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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?



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  • 0
    John Hill TMIET:

    This issue came up on the 18th edition course. The testing procedure for rcd's hasn't changed, it's just the requirement to record the 1x IΔn value that has been removed.



    It may be the case where RCDs are specified for additional protection that the specific requirement in BS 7671 to test and record at x5 is not stated.


    However, RCDs may also be used for ADS to achieve a disconnection time in accordance with Chapter 41.


    In a TN system, the disconnection time is 0.4 s (or 5 s for distribution circuits and some larger-rated circuits), and therefore the x1 test would meet the requirement - is there any need to conduct a x5 test?


    In TT systems, a disconnection time of 0.2 s may be required. The x1 trip time could be within "spec" and greater than 200 ms - would you omit the x5 test if the x1 test was below 0.2 s? I've occasionally come across x5 tests which are a longer trip time than x1. if you look at Table 3A, it's actually the x2 test which is required for ADS in circuits requiring 0.2 s disconnection time.


    Very pertinent on circuits protected by an RCD larger than 100 mA (say to prevent nuisance-tripping with a downstream RCD), where 0.2 s disconnection time is still required.

    On a 30ma device for additional protection you still need to test at 1/2 x IΔn to rule out nuisance tripping, 1x IΔn to confirm normal operation and the 5 x IΔn value is recorded. If for some strange reason the device does not trip out within the required time at 1 x IΔn then it would be recorded as a C2 in the observations and would need to be repaced, regardless of the 5 x value.



    Agreed for Additional Protection and where a disconnection time of 0.3 s (300 ms) or less is required, with 30 mA and 100 mA RCDs.


    The conclusion of all this, is that probably the x5 value is most pertinent to record with 10 mA, 30 mA and 100 mA RCDs, but with RCDs with a greater residual current rating, it may not be safe (or if there's a high earth electrode resistance, perhaps not possible) to get the x5 reading !



    This of course assumes it's correct to test the functionality of the RCD with the associated protective conductor. Certainly the test to BS 61557-6 is really a manufacturing test for the RCD stand-alone, not really a performance requirement for RCDs in an installation. Consider the two following cases, are you happy with the statements based on the previous sentence:
    • Most 30 mA RCDs provide additional protection, and for additional protection we cannot assume there is a protective conductor ... we are protecting against (among other things) accidental failure of basic protection (e.g. cut 2-core lawnmower cable where there is no protective conductor).

    • Where the RCD is required for ADS, it is feasible to rely on the earth fault loop impedance, or Ze+(R1+R2), being low enough to achieve the relevant residual current for x1 or x2 (or x5) based on the disconnection time required, as discussed above, if the RCD is tested with an instrument to BS EN 61557-6, using the "up/down" method (as one test equipment manufacturer calls it)?

Reply
  • 0
    John Hill TMIET:

    This issue came up on the 18th edition course. The testing procedure for rcd's hasn't changed, it's just the requirement to record the 1x IΔn value that has been removed.



    It may be the case where RCDs are specified for additional protection that the specific requirement in BS 7671 to test and record at x5 is not stated.


    However, RCDs may also be used for ADS to achieve a disconnection time in accordance with Chapter 41.


    In a TN system, the disconnection time is 0.4 s (or 5 s for distribution circuits and some larger-rated circuits), and therefore the x1 test would meet the requirement - is there any need to conduct a x5 test?


    In TT systems, a disconnection time of 0.2 s may be required. The x1 trip time could be within "spec" and greater than 200 ms - would you omit the x5 test if the x1 test was below 0.2 s? I've occasionally come across x5 tests which are a longer trip time than x1. if you look at Table 3A, it's actually the x2 test which is required for ADS in circuits requiring 0.2 s disconnection time.


    Very pertinent on circuits protected by an RCD larger than 100 mA (say to prevent nuisance-tripping with a downstream RCD), where 0.2 s disconnection time is still required.

    On a 30ma device for additional protection you still need to test at 1/2 x IΔn to rule out nuisance tripping, 1x IΔn to confirm normal operation and the 5 x IΔn value is recorded. If for some strange reason the device does not trip out within the required time at 1 x IΔn then it would be recorded as a C2 in the observations and would need to be repaced, regardless of the 5 x value.



    Agreed for Additional Protection and where a disconnection time of 0.3 s (300 ms) or less is required, with 30 mA and 100 mA RCDs.


    The conclusion of all this, is that probably the x5 value is most pertinent to record with 10 mA, 30 mA and 100 mA RCDs, but with RCDs with a greater residual current rating, it may not be safe (or if there's a high earth electrode resistance, perhaps not possible) to get the x5 reading !



    This of course assumes it's correct to test the functionality of the RCD with the associated protective conductor. Certainly the test to BS 61557-6 is really a manufacturing test for the RCD stand-alone, not really a performance requirement for RCDs in an installation. Consider the two following cases, are you happy with the statements based on the previous sentence:
    • Most 30 mA RCDs provide additional protection, and for additional protection we cannot assume there is a protective conductor ... we are protecting against (among other things) accidental failure of basic protection (e.g. cut 2-core lawnmower cable where there is no protective conductor).

    • Where the RCD is required for ADS, it is feasible to rely on the earth fault loop impedance, or Ze+(R1+R2), being low enough to achieve the relevant residual current for x1 or x2 (or x5) based on the disconnection time required, as discussed above, if the RCD is tested with an instrument to BS EN 61557-6, using the "up/down" method (as one test equipment manufacturer calls it)?

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