The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 30 replies
  • Subscribers 31 subscribers
  • Views 17808 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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
    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.


    100 mA RCDs are not being installed in the quantity that they used to be as lighting circuits and cables concealed in walls have virtually eradicated the installation of new split load consumer units with a 100 mA RCD main switch and a 30 mA RCD for sockets on the split, with people opting to install consumer units with 30 mA RCBOs or with dual  30 mA RCDs instead, though the technicalities of doing so is open to discussion.


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

    17a55b727d9e8c65b1f509028daf6292-huge-d91aad8c-ee2f-4e7b-b916-d9b1298d4280.jpg

     

    Don't forget for time delayed RCDs there is a minimum trip time as well as a maximum trip time.


    Andy Betteridge
Reply
  • 0
    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.


    100 mA RCDs are not being installed in the quantity that they used to be as lighting circuits and cables concealed in walls have virtually eradicated the installation of new split load consumer units with a 100 mA RCD main switch and a 30 mA RCD for sockets on the split, with people opting to install consumer units with 30 mA RCBOs or with dual  30 mA RCDs instead, though the technicalities of doing so is open to discussion.


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

    17a55b727d9e8c65b1f509028daf6292-huge-d91aad8c-ee2f-4e7b-b916-d9b1298d4280.jpg

     

    Don't forget for time delayed RCDs there is a minimum trip time as well as a maximum trip time.


    Andy Betteridge
Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines