You need an experienced electrician to troubleshoot this problem. Apparently the one you called is not that good at it. 

Good luck!