X5 test on RCBOs

Discussion in 'Electricians' Talk' started by P.A.H, May 20, 2023.

  1. P.A.H

    P.A.H Member

    Hi all.

    I have done x5 RCD tests, plugging into a socket outlets when the CU only has MCB with RCD.
    However, If a consumer unit has all circuits on RCBOs, when testing tripping, 5x test, do you have to test each RCBO individually by connecting a croc to earth terminal and L/N probes on each individual RCBOs.? As clearly plugging in won’t work under this method.?

  2. Philip Hyde

    Philip Hyde Screwfix Select

    I test at a different point in the circuit like a ceiling rose or inside fuse spur.
  3. P.A.H

    P.A.H Member

    Thanks. Was never taught, domestic installer, that was a way you could also do RCD tests
  4. The Happy Builder

    The Happy Builder Screwfix Select

    You are doing unnecessary tests, wasting time and shortening the working life of the RCBOs, and the test results are possibly invalid anyway due to testing with the circuits connected to the RCBOs, not testing at the RCBO outgoing terminals and quite possibly the settings you have selected on your tester.

    You need to keep up to date with testing requirements and procedures.

  5. The Happy Builder

    The Happy Builder Screwfix Select

  6. The Happy Builder

    The Happy Builder Screwfix Select

    Sorry to say it, that is incorrect as well as the test results are not accurate due to capacitance in the circuit conductors.
  7. The Happy Builder

    The Happy Builder Screwfix Select

    Yes, but only test each RCBO once.
    Last edited: May 21, 2023
  8. MGW

    MGW Screwfix Select

    It seems we now do far less testing, if the RCD trips within the 40 mS with X1 test there is no point doing X5, and in real terms we should be doing the test with worst situation, so X1 with nothing connected and X½ with all connected, if we are not doing latter then should be measuring back ground leakage.

    However we don't test the MCB, so why test RCD? Well experience has shown me over the years that strain on the terminals can stop them working within prescribed limits, so one reason is to check that any strain is not going to cause that, so clearly testing must be done after all terminals are torqued up. To remove cables from the RCD/RCBO to test, rather defeats one reason to test.
  9. adgjl

    adgjl Screwfix Select

    If you test the rcd / rcbo with the outgoing wiring still connected and it passes, then fair enough. If not, then it needs to be disconnected to prove a device fault or a wiring fault. With a circuit with an rcbo on it, you are still going to need to disconnect its wiring for an insulation resistance test or you risk damaging the rcbo.
    The Happy Builder and Ind spark like this.
  10. The Happy Builder

    The Happy Builder Screwfix Select

    We don’t have to test at X5 at all, utter regardless of if it trips in 40 milliseconds or not on the X1 test.

    The testing current for a Type A RCD involves Pythagoras' theorem.

    Pythagoras' theorem states that for any right-angled triangle, the area of the square on the hypotenuse is equal to the sum of the areas of the squares on the other two sides. It can be thought of as ² + ² = ² where and are the shorter sides of the triangle, and is the hypotenuse (longest side).

    That gives you a multiplication factor of 1.414.

    So a X1 RCD test on a 30 mA Type A RCD requires 42 mA of pulsating DC and the X5 212 mA, but the RCD type approval allows a up to 350 mA, so it may not trip fast enough anyway.

    The IET have simplified the RCD testing because it’s now all too complicated and electricians cannot be expected to know the type approval specifications for all RCDs made by different manufacturers.

    All RCDs utter regardless of what type they are should trip in less than 300 milliseconds when tested as a Type AC at 30 mA, so that is the test required and if if passes the jobs a good one, no need for 180 degree tests or lots of faffing about with ramp tests.

    Some older and RCD sockets/FCU should trip faster, but that’s the only variation in test results to be expected.

    You do not need a new fancy tester, your old tester that only tests Type AC RCDs is perfectly adequate.
  11. Ind spark

    Ind spark Screwfix Select

    Yes, disconnect the outgoing cables and test at the rcbo.
  12. The Happy Builder

    The Happy Builder Screwfix Select

    Generally a fault that trips a MCB is fairly catastrophic and involves a big flash and a bang, resulting in the MCB being suddenly hit with a large fault current.

    Faults that trip RCDs are much more subtle and give them a gentle tickle with a few milli amps resulting in them being far more likely to stick as the mechanical parts bind.

    We don’t test fuses for a rather obvious reason :)
    Jimbo likes this.
  13. The Happy Builder

    The Happy Builder Screwfix Select

    From the IET article by Michael Peace

    The requirements for RCD testing in BS 7671:2018+A2:2022 are given in the notes to Regulation 643.7.1 for fault protection and Regulation 643.8 for additional protection. A note in each section states the requirements and the key points are highlighted below.

    Regardless of RCD type, e.g. AC, A, F or B, an alternating current test shall be used at the rated residual operating current (IΔn), with a maximum operating time not exceeding 300 ms for general non-delay type RCDs.

    For ‘S’ Type time-delayed RCDs, the operating time shall be between 130 ms (minimum) and 500 ms (maximum). S Type time-delayed RCDs are not applicable for additional protection, hence, the operating times are not included in Regulation 643.8.

    There is no longer a requirement to perform a test using a test current equal to or higher than five times the rated residual current.


    There is also a note on the bottom of the schedule of test results on page 531 in the latest edition of BS7671 with the brown cover:

    ** RCD effectiveness is verified using an alternating current test at rated residual operating current (IΔn)
  14. The Happy Builder

    The Happy Builder Screwfix Select

    If you have the latest IET Guidance Note Three with the brown cover refer to Table 2.17 on page 96.

    You only need to do the recommended tests in part (a) not the optional tests in part (b).

    So a half test is still recommended, but not actually required and the test result is not recorded on the Schedule of Test Results.

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