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 37 replies
  • Subscribers 32 subscribers
  • Views 7563 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

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
    400d75ebd20dc5b4b43e901199921994-original-4cc58ae8-aca3-4e0c-ad4f-4b81b707737e.jpg
  • 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
    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
    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
    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
    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
    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
    Agreed.


    The situation here, though, is that really doing an RCD functionality test, we are really testing the RCD itself, not the installation. Even the OSG says the RCD is tested as close as possible to the device - what I'm not convinced about is the use of the "associated protective conductor", because I think that may well affect the test results


    It's not the protective conductor "close to the RCD" we are concerned about when we are talking about RCDs for ADS - in fact, to verify ADS using an RCD, you will need to do much more than simply "run an RCD test" as discussed in the OSG:. The requisite steps for verification of ADS are:


    (a) To verify the correct RCD has been selected for the relevant disconnection time from Chapter 41, in accordance with Table 3A

    (b) To verify that measured Zs - or measured Ze+(R1+R2) at all points of utilization in the circuit, ultimately highest at the furthest points:
    • Where relevant (Regulations 411.4.204 for 0.4 s disconnection in 230/400 V TN systems, or 411.5.3 for 0.2 s and 1 s disconnection times in 230/400 V TT systems, for non time-delayed RCDs), meets the requirements of Table 41.5

    • Otherwise, is less than U0 divided by the current shown in Table 3A relative to the disconnection time required by Chapter 41.


    (c) To verify the RCD is operating functionally using a test instrument according to BS EN 61557-6, and pushing the test button.


  • 0
    I would be quite suspicious of an RCD which had a greater x5 trip time than x1. This is not sensible from any point of view, and probably means that at say 5A the trip time would be longer still. For Additional Protection the trip time at the lowest possible current is important for the safety of persons, people rapidly get much worse outcomes as the shock current increases, or the exposure time increases. The RCDs I was discussing in another post with pictures have trip times at x1, x5 and x50 which are very similar, which I see as a good thing. Older designs may not be as good, but even so this change to the spec is in the wrong direction and we should object strongly to the IEC etc. There is no point in RCDs or RCBOs which provide inadequate protection, that simply gives a false sense of security to everyone. I wonder how many of you would deliberately expose yourself to one of these RCDs with curious characteristics as an experiment? If the reason for this spec change is so that manufacturers can say that inferior product now passes the spec, it is foolish in the extreme. When the device was manufactured it should have passed the spec. in position then, and should be tested as such, with faulty units replaced.
  • 0
    I agree that the x5 trip should in general not be less than the x1, but there are two things which I believe cause this to happen:
    1. Installation conditions or test conditions. As a simple example, using the protective conductor to sink the residual current, as per latest OSG and GN3) when the earth return path upstream is subject to voltages/currents from other sources / circuits - affects the test equipment - this sort of thing might happen more often in a caravan maintenance centre, or a mobile / portable buildings depot, although on industrial sites with shared transformer or a DNO TN-S supply, you might get the same thing.

      This is one problem with a test which is supposed only to check RCD operation against the RCD standard ... I just want to make it plain here that, in this case, appearing to fail the test "on the nail" of x5 test or x1 test does not mean the RCD will fail to trip in a real fault ... that's another debate.

       

    • Faulty RCD.



    In my opinion, the purpose of the RCD test is to check its functionality and not to verify ADS or additional protection, as it can't do that on its own - as my previous posts - which means that an "up/down" test is just as valid as a test to the 'associated protective conductor'

Community Rules & Guidelines