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db/cu and 521.5.1 Ferromagnetic enclosures: electromagnetic effects

psychicwarrior

a good day wishes to all

 

‘they’ do not write these things for no reason; there is science present, so …

using this example: what's the non-compliance issue, if any in reality, with meter tails entering a db/cu through the same opening (fair enough), but a main earthing conductor being glanded/bolted to the housing inside (via a different aperture to the tails, to the earthbar attached to the case, along with bonding etc),  or even on the outside  ?   

 

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    I wonder if the intention behind 521.5.1 is only about thermal heating under normal conditions? The inclusion of the protective conductor in the requirement (which normally would only carry thermally insignificant currents)  might suggest they might have had something else in mind too.

    I've been wondering if separating the line conductor and c.p.c. with ferrous material might increase the reactance of the circuit when large currents are involved? (Those here with a better knowledge of the underlying theory might be able to enlighten us). If so that might have the potential to reduce fault currents/increase disconnection times beyond what would be estimated from R1+R2 tests or low current Zs tests.

        - Andy.

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    Zoomup: 
     

    What is the implication of all this then and with regard to the specific example(s) I used, that it is a non-compliance but ‘science’ says it doesn't really matter ?  or that it is compliant … !

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    I recon that the line and neutral conductors should enter by the same hole in a ferrous enclosure to reduce the chance of heating of the cables or enclosure, but tests have proved that the heating effect of a single conductor carrying a fairly large current has a very small heating effect. Mike points out that the metal is quite thin and the heating effect is quite small if we look at it as an A.C. magnetic field cutting a ferrous conductor as in a transformer effect. Normally the earthing conductor will not carry a heavy enough current to produce any dangerous damaging heat.

     

    Z.

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    Chris Pearson: 
     

    I don't think that there is a non-compliance.

    521.5.1: Where such conductors enter a ferrous enclosure …

    If the earthing conductor is bolted to a terminal which has been provided for that purpose on the outside of the enclosure, the cable does not enter the enclosure.

     

    if this all about balancing current entering leaving…

    if it [mec] goes inside to the earth bar (along with bonding etc) via a different knockout panel ?

    and what is the balancing issue, if there is a problem with the above internal earthbar connection, set against bolted/glanded to the case ?

     

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    I don't think that there is a non-compliance.

    521.5.1: Where such conductors enter a ferrous enclosure …

    If the earthing conductor is bolted to a terminal which has been provided for that purpose on the outside of the enclosure, the cable does not enter the enclosure.

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