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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).


  • 0
    Hence doing a earth loop test before a RCD test so you are aware of possible touch voltage issues.


    Andy Betteridge
  • 0
    Oh yes. Of course. Forgot about that.
  • 0

    I don't see any issue with half an amp through a domestic earthing system.



    On a TT system with say a 200Ω electrode half an amp will raise the earthing system to about 100V about true earth ... and if neither the RCD nor tester disconnects promptly things might get hazardous.


       - Andy.
  • 0

    Sparkingchip:

    The other issue is that when I did the testing course and we were told that we only test 30 mA RCDs at x 5, because generally shoving half an amp through a domestic earthing system and electrode to test a 100 mA RCD is getting risky.


    Andy B



    The people running the courses are just people like you and me. They quite often get things wrong as well. I don't see any issue with half an amp through a domestic earthing system.

    When I was originally at collage all those years ago the lecturer said that it was bad practice to take a neutral to a switch. The reason? Because you could get a short circuit from phase to neutral. When I pointed out that you were more likely to get a fault to the earthed back box than to neutral and what about all the equipment that has to have neutrals in he realised what a stupid reason for not taking neutrals to switches that was. Nowadays, it's the done thing.

    Therefore, I would take some things they say with a pinch of salt.
     

  • 0

    Sparkymania:




    Sparkingchip:

    And testing Hager Type A 30 mA RCDs has not been mentioned. ?




     

    What's the issue with these?

     




    it is a non issue. They have opted for their product testing standard to be at 250 mA .  If you have a VAR facility, no problem, if not there is the alternative of test at 300mA. 

    This has gone a bit off topic wrt OP (TT ADS) and is delving into Additional Protection.


    The Hager Doc posted by Sparking covers all this and how to use VAR....( 5x50 =250) .Sparking...have you not read the doc you posted?

  • 0
    Reading that guidance from Hager it seems strange that they are saying that if the 30 x 5 test fails to give a result of 40ms then you can test at 300. That's effectively a x 10 test.

    Mind you, they are saying that according to the 61008 specs it is not 150mA but 250mA that it should be tested at.

    If that's the case the OSG, BBB, GN3 and everything else should be updated. So should the testers.


    Setting the VAR is not immediately obvious. After selecting VAR you have to use the double arrow key on the left of the display. An up/down arrow will appear in the right of the display. Then use the right 2 buttons with the small arrows to change the value to 50. When done press the double arrow key again and the up/down arrow on the display will disappear and the VAR will be set. Then just set the x 5 and off you go.
  • 0
    Hager say if their 30 mA Type A RCDs don’t trip fast enough, less than 40 mS at 150 mA (x 5) whack 250 mA through them, as they will still be with specification.


    Can you tell me how to set my Megger to test 50 mA x 5 to get a 250 mA test current as per Hager’s instructions?

    https://hager.com/uk/support/regulations-18th-edition/updated-rccb-testing


    Andy Betteridge
  • 0

    Sparkingchip:

    And testing Hager Type A 30 mA RCDs has not been mentioned. ?




     

    What's the issue with these?
  • 0

    500 mA should be more than enough to fire a 300mA RCD in < =200mS.  If not, then there will be something else wrong.  The "2 times" is arbitrary. The name of the game is that the RCD [ s type ] is quick enough.



     



    Yes. Good Point. I was being to rigid in my thinking.
  • 0

    Sparkingchip:

    The other issue is that when I did the testing course and we were told that we only test 30 mA RCDs at x 5, because generally shoving half an amp through a domestic earthing system and electrode to test a 100 mA RCD is getting risky.


    Andy B




     

    And even more so down on a farm or the like.


     Andy B.

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