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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
    lyledunn


    Your absolutely right. I must have tested thousands of RCDs and I didn't know that. 

    I put your assertion to the test and this is what I found.

    Using a MFT1711 and my kitchen circuit's RCBO (30mA obviously)

    Initial normal auto test x 1 and x 5 to make sure it was operating normally 18.7ms max for both.  Ramp test 0deg 22mA. 180deg 24mA.

    Then:

    Set at 10ma x 5 giving 50mA tripped 18.6ms

    Set at 100ma x 1/2 giving 50mA displayed trp. Expected as it tripped at a half setting.


    Set at 100ma x 1 gave trp just as you said. I must have set this on 100mA when testing 30mA RCDs by accident before but have never noticed it said trp.


    Then I tested with a setting I completely forgot was there. The VAR. Had I remembered this I would not have said that most testers don't have a x 2 setting as you can just set this to 60mA. However, the following shows this does not work because, as you say, it tests at 30mA first preventing a direct 60mA test.

    Set at 55ma x 1 tripped with a time reading.

    Set at 60ma x 1(effectively a x2 setting) it gave trp. So you can't even do a x2 test this way. Again, as you say, seems to be testing at 1/2 with 30mA before testing at 60mA.

    However, it should have also tripped when set at 55mA x 1 as half of this is 27.5mA and the ramp test mentioned above shows this is above the threshold.

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  • 0
    lyledunn


    Your absolutely right. I must have tested thousands of RCDs and I didn't know that. 

    I put your assertion to the test and this is what I found.

    Using a MFT1711 and my kitchen circuit's RCBO (30mA obviously)

    Initial normal auto test x 1 and x 5 to make sure it was operating normally 18.7ms max for both.  Ramp test 0deg 22mA. 180deg 24mA.

    Then:

    Set at 10ma x 5 giving 50mA tripped 18.6ms

    Set at 100ma x 1/2 giving 50mA displayed trp. Expected as it tripped at a half setting.


    Set at 100ma x 1 gave trp just as you said. I must have set this on 100mA when testing 30mA RCDs by accident before but have never noticed it said trp.


    Then I tested with a setting I completely forgot was there. The VAR. Had I remembered this I would not have said that most testers don't have a x 2 setting as you can just set this to 60mA. However, the following shows this does not work because, as you say, it tests at 30mA first preventing a direct 60mA test.

    Set at 55ma x 1 tripped with a time reading.

    Set at 60ma x 1(effectively a x2 setting) it gave trp. So you can't even do a x2 test this way. Again, as you say, seems to be testing at 1/2 with 30mA before testing at 60mA.

    However, it should have also tripped when set at 55mA x 1 as half of this is 27.5mA and the ramp test mentioned above shows this is above the threshold.

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