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Application of BS7671 Table 54.7

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.


  • I have another current post running  about earth leakage protection on a PFS site. I note the consultant has specified that protective conductors other than MPB should comply with Table 54.7 throughout. The consultant is only responsible for the site supply arrangements and has specified a 120mm2 4-core swa with 70mm2 cpc to our unit. For a TT system I think that is overkill. Table 54.7 would certainly allow use of the wire armour for both cpc and MPB.

  • I'd agree - I suspect the consultant is being lazy, and copying from some previous document and in effect over specifying the CPC size require for safety, and quite considerably unless your electrode resistances and those of the substation are both so low that it is really TNS.


  • 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 

    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

  • I think I have provided a simple question that the IET finest should not have a problem with. I have nothing more to add.