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

  • State Helpful Answer
  • Locked Locked
  • Replies 9 replies
  • Subscribers 33 subscribers
  • Views 1257 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

RCD testing for additional protection

Horace Horace

Hi all, im trying to get my head round this question if anyone can help. Amendment 2 no longer requires us to record  RCD x5 results; I understand this is due to manufacturers of type A RCD's utilising various test currents to achieve disconnection times, and that due to some meters not operating at these specific test currents many were returned as defective when they were okay, and it was the testing method at fault.

In the old 18th ed x5 tests operating within 0.04 s used to be recognised in an AC system (as well as other methods such as bonding ect) as providing additional protection. However, because we no longer need to verify this test, how can we be sure the RCD will offer this additional protection ?? The x1 test may fulfil the disconnection time of 0.4s for a TN system for example, but this test verif'ys fault protection only does it not ?. As an RCD is generally used for additional protection (in a domestic sense at least) then how can we be sure it offers additional protection without the x5 test ?

I realise the x5 tests on type A RCD's pose problems as mentioned above. Has it been decided then that if a 30mA RCD passes the x1 test then its more than likely to pass the x5 as well and so no need to verify ??  I cannot find a definite answer to this !

Parents
  • 0

    Ive another question!! or to put it another way its the same question but a different way of asking !

    In the 18th ed (2018) what were the principles that recognised tripping times of <40ms on a x5 test provided additional protection. How did they arrive at that ?

  • 0 in reply to Horace Horace

    It comes back to testing the metallic fault path versus human in the loop. It is of course impossible  to make a device that trips never at 29,99mA and yet trips at full speed at 30mA, so tests were derived in a way that mimicked the behaviour of a typical device of the time, where the imbalance in the sense windings drove the actuator, so lower fault current > slower operation, and somewhere below 30mA no operation at all.

    The modern way is to use some electronics to look at the voltage on the sense winding, and compare  it with a preset threshold. if it exceeds it at all, then a triac or SCR fires and the mains voltage L-N is used to operate the firing mechanism with a force that is independent of the fault magnitude, and has everything to do with the mains voltage. 

    So trip times in a modern device are essentially constant above threshold,* and less prone to jam due to corrosion or grease stiction.

    It does mean that this kind of newer device does not work properly if the neutral is interrupted  or the supply voltage is low on the supply side.

    Mike.

    * some very new and even fancier ones fake delay in inverse proportion to fault current, but it is not a mechanical thing but an electronic integration to achieve a similar end, and  reduce false alarm tripping

Reply
  • 0 in reply to Horace Horace

    It comes back to testing the metallic fault path versus human in the loop. It is of course impossible  to make a device that trips never at 29,99mA and yet trips at full speed at 30mA, so tests were derived in a way that mimicked the behaviour of a typical device of the time, where the imbalance in the sense windings drove the actuator, so lower fault current > slower operation, and somewhere below 30mA no operation at all.

    The modern way is to use some electronics to look at the voltage on the sense winding, and compare  it with a preset threshold. if it exceeds it at all, then a triac or SCR fires and the mains voltage L-N is used to operate the firing mechanism with a force that is independent of the fault magnitude, and has everything to do with the mains voltage. 

    So trip times in a modern device are essentially constant above threshold,* and less prone to jam due to corrosion or grease stiction.

    It does mean that this kind of newer device does not work properly if the neutral is interrupted  or the supply voltage is low on the supply side.

    Mike.

    * some very new and even fancier ones fake delay in inverse proportion to fault current, but it is not a mechanical thing but an electronic integration to achieve a similar end, and  reduce false alarm tripping

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