What non-electrical people need to understand is that a circuit breaker is selected to match the thickness of the wire used along with other factors. It's purpose is to protect that cable. Usually in the main DB the breaker is stronger for the supply to a sub-DB because the cable is thicker. The sum of the breakers is never a consideration. Many sub-DBs' have a supply rated at below 60A in the main board, but the main switch in the sub-DB is rated at 60A. This is because a 60A Double pole isolator is cheaper. The pupose of the main switch is to isolate the sub-installation. Each circuit fed from the sub-DB should be protected by a lower rated CB than the Sub-DB supply CB in the main board. This will localise tripping to the sub-DB.

The only device in the DB that protects the user is the ELCB. It is good practice to once a month switch off all electronic devices and press the test button on the ELCB. This will be an indication that it it still functioning. It is also wise to have it tested every two years to determine whether it still functions within the rating.