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MCB/RCD Fault Current Rating

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  1. #1
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    Default MCB/RCD Fault Current Rating

    I am about to buy some MCB/RCD combo units for my distribution board. Domestic house with remote meter box. Cable between them is 80 amp and there is an 80 amp HRC service fuse in the meter box.

    MCB/RCD combos seem to be fault current rated at either
    1. 30,000 amp
    2. 45,000 amp
    3. 60,000 amp
    Other than bigger is best: what rating do I need?


    I know that I need 30 milliamp tripping C curve units but there also seem to be a choice of either electronic tripping or magnetic tripping. Does anyone have any opinion on which is "better".

  2. #2
    1K Club Member jago's Avatar
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    Default Its all cricket too me...

    Stop stirring the Aussie sparkies up, talk about the Cricket or something pleasant like that.

  3. #3
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    Jago. I am not trying to stir up the sparkies..

    I understand that it is a legal responsibility to engage a competent qualified electrician to wire my house. I don't fully trust "paper qualifications" (have engaged some shonks in the past) so I try to educate myself so I might judge competency.
    No electrician who is competent will feel threatened by my questions. Only the incompetent paranoid need be afraid. I have been reading this forum for some time and some of the electricians who reply seem to be very competent and are very willing to help.

    By the way: nice renovation. Too much manual labour for me. Don't like cricket (10cc)
    Last edited by Johning; 6th Dec 2010 at 12:07 PM. Reason: spelling error

  4. #4
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    Assuming that the fuse is a BS88 type, it will limit the Fault Current to about 10kA.
    Assuming the Prospective Fault Current of the distribution transformer is not more than 15kA & also assuming that there is reasonably long distance between the fuse & your installation, all you would require is a 6kA device.

  5. #5
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    Default

    Not sure of the specific fuse type. It is ETSA sealed. Side of fuse holder states 80 A, 500 V and AS2005.
    Distance between the meter box and the distribution box in house is 20 Metres.

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  7. #7
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    Applied.
    Yes it is the same fuse.
    Thanks for the link to the RCD/MCB combo. Do you know how much this combo costs?
    I intend to specify 3 power circuits each with a RCD/MCB and 3 light circuits each with a RCD/MCB.
    Am I correct that only GPOs and lighting circuits need be protected with a RCD?

  8. #8
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    There verry new products but the rep told me they're about $35-50 trade I don't know what your price is going to be.
    As far as what needs and doesn't need rcd protection I would protect everything but i can't comment on your needs without being there.

  9. #9
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    With the BS1361 fuse, you could feel free to use circuits breakers etc that have a fault current capacity of no less than 2kA. The smaller the kA rating, the cheaper they are.

  10. #10
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    Default Error in original post (to much port)

    Should read:

    I am about to buy some MCB/RCD combo units for my distribution board. Domestic house with remote meter box. Cable between them is 80 amp and there is an 80 amp HRC service fuse in the meter box.

    MCB/RCD combos seem to be fault current rated at either
    1. 3.0 ka
    2. 4.5 ka
    3. 6.0 ka
    Other than bigger is best: what rating do I need?


    I know that I need 30 milliamp tripping C curve units but there also seem to be a choice of either electronic tripping or magnetic tripping. Does anyone have any opinion on which is "better".

  11. #11
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    In sa I am pretty sure our rules specify not less than 5700A and 10000 if your within spitting distance of a transformer.

    6000 is pretty much standard and the most common don't bother with anything smaller just get clipsal there all atleast 6ka

    heres an example of my all clipsal board

    http://www.facebook.com/album.php?ai...6&l=a77132c927

  12. #12
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    As Applied said, 6kA is a safe bet.

    Nonetheless, if BS1361 fuses are used, the Fault Current for an 80 amp fuse will not exceed about 2kA (2000 amps) providing that the Prospective Fault Current is less than about 20kA.

    Most domestic installations usually have a Prospective Fault Current of less than 15kA...unless you live amongst industrial/commercial zones.

    Go for the 6kA RCCB's. This will certainly cover your bum.

  13. #13
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    Quote Originally Posted by applied View Post
    heres an example of my all clipsal board

    December 7, 2010 | Facebook
    Am I missing something? Is that black wire on the right from the N/L bars to the bottom of the rightmost RCD carrying all the neutral current for all circuits?

  14. #14
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    Quote Originally Posted by RustyNail View Post
    Am I missing something? Is that black wire on the right from the N/L bars to the bottom of the rightmost RCD carrying all the neutral current for all circuits?
    looks like it
    what for all the RCD's!
    It's better to be dead and cool than alive and uncool

  15. #15
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    Why not use all rcds total cost was under $300

  16. #16
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    Default

    Quote Originally Posted by applied View Post
    Why not use all rcds total cost was under $300
    No argument about putting RCD/MCBs on each circuit. What I am concerned about is a bit of 4mm2 carrying the entire service load.

  17. #17
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    Quote Originally Posted by RustyNail View Post
    No argument about putting RCD/MCBs on each circuit. What I am concerned about is a bit of 4mm2 carrying the entire service load.

    yeah i would be concerned if that was the case.

  18. #18
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    Quote Originally Posted by applied View Post
    As far as what needs and doesn't need rcd protection I would protect everything but i can't comment on your needs without being there.
    Did I quote right?

    Does not AS 3000 specify what circuits are mandated to be protected with RCDs?
    Specifically in addition to 3 power circuits and 3 light circuits (RCD protected) I will have an electric hot water system (peak and off peak tariff) Electric oven and in the future aircon. I would prefer that these circuits are only protected with circuit breakers if it is within the rules.

  19. #19
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    If they are hard wired then it would be acceptable but there's no advantage to not protecting them but each to there own.

    I would not do it but that's me, 20% on a rcd/mbd is more than 20% on a $3 breaker.

    Hey how much of this work are you planning to do yourself these seem to be very basic but specific questions?

  20. #20
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    Quote Originally Posted by applied View Post

    I would not do it but that's me, 20% on a rcd/mbd is more than 20% on a $3 breaker.

    Hey how much of this work are you planning to do yourself these seem to be very basic but specific questions?
    I am not sure what you mean 20% on a rcd/mbd is more than 20% on a $3 breaker.

    I do not plan to do any of the installation. I am not an electrician. I would like to plan the installation and have it checked, and approved, by an electrician before installation.

  21. #21
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    I make money on what I sell the more the parts cost the more I make if one product is superior to another but costs a bit more I would use the more expensive part.

    I don't waist time with people who try to do jobs as cheaply as possible because they either don't pay anyway or when something unrelated stuffs up they call you back and expect you to fix it for nothing.

    That's what I mean.


    Goodluck. You must be doing some serious renos to need all this stuff.

  22. #22
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    Quote Originally Posted by applied View Post
    In sa I am pretty sure our rules specify not less than 5700A and 10000 if your within spitting distance of a transformer.

    6000 is pretty much standard and the most common don't bother with anything smaller just get clipsal there all atleast 6ka

    heres an example of my all clipsal board

    December 7, 2010 | Facebook
    How far can you spit . I am 190 metres from the distribution transformer and live in a purely residential area. I had a look you board and I noticed that you have installed a surge arrestor. Is this now standard or something you would recommend? Have you ever had to replace the arrestor?. Is this "board" mounted in a meter box? If so how often do you check if the arrestor needs replacing?

  23. #23
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    Quote Originally Posted by applied View Post
    I make money on what I sell the more the parts cost the more I make if one product is superior to another but costs a bit more I would use the more expensive part.

    I don't waist time with people who try to do jobs as cheaply as possible because they either don't pay anyway or when something unrelated stuffs up they call you back and expect you to fix it for nothing.

    That's what I mean.


    Goodluck. You must be doing some serious renos to need all this stuff.
    I understand that it is essential that tradesmen judge the integrity of their prospective customers if they are to stay in business. But customers also need to feel confident in the capability and integrity of any tradesman they employ. Educated customers will be able to discriminate between related and unrelated stuff ups.

    I am here to learn.

    Yes! serious renovation.
    Last edited by Johning; 9th Dec 2010 at 11:13 PM. Reason: spelling error

  24. #24
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    Well the transformer is in front of my neighbors house probably less than 50m but the supressor was just for the plasma and hifi at Christies we get reasonable lightning.


    I never check it there pretty rare unless you are a sparky with too much time on your hands definitely not essential for most SA metro

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