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TT systems and RCD trip times

Sparkymania
Hi all


Just reading through the disconnection times in the regs and noticed something I've never thought of before.


On table 41.1 there is a note under the table that states that the disconnection times of a TT system can be the same as a TN system if disconnection is achieved by an OCPD and bonding is in place. However, I've not come across a TT system with a Ze low enough where only an OCPD can be used. The lowest Ze I've seen was 20 ohms.


Therefore:

On a TT system the disconnection times are 1 second and 0.2 of a second. Reg 411.3.2.2 & 411.3.2.4.

You would normally use an RCD for fault protection due to earth rod resistance causing higher than max Zs on final circuits as well as additional protection. Reg 411.5.3.


However, according to the OSG 11.2 and 11.3 the max allowable time to trip for a 61008 and 61009 can be up to 300ms (or, as stated, less then 300ms so 299.99ms).

If you are using RCBOs on circuits requiring 30mA protection and a selective type on the incoming it is 150 to 500ms on circuits without the RCBOs, so even worse.


This will mean it may not comply with the disconnection times. If you test the RCD and it trips within the 200ms it will comply but the fact that the BS standards allow for a longer time means that technically it does not comply.


(On a side note it seems strange that the older BS4293 RCD specifications do have a max trip time of less that 200ms, although it would still be the same issue with the selective type).


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  • 0
    I think you may have become a little confused about TT. It is exceptionally unlikely that a TT installation will have a Ze low enough not to have RCD protection. The RCDs for all circuits which need additional protection as well as Earth fault protection need 30mA RCDs or RCBOs. This is circuits with sockets, lighting circuits, circuits with buried cables, etc. Exposed (in that the route in accessible areas can be seen) still need Earth fault protection but not additional protection. The original TT designs has an overall 30mA RCD in domestic premises, but this has changed in some cases to a delayed type overall RCD, and RCBOs where required for additional protection. The trip current of this overall device could be higher than 30mA as it is only for Earth fault protection due to excess Zs. However it is as well to remember that the potential reached during a fault could be nearly the supply voltage to real Earth, and sufficient bonding should be in place to eliminate any possible danger while waiting for disconnection. Because of this higher risk the disconnection times are reduced to half that of TN systems. the numbers are in the BYB, but basically 40 mS at  5 delta In for additional protection. Earth fault protection time is 1 second whatever.

    Refer to section 411 onwards, but I must admit that it is not the clearest part of the regs and has changed slightly in the amendment.


    I hope that makes the situation clearer.
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  • 0
    I think you may have become a little confused about TT. It is exceptionally unlikely that a TT installation will have a Ze low enough not to have RCD protection. The RCDs for all circuits which need additional protection as well as Earth fault protection need 30mA RCDs or RCBOs. This is circuits with sockets, lighting circuits, circuits with buried cables, etc. Exposed (in that the route in accessible areas can be seen) still need Earth fault protection but not additional protection. The original TT designs has an overall 30mA RCD in domestic premises, but this has changed in some cases to a delayed type overall RCD, and RCBOs where required for additional protection. The trip current of this overall device could be higher than 30mA as it is only for Earth fault protection due to excess Zs. However it is as well to remember that the potential reached during a fault could be nearly the supply voltage to real Earth, and sufficient bonding should be in place to eliminate any possible danger while waiting for disconnection. Because of this higher risk the disconnection times are reduced to half that of TN systems. the numbers are in the BYB, but basically 40 mS at  5 delta In for additional protection. Earth fault protection time is 1 second whatever.

    Refer to section 411 onwards, but I must admit that it is not the clearest part of the regs and has changed slightly in the amendment.


    I hope that makes the situation clearer.
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