Elcb keeps tripping even with all MCBs turned off
Sorry i couldnt provide answers to the above comments (not enough points!), but finally got the issue fixed. Posting here in case anyone needs to know. 

So two things i found out from my friend who is an electrical maintenance man at factory near my house:
1) if the earth leakage is from a live wire, then the earth leakage circuit breaker will trip immediately upon reset - u can easily find the culprit by turning on your mcb one by one
2) if the earth leakage is from a neutral wire, then things can get messy, the breaker will not trip immediately, and turning on the MCBs one by one will not have any effect. In fact turn off all MCBs and the ELCB will still trip. 

The second one was the issue i had. An electrician came by and looked at all our neutrals and got it fixed in half a day's time