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Type A rcd . EICR coding ? etc

aligarjon
Hi Guys.   Not been on for a long time, just had a bit of a search and couldn't really find anything so thought i would ask and see what you all thought.


1.  Are we or will we be coding type AC rcd's if there are LED's or induction hobs, lots of electronics  etc  present.

2. How much DC leakage does it actually take to saturate an rcd and cause  problem?

3. How much does a standard LED lamp or induction hob  leak ?

If we test an AC RCD with no load and it's fine then re-test it with all LED lights, induction hobs etc turned on and it operates correctly could we then say that it is ok with a note on EICR  OR EIC if installing any of the above.  


Obviously also on an EICR if the RCD then doesn't operate with it all on it becomes a C2 ?


Any thoughts



Gary
  • 0
    whjohnson:

    Some salient points made here. The thing which strikes me is that the pyramid of responsibility has been inverted ...


    I have certainly benefitted much more from the discussion in this forum than from the manufacturers' advice.


    So it will have to be a type A RCD on the assumption that the boiler will trip a type AC one in normal service.


    In this instance, the extra £25 or so isn't much compared with a whole CH system, but a customer would certainly be unimpressed if a boiler change required a new CU and all that goes with it. At the end of the day, it will be the tradesman who gets the blame and not the manufacturer.


  • 0
    Type AC, in terms of standards-requirements or modern designs, shouldn't trip in any case where type A would not.  Rather, it's the failure of AC to trip in certain cases that makes A preferable. 


    However, modern RCD standards (whether for A or AC) require a test of immunity to false trips during transient residual currents. Some older RCDs that didn't have this requirement can trip easily on the brief residual currents through, e.g., a power-supply filter during turn-on or rapid changes in voltage.  I'm not sure where the boundary in time lies, but certainly the current IEC61008 specifies tests of transient immunity, whereas I've come across very [over]sensitive RCDs from the 1990s. 


    So it's true that changing an old RCD (which incidentally happens to be type AC) to a modern one (that's type A), would [edit: could] help avoid the false trips.  But I'd be surprised if it's because of the "type" in itself: it's because of another feature (transient immunity) that varies between old and new models. 


    See for example this thread elcb and borehole pump where another motor-type load caused an old RCD to trip.  


    As far as I'm aware, the suggestions that manufacturers make (whether "use this type of RCD" or "this contains a rectifier") are concerned more with the problem of RCDs not responding to particular waveforms.
  • 0
    Chris Pearson:


    This is the reply:


    Thank you for the enquiry effectively you are looking for an A Type RCD which features the characteristics of a smooth DC fault current is less than 6 mA, as the standard RCDs we offer in this range as all AC type and dont have this feature,


    Why do Schneider mention smooth DC when a type A detects pulsating DC? Do they know their As from their elbows?


  • 0
    The wording of 531.3.3 Note 1;

    "NOTE 1: For RCD Type A, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current up to 6 mA."

    (and the other notes) could be read that either just the smooth component of the residual current could be up to 6mA or that the combined smooth and pulsating components will trip a 30mA RCD when the total peak residual DC current exceeds 6mA.

    If the former, then i assume the RCD will trip when a DC pulse peak exceeds 30mA in a 30mA RCD. What would happen if the smooth DC component is 7mA?

    If the latter then why 6mA? It seems very low. Perhaps the type A is blinded? 


    These questions were probably answered in John and Graham's recent webinar which i haven't had a chance to see yet, apologies if so.
  • 0
    A couple of weeks ago I went to look at an old Crabtree upfront 30 mA RCD, it had started tripping since a new freezer arrived and had tripped four times the previous day.


    When tested the old Crabtree RCD tripped at 31 mA and within the required times, my leakage clamp measured around 6 mA constant leakage, but pinged way up when the freezer kicked in.


    I swapped it for a new BG 30 mA Type A RCD and it has been fine, I must send the bill though I’m not exactly sure what problem has been solved.


    I think the old RCD was getting a bit trip happy in its old age, unlike its new fresh out of the box cousin and did not like the freezer kicking in.
  • 0
    What would happen if the smooth DC component is 7mA? .... Perhaps the type A is blinded?

    As I understand it, that's exactly the issue. When smooth d.c. of more than 6mA might be involved then you either need a B-type RCD (which isn't blinded by d.c. but might not trip until the d.c. reaches 60mA) or an A-type (or similar) backedup with an RDC-DD device that'll trip if the smooth d.c. exceeds 6mA (e.g. see 722.531.3.101).


       - Andy.
  • 0
    The wording of 531.3.3 Note 1;

    "NOTE 1: For RCD Type A, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current up to 6 mA." (and the other notes) could be read that either just the smooth component of the residual current could be up to 6mA or that the combined smooth and pulsating components will trip a 30mA RCD when the total peak residual DC current exceeds 6mA.

    If the former, then i assume the RCD will trip when a DC pulse peak exceeds 30mA in a 30mA RCD. What would happen if the smooth DC component is 7mA? If the latter then why 6mA? It seems very low. Perhaps the type A is blinded? 

    Yes - not clear there. The requirement is that the usual test with pulsating current is repeated, but with an additional smooth dc current that is not included in the measured current; the test must be passed in spite of the presence of this smooth dc. This is in IEC61008(2013), 9.21.1.4 "Verification of the correct operation in case of residual pulsating direct currents superimposed by smooth direct current of 0,006 A". 


    The test with 6 mA dc (smooth) is only used together with a pulsating dc residual current. This is the case in which the core could already be taken towards saturation by the smooth dc, and then a 'pulsating dc' fault could happen in the same direction, causing little change in magnetization and so not tripping the RCD even with much more pulsating current.  I.e. it's blinded to this pulsating dc. (Meanwhile, type AC is permitted congenital blindness to the pulsating dc.) My earlier rather long description of RCD cores and trip coils might make this clearer. In a type-A RCD that's near the boundary of what is permitted, a 7 mA smooth dc could potentially make it fail to trip even with a large pulsating residual current in the same direction. But a balanced ac residual current would still trip it at not too very different a level from the usual, as it would push the magnetization out of saturation in some half-cycles.


    I've not seen a rationale for the choice of 6 mA. A standard number, around the level that's been thought to be a reasonable limit for what smooth dc might leak out in a circuit except in special cases? Note that the smooth dc needed to saturate an RCD core can be less than the pure ac, as the ac induces current in the core's secondary (feeding the tripping coil), and this current opposes the core's field, so the overall current magnetizing the core is less than one would think from the residual current. Smooth dc doesn't have this opposing current in the secondary. 

  • 0
    I think the old RCD was getting a bit trip happy in its old age, unlike its new fresh out of the box cousin and did not like the freezer kicking in.


    I'd be surprised and interested if RCDs get more trip-happy.  I can't imagine a mechanism for it. 

    I think it's just that they were always trigger-happy [edit: trip-happy], but that:  (a) modern loads are better at stimulating the trip-happiness, and (b) modern RCDs are less trip-happy because of the standards requirements.  How much (a) caused (b) or vice versa I don't try hard to guess .. it was sensible to do (b) anyway as there were cases of RCDs tripping on transients from the supply that caused currents in L-PE or N-PE capacitance. 


  • 0
    Nathaniel:

    Type AC, in terms of standards-requirements or modern designs, shouldn't trip in any case where type A would not.  Rather, it's the failure of AC to trip in certain cases that makes A preferable. 

    That is my understanding. Perhaps type A was specified by the pesky boiler manufacturer to protect against faults after a rectifier, but I think that we are agreed that this is not necessary.

    However, modern RCD standards (whether for A or AC) require a test of immunity to false trips during transient residual currents. Some older RCDs that didn't have this requirement can trip easily on the brief residual currents through, e.g., a power-supply filter during turn-on or rapid changes in voltage.  I'm not sure where the boundary in time lies, but certainly the current IEC61008 specifies tests of transient immunity, whereas I've come across very [over]sensitive RCDs from the 1990s. 


    So it's true that changing an old RCD (which incidentally happens to be typeier AC) to a modern one (that's type A), would [edit: could] help avoid the false trips.  But I'd be surprised if it's because of the "type" in itself: it's because of another feature (transient immunity) that varies between old and new models.


    So what happens when e.g. a motor's controller fires up? I have a mental image of current in the line conductor going through a rectifier and then stopping in a capacitor until it is charged up. For that brief period, less would return via the neutral and the RCD might trip. Does that make any sense at all? ?


  • 0
    So what happens when e.g. a motor's controller fires up? I have a mental image of current in the line conductor going through a rectifier and then stopping in a capacitor until it is charged up.

    But wouldn't you normally get an equal current flowing from the capacitor's -ve plate back to N? The Charge stays in the capacitor, rather than the current. In my head a capacitor is like an expansion vessel - a box with a flexible membrane in it and holes at opposite sides - pushing water in one side would push the same volume of water out of the other - but drop the pressure and the elasticity of the membrane and the water originally pushed in now flows back out - drawing back in an equal amount of water from the outlet side.


    Or if you prefer - like charging a battery - you still need both +ve and -ve connections to charge it - just like you need both to discharge it.


       - Andy.

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