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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. 

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  • 0

    RCD trip time is 20ms 

    Just to add that test results can't really be used for calculations - just because a device tripped in 20ms a week last Wednesday in warm dry weather doesn't mean it'll necessarily do the same under different conditions (cold, damp, the device 5 years older and not been exercised for ages) - all the test really tells you is that it's likely to be working within specification - so you still need to use the general data for calculations - i.e. 300ms at 30mA, 150ms at 60mA and 40ms at 150 or 250mA.

      - Andy.

  • 0 in reply to AJJewsbury

    Thank you so much

  • 0 in reply to Pabindu Gamage

    Seems like a bad design to rely on (RCD) Residual Current Device unless there is no other option,I have run parrellel cpc in the past, does it need to be C curve.

    Also you can't use 100% values of 1.09,you need 80% values at 0.87 unless your conductors are at 70C

  • 0 in reply to Newfutile
    Newfutile said:
    Seems like a bad design to rely on (RCD) Residual Current Device unless there is no other option,I have run parrellel cpc in the past, does it need to be C curve.

    Why? Since 2008 (introduction of 17th Ed), there has been no problem using RCD for automatic disconnection of supply, and Table 7.1(ii) of the OSG - which before 2018 used to be 7.1(i) - includes a "with RCD" max circuit length, as well as a "without RCD".

    Provided fault current protection is OK (adiabatic), and volt drop, there really is no problem as long as the RCD is correctly selected and is of a type that provides fault current protection (some voltage-dependent RCDs, with marking E2 or E3, whilst they do provide additional protection, might not provide protection for ADS when the voltage collapses during a fault of negligible impedance, this being one reason why the FE lead is present on the ones that do ... also, as well as their position in the circuit, a reason why in general SRCDs and FCURCDs are only suitable for additional protection, not ADS).

  • 0 in reply to gkenyon

    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.

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

    Most remedial repairs I do are for failed rcd, probably because no one has ever pressed the test button.

  • 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.

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  • 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.

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