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Max Zs for AFDD RCBO?

Hello, there is a debate on site as to which table should be referred to when populating the max. Zs column on test sheets. For an AFDD RCBO, should it be table 41.5 or 41.3 from BS7671?

  • Depends on which means the designer has chosen to provide ADS. If it's via the MCB element (typical on TN systems) then 41.3 (the RCD element is then just for additional protection), but if relying on the RCD element for ADS (typical in TT systems) then table 41.5.

       - Andy.

  • Thanks Andy, much appreciated.

  • It is acceptable to use RCD for ADS in TN systems, if the circuit is longer than that which would operate the overcurrent (mcb) part of the breaker, provided volt drop requirements are met - in which case Table 41.5 is again the required maximum. This is illustrated in Table 7.1(ii) in 8th Ed On-Site Guide (I think it was Table 7.1(i) in previous recent Editions).

    Of course, the maximum loop impedances in the Tables in Chapter 41 need to be adjusted for temperature when comparing a measured value (usually the values in Chapter 41 are multiplied by 0.8, see Appendix 3 in BS 7671:2018+A2:2022, page 410).

    Alternatively, already-adjusted tables of maximum measured earth fault loop impedance for overcurrent protective devices are included in Appendix B of the OSG, and  Appendix A of GN3.

  • Thanks gkenyon, much appreciated

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