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Not testing RCDs at x1 is omitting an essential test

Sparkymania
Hi all


Following the last two weekends posts about RCD testing and trip times, in which I learnt a few things that I would never have known

as they are not documented in most tester manuals, a few more thoughts have come up.


On the hager site where they have "updated guidance on testing" their 30mA RCDs at 250mA they stated 2 things that were wrong.

This same mistake had been made in 2 videos as well.


They state that if you don't have a tester with a VAR that can be set to 50mA at x5 to give 250mA then you can use 300mA setting at

x1.

This is wrong. As I've found out over the last two weekends the tester does an unseen pretest before the main test. 300mA x1 will

pretest at about half 300mA and trip the RCD with the diplay showing "trp" and abort the test.



They also state "The x1 test is no longer a requirement but could of course be carried out".

I can't find anywhere that states it is no longer a requirement.


Regulation 643.8 requires that the instrument used complies with BS EN 61557-6.

There is a ‘Note’ to this regulation but Notes to Regulations only provide guidance and are not regulations.

The Note says: “Effectiveness is deemed to have been verified where an RCD meeting the requirements of Regulation 415.1.1 disconnects

within 40 ms when tested at a current equal to or higher than five times its rated residual operating current”.

Is this reg stating x 1 doesn't need to be done or is this being misinterpreted?


On the new test forms there is no longer a column for x1.

The other sparks I work with now only do x5 tests unless doing a MWC where it still has a x1 entry. However I still do all tests.

A 30mA RCD is supposed to trip when 30mA is detected. How are you going to know if it does that if you don't do a x1 test?

I tested one this week that passed x5 at 16.9ms but failed x1 with >300. When I ramp tested it it tripped at 75mA.

This proves that it needs to be tested at x1 as well, especially when used for additional protection as it must trip at 30mA when

going through the human body, not at the 75mA it was ramp tested at.


Also, as someone pointed out on another post, If someone mistakenly installed a 100mA (non-delayed) unit instead of a 30mA one -

chances are it would pass if only subjected to a 40ms/150mA test - yet it would hardly provide adequate additional protection.



As a side note and for the information to those who replied to my post about this pretest setting of half the selected current:

I don't think it's a half current pretest.

I have tried the VAR setting of 50mA x5 and it works. However, if it did pretest at half current then the 30mA RCD would trip at 25mA

as that is over the ramp test result of 22mA at 0 and 24mA at 180.

It even worked at 55mA without tripping and that would have been 27.5mA if it was half.

It did trip, though, set at 60mA, displaying "trp" so must have pretested at over 22/24mA.

Therefore I think this pretest current is somewhat less than half.

Too knackered after today's work to try to work out what the likly percentage of pretest current is but I bet some here will be able

to.


Any thoughts on this?



  • 0
    as I've discussed the x5 test is required because we don't have a x2 test and I'm not (personally) satisfied that passing the x1 test is OK, as I've had x5 with longer trip times than the x1 test.

    If some RCDs get slower with increasing current, how can testing give us confidence that the RCD will trip within 0.2s or whatever under actual ADS conditions, given a L-PE fault of negligible impedance will likely cause a residual current of several amps? Or indeed for additional protection when the actual shock current is very unlikely to be exactly 30mA or 150mA (or 250mA).


    Doesn't there come a point where we must admit that we can't just "test-in" quality - but we need to rely on correct RCD operation across the range having been "designed-in" and "built-in" and the testing need only provide some reassurance that the individual unit hasn't been significantly damaged.


    Nor should we let perfection become the enemy of the good - I think we need to admit that we can't practically test for all possible situations on site - so we shouldn't be even aiming to "prove" correct operation under all conditions - when all we can really achieve is to show that it's "reasonably likely" that the RCD will behave as required.


    We don't normally take micrometers to wires to double-check conductor sizes as as they should be, nor test MCBs or fuses on site, or disassemble isolators to check for 3mm contact clearances - are we really taking a proportionate position with RCDs? How many RCDs that appear to trip OK on the T button are really unsafe? (and I'm not thinking that this needs to be quite zero either). Also keep in mind that no testing can prove how the RCD will behave - it can only examine how it behaves at moment of the test - it may behave quite differently next year, or next month or even next day. We can't eliminate all risk - we should only be trying to limit it.


       - Andy.
  • 0
    Field 22 of the Generic Schedule of Test Results (p483 of BS 7671) form says "disconnection time", but that is used to record the measured disconnection time, because it comes under "Test Results".


    If it were the required disconnection time (from Chapter 41), that would be a column under "Circuit Details" - whilst the model form in Appendix 6 does not have that column, I guess there's no problem with someone adding it in their own form. The model forms in Appendix 6 are just that - they might not be suitable for every organisation, every installation, or indeed every type of test activity.


    A good example is the condition report inspection schedule (which is stated as only applying to domestic and similar premises with supplies up to 100 mA (and similar) - items under 6.0 relate to Section 701, but there are a number of Part 7's missing that are increasingly common in those premises, such as Sections 714, 715, 722, and there's nothing about SELV/PELV etc. except in Section 701 ...
  • 0
    Good point,  the certification I use prepared by NAPIT asks you to record the disconnection time so the person reading the certification knows what it should be.


    I thought the IET model forms used to ask for the disconnection time to be stated as well?


    Andy Betteridge




  • 0
    Sparkingchip:

    If you were required to record the X5 test result it would ask for it. Have a look at section 11 of the IET OSG.


    Yes, 11.5 - Additional protection - requires x5 test. QED.


    Agreed that where a BS 4293 RCD only provides ADS in TT systems, 11.2 x1 test may be acceptable, but if it provides additional protection, 11.5 applies also.


    Following only Section 11.3 would not necessarily verify ADS in TT systems (but would be OK for TN systems) - as I've discussed the x5 test is required because we don't have a x2 test and I'm not (personally) satisfied that passing the x1 test is OK, as I've had x5 with longer trip times than the x1 test.


    Section 11.4 is unlikely to apply, as regardless of the debate on whether BS 7288 is accepted in 18th Edition, for the following reasons:



    1. It's a socket-outlet, this RCD needs to provide ADS, and 11.5 applies.

    • If we use the latest standard BS 7288, the standard itself says they are only suitable for additional protection (and 11.5 applies).

    • If its an FCU-RCD connecting an appliance, the appliance manufacturer may require RCD for additional protection (and 11.5 applies).

    • Neither SRCD nor FCU-RCD can provide ADS for the circuit to the accessory, so it's more likely to be providing additional protection and 11.5 applies

  • 0
    Sparkingchip:

    The test results sheets ask for the rating of the RCD IΔn mA and the disconnection time in mS.


    That is what you need to record, it is the 5Δn disconnection time test result that you don’t need to record on the certificates.


    Andy B


    That can't be true if the RCD is used for ADS in most final circuits TT systems where the circuit requires a disconnection time of 0.2 s, and the RCD trip time exceeds 0.2 s - as it doesn't verify ADS. Even if the 1x test is below 0.2 s, it doesn't mean the 2x test or 5x test would pass.


    It's also not true where a 10 mA or 30 mA RCD provides Additional Protection, as the required test is "at least 5x".


    Therefore, more often than not, the 5x trip time value is required for verification as per BS 7671 requirements.


  • 0
    If you were required to record the X5 test result it would ask for it. Have a look at section 11 of the IET OSG.
  • 0
    400d75ebd20dc5b4b43e901199921994-original-4cc58ae8-aca3-4e0c-ad4f-4b81b707737e.jpg
  • 0
    The test results sheets ask for the rating of the RCD IΔn mA and the disconnection time in mS.


    That is what you need to record, it is the 5Δn disconnection time test result that you don’t need to record on the certificates.


    Andy B
  • 0
    Have a look at page 127 Guidance Note 3. Column 23 has been completed with figures that exceed those required for additional protection. It is clear from column 8 that the mcbs are providing the necessary automatic disconnection although circuit 10 would appear to rely on the RCD as Zs measured exceeds the permitted Zs stated in column 8. Small wonder sparks are bewildered at the RCD testing requirements!
  • 0
    Sparkingchip.


    I thought you were referring to the fact that they didn't just say x5 in the column on the schedule of test results.

    When I checked after your first reply I found that it just says "disconnection time (ms)".

    I thought that's what you meant.

    I checked both the minor works and the column in the schedule of test results.

    I don't understand when you say "Oh yes they do!"

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