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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).


Parents
  • 0

    The outcome is that many electricians rarely, if ever, test time delayed RCDs so don’t concern themselves about the issues regarding testing them.

     



    The OPs last issue was about achieving 200mS disconnection with an RCD tester with an S type on a TT system. If there is a final circuit protected by the s type, the 200mS may need to be demonstrated.

    If you are testing a 100 mA S-type time delayed RCD you need to use the correct test with the countdown timer, if you start fiddling about testing at x 0.5, x 1 and x 5 you will end up with different test results which are not correct, I have been told that by both RCD manufacturers, tester manufacturers and other reputable sources.

     



    Not necessarily so. The countdown test for S type is simply to check there is discrimination with down stream devices. It is more of a gimmick, as you would quickly find if there is no discrimination if you tested a downstream RCD first. Overall, a lot more efficient than waiting for the countdown test to do its thing.


    The Variable test [VAR] on megger [maybe others also] is designed to work with the 5x  , simply as you will mostly fall foul of the notional "fail" [ = "trp" ] if you try this on 1x.
Reply
  • 0

    The outcome is that many electricians rarely, if ever, test time delayed RCDs so don’t concern themselves about the issues regarding testing them.

     



    The OPs last issue was about achieving 200mS disconnection with an RCD tester with an S type on a TT system. If there is a final circuit protected by the s type, the 200mS may need to be demonstrated.

    If you are testing a 100 mA S-type time delayed RCD you need to use the correct test with the countdown timer, if you start fiddling about testing at x 0.5, x 1 and x 5 you will end up with different test results which are not correct, I have been told that by both RCD manufacturers, tester manufacturers and other reputable sources.

     



    Not necessarily so. The countdown test for S type is simply to check there is discrimination with down stream devices. It is more of a gimmick, as you would quickly find if there is no discrimination if you tested a downstream RCD first. Overall, a lot more efficient than waiting for the countdown test to do its thing.


    The Variable test [VAR] on megger [maybe others also] is designed to work with the 5x  , simply as you will mostly fall foul of the notional "fail" [ = "trp" ] if you try this on 1x.
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