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Higher permitted zs on a rcbo protected circuit

Highspark

Hi all am looking for thoughts on a zs slightly higher than max permitted zs 

I was testing a large holiday let  with a 100 amp three phase supply  and a 3 phase distribution board with all rcbo c type  installation was tidy bar a few items needed  addresses and the socket circuits are poorly design 2.5 ring final circuit on c type rcbo and are wee bit over the max zs  just looking for thoughts and regulation on how to code this I would recommend change for b type  but there are 7 crabtree rcbo what are around  £30+ each and just looking for justification for them for client 

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    It's perfectly compliant BS 7671 wise to select a residual current device for ADS even on a TN system - so as long as Zs is below 1667Ω I don't see an issue.

    I might double check that the C type provides adequate (thermal) protection to the conductors (both live and c.p.c.) under fault conditions though.

       - Andy.

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  • 0

    It's perfectly compliant BS 7671 wise to select a residual current device for ADS even on a TN system - so as long as Zs is below 1667Ω I don't see an issue.

    I might double check that the C type provides adequate (thermal) protection to the conductors (both live and c.p.c.) under fault conditions though.

       - Andy.

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  • 0 in reply to AJJewsbury
    AJJewsbury said:
    It's perfectly compliant BS 7671 wise to select a residual current device for ADS even on a TN system - so as long as Zs is below 1667Ω I don't see an issue.

    And volt-drop is met.

    Of course, though, we'd be worried if Zs approached that value in a real installation.

    Happier, if (R1+R2) were as expected (for the assumed length of the circuit), then, ignoring parallel paths, quite roughly, Zs should be of the order of Ze+(R1+R2) give or take a little, and particularly if Ze is very low, some instrument error.

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