I am hoping I have understood your question. If not, please come back and correct me. 

This is what I think is the situation.
The loop impedance is > 0.53 ohms, so (assuming it is a 230V system in your place ?) the maximum fault current would be something rather less than ~ 500A . A 63 A breaker will trip on this, but not always instantly. Depending in the actual fault current it it may take several seconds.
But the  400A MCCB has some sort of earth fault detection.

What is the tripping current of that earth fault detector ?( not the 400A but the earth current)

When it is tripped is it instant or is there a delay ? -  Sometimes earth fault relays have a delay so that faults on smaller circuits have time to operate their own trips without cutting out the whole installation.

If I understand it right, then yes  the MCCB will trip for earth faults above its tripping limit, that do not cause something else to trip first.
This may be an example of a good economic design, or it may be very unwise, that all depends on what else loses power when the big trip operates and how important it is that the rest of the installation has power.

Do come back with more info if you can.

Mike

I am hoping I have understood your question. If not, please come back and correct me. 

This is what I think is the situation.
The loop impedance is > 0.53 ohms, so (assuming it is a 230V system in your place ?) the maximum fault current would be something rather less than ~ 500A . A 63 A breaker will trip on this, but not always instantly. Depending in the actual fault current it it may take several seconds.
But the  400A MCCB has some sort of earth fault detection.

What is the tripping current of that earth fault detector ?( not the 400A but the earth current)

When it is tripped is it instant or is there a delay ? -  Sometimes earth fault relays have a delay so that faults on smaller circuits have time to operate their own trips without cutting out the whole installation.

If I understand it right, then yes  the MCCB will trip for earth faults above its tripping limit, that do not cause something else to trip first.
This may be an example of a good economic design, or it may be very unwise, that all depends on what else loses power when the big trip operates and how important it is that the rest of the installation has power.

Do come back with more info if you can.

Mike

Thanks for the quick reply, the system is running 220V (Hong Kong). And that 400A MCCB is currently equipped with a delay earth fault ( the tripping current should be around 40A (if i remember it correctly) , CT 400/5A, 0.5 setting , 0.1S trip second)

Your reply actually helps me a lot :) Thanks mate!