Sub DB Rating

[AndyD]

This was just a drawing I had kicking around so I edited it for illustration.

The garage circuit breaker would be sized according to the size of cable feeding the garage and several other factors including the installation method used for the cable and the length of the cable etc.

It could be okay to use the same size breaker for the garage as the main breaker if the cable size was sufficient but it would be good practice to introduce 'discrimination' by making the garage breaker a faster curve than the main breaker ie the garage breaker might be a 'B curve' and the main breaker could be a 'C curve'.
0600DB0105.pdf (first page for brief explanation)

[Sparks]

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.

[Foot]

The supply for the Sub DB would be via the main breaker in the main DB and a second breaker in the main DB for the subDB cable.

[ATTACH=CONFIG]6875[/ATTACH]

If the Sub-Db is fed from the main breaker Pre-Earth Leakage in the main DB, does this mean that the cable from the main DB to the garage sub-DB is not protected by an earth leakage device and that only the wiring and devices after the Sub-DB's earth leakage device are protected by the EL? Is the cable from the main DB to the Sub-DB only protected from over-current by the MCB and not protected against earth leakage?
Could one, for example, have the EL for the sub-DB situated in the main DB enclosure as shown in this diagram?

[Derlyn]

One could have the sub db fed from the earth leakage in the main db as in the diagram, however, I never do it that way. I always ensure that the sub db has it's own earth leakage. That way, if there's an earth fault on the sub db, it doesn't affect the main db.