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Application of BS7671 Table 54.7

McN41R

Can anyone offer experience on the application of Table 54.7 for Main Earth size on non PME supplies for supplies that have parallel incoming cables.

For example, 2 x 185mm2 PCV/SWA installed in separate ducts.

Thanks

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    Hello McN41R, I am a domestic installer and don't even know What a 185mm2 cable looks like, so have a pinch of salt handy.

    Your first example 

    McN41R said:
    2 x 185mm2 PCV/SWA installed in separate ducts

    Would surely need 2x "main" earthing cables, one in each duct. I know you said these examples are hypothetical, just pointing it out.

    Also, the way I read 543.1.1, if the choice of CSA of line conductor is determined by considerations of PSSC, then it must be calculated. Parallel conductor CSA is determined by "considerations" of PSSC, in that this is the most likely fault scenario since we are talking about a separate conductor for earthing. 

    Just my take on it

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  • 0

    Hello McN41R, I am a domestic installer and don't even know What a 185mm2 cable looks like, so have a pinch of salt handy.

    Your first example 

    McN41R said:
    2 x 185mm2 PCV/SWA installed in separate ducts

    Would surely need 2x "main" earthing cables, one in each duct. I know you said these examples are hypothetical, just pointing it out.

    Also, the way I read 543.1.1, if the choice of CSA of line conductor is determined by considerations of PSSC, then it must be calculated. Parallel conductor CSA is determined by "considerations" of PSSC, in that this is the most likely fault scenario since we are talking about a separate conductor for earthing. 

    Just my take on it

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