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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

    The only issue is testing the RCD at x2 to make sure it trips within 150ms.



    You don't necessarily have to test at exactly 2x. If you test at 1x and it disconnects within 200ms (which likely it often will in practice) then that's fine.


    Remember that the actual earth fault current during a fault will likely be much higher (Uo/Zs) - we can assume a L-PE fault of negligible impedance for ADS - given an electrode of no more than 200Ω it's likely to be well over an amp. So any test current up to that would be acceptable. If it doesn't trip within 200ms at 1x you'll need to try a higher setting. It doesn't have to be 2x or 5x setting - you can get the same effect using a 1x setting for a higher rated range - e.g. 1x 100mA (for 30mA RCDs) or 1x 300mA or 1x500mA (for 100mA devices).


       - Andy.
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  • 0

    The only issue is testing the RCD at x2 to make sure it trips within 150ms.



    You don't necessarily have to test at exactly 2x. If you test at 1x and it disconnects within 200ms (which likely it often will in practice) then that's fine.


    Remember that the actual earth fault current during a fault will likely be much higher (Uo/Zs) - we can assume a L-PE fault of negligible impedance for ADS - given an electrode of no more than 200Ω it's likely to be well over an amp. So any test current up to that would be acceptable. If it doesn't trip within 200ms at 1x you'll need to try a higher setting. It doesn't have to be 2x or 5x setting - you can get the same effect using a 1x setting for a higher rated range - e.g. 1x 100mA (for 30mA RCDs) or 1x 300mA or 1x500mA (for 100mA devices).


       - Andy.
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