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RCD types and tripping versus blinding

Ok, so I'm still a bit confused about the characteristics of the various types of RCDs, i.e. AC/A/F/B. I've concluded that's because there are two classes of effects: a type of residual current that will cause a trip, and a type of residual current that will blind the device against other types of current it would normally trip on. I find that online discussions and 531.3.3 don't clearly distinguish these cases. Can anyone provide, or give a link to, a clear description of what each type RCD is designed to trip on, and what can blind it? For example, might a smooth DC residual current >6ma blind a type A? And if so, is that completely blind, or just blind against pulsating DC (AC still works)? Etc.
  • This might help.

    https://www.hager.co.uk/news-exhibitions-case-studies/18th-edition/selection-of-type-of-rcd/90066.htm


    Z.
  • Blinding of R.C.D.s

    https://www.westernautomation.com/rcd-blinding/


    Z.

  • Zoomup:

    This might help.

    https://www.hager.co.uk/news-exhibitions-case-studies/18th-edition/selection-of-type-of-rcd/90066.htm




    Although it doesn't seem to discuss the blinding aspect

  • Blinding is a far less obvious problem than tripping and will usually not be noticed, so not observed as a problem.
  • The problem is the blinded behaviour is not specified, as it depends on aspects of the RCD design that are not part of the normal use.

    So an AC type, may be blinded by DC, but in practice many are not totally blinded unless the DC is smooth, but they are nearly all  desensitised to a degree.

    An A type is really an AC with a large enough ring core inside that the ripple of unsmoothed DC operates the mechanism, in the presence of the average DC does not saturate the larger core.

    Most AC and A types can be  totally blinded by a smooth DC, if it ramps up slowly enough.

    All the designs for the more complex classifications have deliberate DC sensing with a hall effect sensor.

  • In particular, 531.3.3 says that type A will trip for pulsing DC imposed on smooth DC up to 6mA. Which I guess is a promise that it won't be blinded by smooth DC as long as its <6mA (and no guarantee about AC not being blinded). Type F has a similar note for 10mA. Then type B has a note that it will trip for pulsing DC imposed on smooth DC up to 0.4 x Idn. This seems to imply that even type Bs can be blinded by a large enough smooth DC added to a pulsating DC.


    And there seems to be no provision for coordination. E.g. a downstream type B won't protect an upstream of any type "less than" B against residual DC currents which could blind it. So that 300mA fire RCD is stuffed.


    It all seems to be a bit of mess, with the only logically safe arrangement being for every RCD to be type B.


    How easy it is it to get blinding currents - e.g. could a fault in a cheap AA battery charger do the job?

  • wallywombat:

    In particular, 531.3.3 says that type A will trip for pulsing DC imposed on smooth DC up to 6mA. Which I guess is a promise that it won't be blinded by smooth DC as long as its <6mA (and no guarantee about AC not being blinded). Type F has a similar note for 10mA. Then type B has a note that it will trip for pulsing DC imposed on smooth DC up to 0.4 x Idn. This seems to imply that even type Bs can be blinded by a large enough smooth DC added to a pulsating DC.


    And there seems to be no provision for coordination. E.g. a downstream type B won't protect an upstream of any type "less than" B against residual DC currents which could blind it. So that 300mA fire RCD is stuffed.


    It all seems to be a bit of mess, with the only logically safe arrangement being for every RCD to be type B.


    How easy it is it to get blinding currents - e.g. could a fault in a cheap AA battery charger do the job?

    .................................................................................................................................................................................................................................................................................................................................................................................................................................................

    Well we'll have to rely upon A.D.S. in the form of M.C.Bs tripping off in the case of an earth fault, that and good bonding should make things safe if R.C.D.s can't be trusted to work reliably.


    Z.




     

  • It is indeed a mess. I reckon in about 10 years all new RCDs will have DC sensing.

    Right now mass production is mainly making AC and A types as this is an established design, and a small variation on it.

    Blinding requires an AC-DC conversion on the mains side of any transformers, and a fault to earth from the DC bus.

    Historically it has not been much of a problem, but high power class one devices that are switch mode like VSD and EV chargers

    are a new problem. Here we do have the mains being rectified and smoothed as the first step, and also a circuit that then slices that DC bus at a supersonic frequency, which is after all how we keep the transformers so small and light.

    But that brings 2 new problems.

    1) there is now a credible DC source in an earthed metal box (smaller switching supplies tend to be double insulated so the earth fault is impossible)

    2) to stop the switching frequency radiating off the wiring like a crude radio transmitter, we now need to add high frequency filtering, so there are now RF capacitors between line and earth, among other things, so there is a standing leakage to earth even when all is well.


    So the RCD designer is being squeezed,  as now they must neither trip nor blind with non-mains frequencies,  they must not blind on DC faults, and must work with a significant leakage and not trip.

    The price and scarcity of devices to the harder specs tells us this is a hard problem, and that situation will remain until the 2nd tier manufacturers can get tooled up for it.
  • Some good stuff here on the 'blinding effect'.


    http://www.doepke.co.uk/download/Techpub-08.pdf


    http://www.doepke.co.uk/download/Techpub-16