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  1. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    It's not quite that simple unfortunately. For large installations it is common for a consultant to produce a design and the electrician will then install to that design. The design instructs the electrician to install Final Circuit Protected By 30mA Type B 16A RCBO on a TN-C-S System. It...
  2. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    BS7671 is not a regulation, it is a Technical Standard. There is a distinct difference. BS7671 clause 411.4.5 states for fault protection the following may be used (i) An overcurrent protective device (ii) An RCD. As an RCBO integrates and RCD and MCB, it would seem a bit unclear to the the...
  3. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    Thanks, I thought I would have to break out the calculator. I am going to go out on a limb here and state that 2.73 Ohm Zs detailed on Table 41.3 is only concerned with fault as described in clause 411.3.2.2 between the line conductor and an exposed-conductive-part or a protective conductor in...
  4. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    I note your comment and yet we still have not detailed the Max permissible Zs earth loop impedance for Final Circuit Protected By 30mA Type B 16A RCBO on TN-C-S System.
  5. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    Pick your favorite option from below: 1. I'm using VPN 2. I'm on holidays in Southern Ireland. 3. I'm manufacturing electric skids in Southern Ireland that are going to be installed in the UK. 4. All of the above
  6. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    Well said James, very insightful. Can people write that on test reports instead of recording Zs vales for pass/fail criteria ?
  7. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    I am not interested in playing the regulation game, maybe some other day :) Lets stick to BS7671 and ignore legalities as it is not relevant to my original post. Generally agree with what you have stated here. However I would disagree with your conclusion. I would say of two values, 2.73...
  8. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    Appreciate the input. Yes, fair catch , I should have stated 2.37 Ohms not 1.37 Ohms for table 41.3. I take your point in regards 80% as per APPENDIX 3. However I intentionally stated Zs in my original instead of Zs(m). BS7671 does not state that a circuit is non-compliant if the 80% equation...
  9. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    I believe the earth loop impedance Zs values for overcurrent characteristics of RCBOs detailed in Table 41.3 should be adhered to so that we do not have BS7671 non-compliance. I realize I'm asking the exact same question again but only because you did not answer the first time. Why are you...
  10. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    Yes, fair catch Tom, I should have stated 2.37 Ohms not 1.37 Ohms for table 41.3. You state mentioning TN-C-S is pointless. I find it surprising that you would make such a clearly false statement. There are going to be quite a few people reading that bemused. The earthing system is indeed...
  11. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    Agreed, Zs is earth loop impedance as stated in my original post. Point noted on Zs not concerning faults between live conductors. As per my original post, at best, Zs can only infer the impedance of line/neutral for overcurrent short circuit scenario, perhaps did not need to be mentioned in my...
  12. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    Agreed, Zs values in Table 41.3 enable disconnection times to be met as detailed on the top of the table. As per my original post it states 1.37 Ohms for Type B 16A RCBO. Agreed, RCD always backed up by MCB (or fuse). However I am keen for people to stay focused on the specific scenario in my...
  13. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    Non-compliance would be exceeding max permissible Zs value permitted by BS7671. I have detailed permissible Zs value in my original post for Final Circuit Protected By 30mA Type B 16A RCBO on TN-C-S System. As per my original post, I have read a few articles stating that under BS7671, max...
  14. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    In regards to my original post, it's not quite that simple. For the circuit described in my original post, the RCBO will typically disconnect in a shorter time duration for line/earth (fault current) due to the RCD integrated into the RCBO. As the RCD only requires a relatively small amount of...
  15. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    Yes, I am aware that an RCD does not provide overcurrent protection, as I have stated in my original post above an MCB/RCBO has to be provided for overcurrent protection. Agreed, a consultant should avoid producing a design that has very high impedances. My post concerns BS7671 Max...
  16. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    Agreed, it is very unlikely. Line/Neutral impedance needs to be much lower to ensure that the RCBO can provide effective overcurrent protection. Perhaps a suitable Line/Neutral impedance would be something similar to the Zs values detailed for overcurrent characteristics of RCBO in Table 41.3
  17. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    I am concerned with BS7671 non-compliance. In a scenario where we have an overcurrent short circuit between line/neutral, it is my understanding that the RCD integrated into the RCBO will not detect the short circuit and will not disconnect. The MCB integrated into the RCBO should detect the...
  18. Jim Specs

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

    TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO I have read a few articles stating that under BS7671, max permissible Zs earth loop impedance for above circuit is 1667 Ohms. 1667 ohm permissible Zs for above circuit protected by 30mA RCD would only seem to satisfy...

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