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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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    The wording of 411.4.204, "Where an RCD is used to satisfy the requirements .....Table 41.5 may be applied..." suggests that it should be a choice of the designer of the circuit. It doesn't say "If an RCD is present then table 41.5 should be applied". In which case, ideally the designer should indicate in the certification that this choice has been made ie max Zs permitted by BS7671=1667

    An inspector doing an EICR is surely not in the position to choose whether to use 41.5, because they are not the designer. They can take a view on whether it is safe since BS7671 allows a designer to use 41.5. 

    As to whether a code is appropriate, don't know, i don't do EICRs. Slight smile

  • 0 in reply to OlympusMons
    OlympusMons said:
    An inspector doing an EICR is surely not in the position to choose whether to use 41.5, because they are not the designer. They can take a view on whether it is safe since BS7671 allows a designer to use 41.5. 

    I think I agree with the statement "They can cake a view on whether it is safe".

    The inspector can only decide whether there is (or is not) now adequate means of protection against electric shock.

    This is very important, because the periodic verification is done against the current standard, not the one in place at the time the installation was originally designed.

    So, in this case, clearly Zs for ADS by OCPD is not met, but is the installation safe? Well, the inspector can apply the current version of BS 7671 to determine (and of course this permits the RCD element in an RCBO to provide ADS).

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  • 0 in reply to OlympusMons
    OlympusMons said:
    An inspector doing an EICR is surely not in the position to choose whether to use 41.5, because they are not the designer. They can take a view on whether it is safe since BS7671 allows a designer to use 41.5. 

    I think I agree with the statement "They can cake a view on whether it is safe".

    The inspector can only decide whether there is (or is not) now adequate means of protection against electric shock.

    This is very important, because the periodic verification is done against the current standard, not the one in place at the time the installation was originally designed.

    So, in this case, clearly Zs for ADS by OCPD is not met, but is the installation safe? Well, the inspector can apply the current version of BS 7671 to determine (and of course this permits the RCD element in an RCBO to provide ADS).

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