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Thread: Sub DB Rating

  1. #11
    Diamond Member adrianh's Avatar
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    Quote Originally Posted by hartdev@hotmail.com View Post
    1. I would say it is always best that the sub Db have its own earth leakage breaker for discrimination , to fault find etc but don't believe it is a law, just good practice, especially since some places have their entire installations on earth leakage protection.
    2. The Main CB is very seldom rated at the sum of all other circuit breakers as we take the estimated load factors into account , ie: Your geyser won't always be on when your stove is on etc you would struggle to find a residential 60amp installation with only a sum of 60amps circuit breakers installed.
    3. It should if done properly.....
    but I just took a look at your logo (diamond member) and I'm wasting my time telling you things you already know x)

    Thanks. I am a Diamond member because I have been a part of this group for many years. I am not an electrical contractor and I really appreciate your insight. And yes, I fully agree that everything should be done properly.
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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by adrianh View Post
    A couple of stupid questions:

    1. Should a sub DB have an earth leakage breaker?
    Depends on the circuits in the SubDB. If there's socket circuits or any other types of circuit that require earth leakage protection then yes, there should be an earth leakage breaker installed (or RCBO's used instead of MCB's).
    Quote Originally Posted by adrianh View Post
    2. Will that breaker be rated at the sum of the other breakers (provided of course that all wiring is appropriately rated)
    No, highly unlikely. We work on a system of 'diversity' where it's assumed (gambled) that every circuit would never simultaneously be under full load conditions. There are rules for working out diversity in the regs.
    Quote Originally Posted by adrianh View Post
    3. Will the tripping of the sub DB EL breaker save the main DB EL breaker from tripping?
    It's poor electrical design to have an earth leakage breaker that's fed from another earth leakage breaker unless one is differently rated to the other ie one is a faster type or they both have different mA ratings otherwise it's always a lottery which one trips first and often both will trip during a fault. It's also bad design to have the supply for a SubDB run from an earth leakage breaker in the main DB.
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  4. #13
    Diamond Member adrianh's Avatar
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    It's poor electrical design to have an earth leakage breaker that's fed from another earth leakage breaker unless one is differently rated to the other ie one is a faster type or they both have different mA ratings otherwise it's always a lottery which one trips first and often both will trip during a fault. It's also bad design to have the supply for a SubDB run from an earth leakage breaker in the main DB.

    - Does this mean that A sub DB board is wired in parallel to the main board rather than being fed off the main db board breaker - so that each board has its own separate supply, main breaker and earth leakage?
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    Diamond Member AndyD's Avatar
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    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.

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    Diamond Member adrianh's Avatar
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    Why are the two breakers in series with the same break current?

    Which one will break if a fault occurs in the garage at 60A?
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    Diamond Member AndyD's Avatar
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    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)
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    Gold Member Sparks's Avatar
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    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.

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