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TN system - Max Zs Vs tripping time in the Presence of RCD protection

Pabindu Gamage

Hi, 
I am trying to confirm a point. 

as long as we meet the max disconnection time (let say 0.4s for a TN system) . for a small power circuit, 
Can we ignore the Max Zs requirement ? 

Example 
Small power  radial circuit ,20A RCBO 30mA , type A C curve, 
RCD trip time is 20ms 

but, max Zs is higher than it should be to Overload trip before 0.4s (lets say Max allowed  Zs is 1.09 but the measured is 1.40)

Can we override the 0.4s overload tripping requirement as its already being met with RCD protection of the RCBO ? i.e. as it trips at 20ms anyway so it meets the 0.4s requirement. 

Or is this a subjective matter? 

Thank you all in advance. 

  • 0 in reply to gkenyon

    Actually my post was two part, the first line related to the provision of supplementary bonding but the second part related to what used to be called the "alternative method" which permitted disconnection times for final circuits that would otherwise require disconnection in 0.4s to be extended to a time not exceeding 5s. The method did not require supplementary bonding but, of course, the cpc impedance was derived from  the final circuit protective device disconnection times plotted against the touch voltage curve. 

    It was a feature of the 16th (413-02-12) and  the maximum resistance of the final circuit cpc was set out in Table 41C. Fuses were prevalent in final circuits back then.

    I wasn't recommending the method to be deployed, just mentioning it out of interest and, to some extent, as an acknowledgement of how we have become slaves to look-up tables rather than understanding what we are trying to achieve. 

  • 0 in reply to Newfutile
    Newfutile said:
    I know BS7671 allows this, perhaps bad design was a poor choice of words.i would still argue that meeting disconnection times with the mcb is a better design,and have rcd for additional protection.

    It was certainly the way prior to 2008

    Newfutile said:
    I always assumed that BS7671 permitted rcd for ADS so that TT systems can be used.

    RCDs were used in TT systems prior to 2008, but in 17th and 18th Editions (BS 7671:2008, and BS 7676:2018 ... both with relevant amendments), Regulation 411.4.5 makes it clear that Regulation 411.4.4 can be satisfied by an RCD as well as an overcurrent protective device.

  • 0 in reply to lyledunn

    Thanks  .

    The reason for my response was that I do hear it commonly put forward that BS 7671 doesn't permit touch voltages exceeding 50 V AC ... and I wanted to make sure no-one thought protective bonding (main or supplementary) actually achieved that in all cases.

    I suppose it's correct to say that BS 7671, for dry conditions, doesn't permit touch voltages exceeding 50 V to exist indefinitely for the faults it considers in the general rules.

  • 0 in reply to gkenyon
    gkenyon said:
    As Lyle points out, supplementary protective bonding is not really useful for B curve and C curve circuit-breakers, but is useful for fuses and D curve circuit-breakers.

    My interest in electrickery post-dates 15th Edn, but at home there is supplementary bonding everywhere (including the kitchen sink), so was it the norm then to install it with MCBs?

    My concern is what to do with it when upgrading to RCD protection. Is it better to leave it in situ (which I have done) or remove it? If removal is an option, is there a risk from leaving some of it behind - there is so much of it that it would be difficult to be confident that it had all been removed?

  • 0 in reply to Chris Pearson
    Chris Pearson said:
    so was it the norm then to install it with MCBs

    It seems supplementary bonding in 15th Ed went well OTT. 

    Chris Pearson said:
    My concern is what to do with it when upgrading to RCD protection. Is it better to leave it in situ (which I have done) or remove it?

    It depends on what else has changed ... for example plastic pipes etc.

    In general, there is probably little harm inside the premises, it's influence outside especially PME (e.g. garden tap)

  • 0

    related to what used to be called the "alternative method" which permitted disconnection times for final circuits that would otherwise require disconnection in 0.4s to be extended to a time not exceeding 5s.

    Arguably that option is still there - tucked away under 419.3 - all the "alternative method" did, in today's language, was ensure that the c.p.c. itself met the requirement for a supplementary bonding conductor.

       - Andy. 

  • 0

    You can’t “ignore Zs,” but on a TN system you may rely on the RCD part of an RCBO for fault protection (ADS) instead of the MCB curve—if you verify to the RCD criteria rather than the MCB tables. In BS 7671 this is allowed (TN: Reg 411.4.5 / 411.4.204); use Table 41.5 (RCD method) instead of Tables 41.2–41.4 (MCB method). electrical.theiet.org+1

    What that means for your example (20 A Type C RCBO, 30 mA):

    • Your measured Zs = 1.40 Ω is higher than the MCB-only limit (e.g., 1.09 Ω), so it fails the MCB route.

    • But for an RCD, the criterion is essentially Zs ≤ 50 V / IΔn → for 30 mA, ≈ 1667 Ω (see Table 41.5). Your 1.40 Ω passes easily, so ADS is met—provided the RCD disconnects within 0.4 s for final circuits ≤ 32 A (which your 20 ms test already shows). electrical.theiet.orgProfessional Electrician

    Don’t forget:

    • Overcurrent (overload/short-circuit) protection must still be satisfied by the RCBO’s overcurrent part—RCDs don’t protect L-N faults. electrical.theiet.org

    Bottom line:
    Yes—you can comply via the RCD route on TN. Verify Table 41.5 (RCD) + 0.4 s disconnection, and keep the RCBO’s overcurrent protection in place. electrical.theiet.org+

  • 0 in reply to Al Muatasem Al Saadi
    Al Muatasem Al Saadi said:
    Bottom line:
    Yes—you can comply via the RCD route on TN. Verify Table 41.5 (RCD) + 0.4 s disconnection, and keep the RCBO’s overcurrent protection in place. electrical.theiet.org+

    And, don't forget, if you use a voltage-dependent RCD/RCBO with E2 or E3 marking, check with the manufacturer regarding suitability for ADS ("indirect contact").

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