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Thread: House DB main switch

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    House DB main switch

    Does the main switch on a single phase DB have to be single pole, single pole and neutral or double pole, I ask this as there seem to be some confusion amongst a lot of local electrical CoC providers in Cape Town.
    Apparently there is nothing that cannot happen today.

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    Sometimes people install a single pole 40 amp circuit breaker before the main switch, for example in a low cost house. But last I checked the main switch in a single phase installation must be a double pole isolator.

    3 phase main switch must break all 3 phase wires, but not the neutral. Then you get a 3 phase earth leakage which puts a spanner in the works and lets not even go into generator switch gear.

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    the main switch must be a single pole + neutral or a double pole circuit breaker or as in the old days a single pole next to a double pole isolator. presuming its the main switch of a house where the council breaker would be 1 step up. e.g. 60 amp up to 70 amp outside.
    if its a sub db it can have a double pole isolator as the main switch . the overload protection will be in main db. that way it protects the supply cable as well.that particular breaker in the main db can be a single pole.
    with a 3 phase main switch you can break the neutral as well if you want. in sans 10142 it actually says that all live conductors must be disconnected where neutral is also classed as a live conductor.
    however its not done normally and is accepted. i wouldnt know why.

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    Diamond Member AndyD's Avatar
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    Isolators must disconnect the neutral in most cases, it should also disconnect the live(s) before the neutral when switched off and reconnect the neutral fisrt when switched on. It also should be easily lockable. Many ELCB's don't truely fulfil the requirements of being an isolator.
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    Personally I would prefer that the Neutral be isolated as well.

    I remember a case many many years ago, in which a cable to a 3 phase training bench was wired incorrectly, they used one of these trailing cables which did not use standard colours, and the bench had a 3 pole isolator, the Neutral was incorrectly connected to a live line, one of the students was horsing around, and had make a connection to the training bench with a cable, and had got himself in a situation in which he was being shocked. The other students switch off the isolator, but because the Neutral was not switched off, it did not help. Fortunately they managed to disconnect the supply cable and were able to prevent the electrocution.

    This was an invaluable lesson to me, and I always insist that any 3 phase connection/installation done for me, must have either an isolator of ELCB that switches off Neutral. I just feel a bit more comfortable with this knowledge of complete isolation, in the case of a fault.
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    Thanks guys, we seem to have the same situation here, I just can't find the specific rule in the regs book.
    Apparently there is nothing that cannot happen today.

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    A circuit breaker combo (overload protection for live and green toggle for neutral) can be used as an isolator, in a single phase installation both live and neutral must be isolated, how you do it is up to you.

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    In a single phase installation, i see that some have a double pole isolator and then a 60 amp MCB.
    Some others have a 60amp SP/N as a main switch. There are some with 63amp.
    As long as you can isolate both poles L and N i think thats what the regulator wants

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    Keep looking, if I find the time i will get it. Main switches must isolate all Live as well as the Neutral. In a single phase installation that means double pole: SP+N, Isolator or ELCB can serve as main switch, whichever is used must be indicated on the COC as well. You may have a SP CB before the main switch often the case when the ELCB is the main switch. For a 3ph supply, surprising to almost all, you are supposed to use a 3ph 4pole isolator, 3pole+N or 3phELCB. It is in the regs but i am afraid I do not know exactly where. If I get a chance I will look for you. A sub DB may be fed from a SPCB but the DB must have a DP as main switch which is also specified in the regs.

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    6.9.2.2 In the case of a single-phase circuit, the disconnecting device
    shall disconnect live and neutral. In the case of a multiphase circuit, the
    disconnecting device shall disconnect all the phase conductors but need
    not disconnect the neutral conductor in an installation connected to a
    supply system in which the neutral conductor is earthed direct (see the
    TN system in annex M).

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