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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
    You could tie yourself in knots.


    If you have a cooker circuit with a socket in the switch you could be recording a different RCD test result than if the socket wasn’t there.


    I don’t need to worry about what to write down as all the certification I use has more than enough boxes to record it all.


    Andy Betteridge
  • 0
    Today I replaced a Schneider RCBO that x5 tested tripped at 28.8 and x1 tested >300 (no trip). Ramp tested at 75mA

    Tested new one and x1 was 28.8 and x5 was 28.9. I have quite often found that on some makes the x5 can be longer that the x1. Not on all from the same manufacturer, but on some. Then they can also be the same at, for example, 28.8 at both x1 and x5.

    However, I tested this at VAR set to 47mA x5 giving 235mA and it tripped at 18.9 so it did trip faster at a higher test current, just not at 150mA.


    As a side note, when I tested at 48mA x5 giving 240mA it displayed "trp" so I was wrong about the pretest being somewhat less than half as the ramp test gave 24mA so it does pretest at exactly half the selected current.


    This test had no external influence. When I install an RCBO I always test it before connecting the conductors and when doing periodic I do an IR test on the circuit then RCD test at the RCBO before reconnecting.


    Section 11 of the OSG only talks about the tests. It does not say anything about recording the results.

    However it does indicate that all RCDs should be tested at x1 and "if" it is being used for additional protection should be tested at x5.

    So the only issue is what to record in the single box, x1, x5 or highest of the two. 

    Remember, this is also a reference for the next test to compare performance to check for deterioration. What did the last person record?

    There should be clarification on this so we all know where we stand.

  • 0
    By the way. I've never heard of an up/down test. I did a search for it but nothing came up.

    What is it?
  • 0
    A traditional current balance RCD with no electronic amplification should get quicker with increasing fault current, at least up to the point of magnetic saturation, as the energy to release the catch comes from the imbalance current itself.

    The more modern kind where the coil is fired by an SCR ( a type of semiconductor switch) connecting the trip coil accross L-N, the pull force has more to do with the mains voltage at the time the 'fire' command came through, rather than the fault level, once the threshold is passed, the SCR is turned on, and that is pretty much as fast as it gets.

    Within that there will be some uncertainty as to when the tester button was  pressed in relation to the AC cycle timing, so the fact that 2 readings are a few milliseconds apart should not be a concern - that may happen between a set of readings at the same test current too, and variations between meters made to different  designs. However, more than half a cycle period (10ms) of timing drift, and I'd be getting a bit wary.


    By the way these designs often draw quite a few amps for the firing circuit giving it a good 'clonk', so loss of sensitivity from 'stiction' should be less of an issue, but of course this type can not fire if the supply side neutral connection is missing (or the supply voltage is too droopy for some reason, which is possible in some fault conditions), and the rectifier arrangements in some designs mean that a highly distorted waveform with say the negative half cycles suppressed, may cause the firing circuit some problems - not the detection, there will still be an AC component in the pick up coil, but generating the DC that is used to power the triggering electronics. Refinements include some circuits with an earth tail that can take power L-E if L-N is missing, but these tend to trigger upstream RCDs, which  may not be a good thing, and they are going out of favour.

  • 0
    Sparkymania:

    By the way. I've never heard of an up/down test. I did a search for it but nothing came up.

    What is it?


    Illustration of methods of testing RCDs without the associated protective conductor, if the test set supports this.

  • 0
    Sparkymania:


    This test had no external influence. When I install an RCBO I always test it before connecting the conductors and when doing periodic I do an IR test on the circuit then RCD test at the RCBO before reconnecting.

    I'm not sure we can say that for definite ... for example, in TN-C-S systems, current from other installations acts in the return path back to the transformer, and in TN-S systems, protective conductor currents from other installations travels back through the protective conductor of the supply also. TT systems may pick up ground currents in the earth return



    Section 11 of the OSG only talks about the tests. It does not say anything about recording the results.

    However it does indicate that all RCDs should be tested at x1 and "if" it is being used for additional protection should be tested at x5.

    So the only issue is what to record in the single box, x1, x5 or highest of the two. 

    Remember, this is also a reference for the next test to compare performance to check for deterioration. What did the last person record?

    There should be clarification on this so we all know where we stand.

     


    I agree - because whilst I'd support what OSG says about x1 test for TT systems, do you "pass" a x1 test at 201 ms in a TT system for ADS without doing a x5 test to check it's within spec?


    I also agree it's confusing about which time to write down for the TT system ADS test above, and also where the RCD provides both ADS and additional protection - which takes precedence?


  • 0
    Never thought of doing an RCD test to supply neutral instead of earth. 


    It's a strange phenomenon, though, that all 6 sparks at the firm I work for have all mistaken the single column to be x5. I can't understand why we have all made the same mistake. The first time I was sent the updated 18th ed Excel sheet and saw that it only had one column for test results I'm sure I would have looked at the heading. It says nothing about the multiplier yet we've all thought it was x5. Will be speaking with the lads when they come back off furlough. (Only me and a mate working at the moment).


    Anyway,

    Thanks for all your replies everyone.

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