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Type A/B RCDs can you help explain the component parts?

Zs
Hello,  Long time no post but I hope you are all OK.


I'm curious about something.  Anyone know what is inside a type B RCD that is different from what's inside a type A RCD? What component parts are different and what's the physics behind the tripping of a type B that is different from the tripping of a type A?


I've heard the terms blinded and flooding being bandied around on the virtual water cooler lately so I thought I'd do a little quiet due-diligence.  The web can be rather conflicting though. Ever wish you'd never started something?  


Am I right that a type A will disconnect ac and pulsating dc and a type B does those two with smooth dc as well?  I can also find a site that tells me type A only does ac.


Also, if a type A is subject to a DC fault in one of the circuits, say a charger, they, at the water cooler,  say it will be 'blinded' and won't trip.  Well, as far as I can see that could be partially true  and it  might be unable to see other (ac) faults in the installation - but  I'm thinking there'd be a level of DC fault at which it would not go 'blind'.


Add to that the conundrum that a pulsating dc surely involves time so it will have a  level of frequency?


I've managed to get myself proper confused now and wonder if you can help with what happens inside the type B that makes it so different, not to mention expensive.

  

Zs


  










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  • 0
    To some extent it's all down to "rigidly defined areas of doubt and uncertainty" if you like.


    AC types have no defined response to d.c. or partial waveform a.c. residual currents - some certainly can be disabled by d.c. (remember the D-LOC loop testers?) but others may (and sometimes do) behave differently - more modern ones seem very close to A types - but the standard doesn't guarantee that behaviour so you can't rely on it in any design.


    A types, as well as responding to partial a.c. waveforms, are required to behave properly with a residual d.c. current of up to 6mA - which gives you some leeway (e.g. where EV charge points have their own trips that'll disconnect at 6mA d.c.) ... provided of course you don't have multiple possible sources of d.c. downstream... What happens if you feed them more than 6mA of d.c. is again "undefined" ... they might work as expected, or they might not.


    B types have much smaller areas of doubt and uncertainly - but still some.


    (Then add in F-types, K-types, EV-types and B+ types just for extra confusion)


        - Andy.
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  • 0
    To some extent it's all down to "rigidly defined areas of doubt and uncertainty" if you like.


    AC types have no defined response to d.c. or partial waveform a.c. residual currents - some certainly can be disabled by d.c. (remember the D-LOC loop testers?) but others may (and sometimes do) behave differently - more modern ones seem very close to A types - but the standard doesn't guarantee that behaviour so you can't rely on it in any design.


    A types, as well as responding to partial a.c. waveforms, are required to behave properly with a residual d.c. current of up to 6mA - which gives you some leeway (e.g. where EV charge points have their own trips that'll disconnect at 6mA d.c.) ... provided of course you don't have multiple possible sources of d.c. downstream... What happens if you feed them more than 6mA of d.c. is again "undefined" ... they might work as expected, or they might not.


    B types have much smaller areas of doubt and uncertainly - but still some.


    (Then add in F-types, K-types, EV-types and B+ types just for extra confusion)


        - Andy.
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