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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
    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.
  • 0
    Dave,

    do I remember a while back TT setups were either:-

    1/ 100mA delayed RCD feeding the whole board and a further 30mA RCD (or RCBOs) for certain circuits   or

    2/ 100mA non delayed for RCD for most circuits and 30mA RCD (or RCBOs) for certain circuits.


    I must admit that if relying on RCD(s) for EF protection I prefer to have 2 RCDs covering each circuit say in a TT install. If an RCD is purely for additional protection then it doesn`t bother me as much.

    I feel happier with 2 RCDs even is one is 100mA and time delayed. OK it will not be classified as personal protection but if it acts before you hold something rather than the fault occouring whilst you holding it and also if the fault is bad enough to trip a 100mA RCD then it gives another fighting chance against failure (often by "Stiction" especially if the two RCDs are in slightly different locations, therefore possibly different enviroments too)
  • 0
    Sparkymania, have a look at Table 3A on page 363 of 7671. The two requirements for TT systems are that disconnection times must be met and that the product of Ra and I delta n must be less than 50v. If you examine the latter and consider a 100mA RCD is in circuit thus requiring Ra to be nmt 500 ohms. Accepting a relatively low impedance for the DNO earth electrode and the rest of the circuit then the fault current would be circa 230v/500 ohms = 0.46A or 460mA. Refer to Table 3A and you will see that said RCD would be expected to operate nmt 150ms.

    Although this is not the purpose of the second requirement cited above, it does in reality serve to effect disconnection on time.
  • 0
    Thanks for your replies


    davezawadi:

    I wasn't a little confused about TT. Everything you said is what I had written but didn't go into the detail. 


    ebee:

    What you mentioned is what I was refering to. 


    I don't know if I had not explained it properly and you have misunderstood what I was getting at.

    I have reread what I had written but I can't see any other way of explaining it. I have to admit it took a while to write it out in the first place.

    Apologies if I haven't made the issue clear. It was the actual RCD and RCBO max trips times compared to the allowable trip times that's the issue.


    lyledunn:

    I think I see what your saying.

    I presume the 500 ohms for the 100mA RCD is taken from table 41.5 on page 64.

    I understand what you are saying as this is the same with 300 or 500mA RCDs and their max allowable Ra from that table doing the calculation you still get a current that will effect a x2 trip time at 150ms so within the 0.2 TT trip time.

    Regarding table 3A it is giving x1, x2 & x5 tripping times for RCDs. However, most testers do not allow x5 testing above 100mA. When I've tried it set to 300 it won't test.

    Most testers also don't test at x2 which is what would be needed to make sure that the RCD did trip within the time you have mentioned.


    It seems there is more to it than just the stated trip times mentioned in the OSG.

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





  • 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.
  • 0
    ajjewsbury


    Good point. Thanks.
  • 0
    Andy,

    I think that if you select 500mA to test a 100mA trip, for example, modern test instruments apply an initial test current of half the rated value of the setting chosen to ensure that touch voltages remain below50v. The message returned on Megger instruments is not a figure but “trp” if the device operates. Will check the “var” setting on my tester tomorrow.
  • 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.

  • 0
    Some years ago I spoke to RCD manufacturers and others about testing RCDs, the consensus of opinion is if the RCD passes all the tests you can do, you assume it is within specification so will pass any tests you cannot do.


    Andy Betteridge
  • 0

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



    On the half setting isn't the meter expecting no trip to be a pass (since RCDs shouldn't trip at 50% IΔn) - so any trip is a failure - so it's not doing quite the same as 50mA x1 test (should such a setting exist)

     

    Set at 100ma x 1 gave trp



    That seems odd - at 100mA even in a TT system with a half decent electrode (200 Ohms say) I wouldn't expect the voltage rise on PE to increase by more than 20V - far below the usual 50V limit. Are you sure it wasn't still on a 1/2 x setting?


       - Andy.

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