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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
    I think at the most basic level, what is important is what the product is tested and certified to.


    I understand Zs asking the question about whether any particular product may or may not be blinded, to perhaps inform a decision, or even technical curiosity.


    However, from the point of view of answering whether an RCD may or may not protect someone [within the bounds of BS 7671 and the product standard of course] is it not more pertinent to understand what the product has been designed and tested for (e.g. Type A residual current detection), not what a product designed and tested for something else might or might not do when subjected to residual currents it outside the specification [standard]?


    I'm sure we've all come across even Type AC RCDs tripping during continuity tests, for example, which are carried out with DC test currents often in excess of 6 mA - but that sort of anecdotal evidence is at best very subjective and probably unreliable, because:
    1. When the test current is applied, it's not "steady DC" at the moment the current is applied.

    • Anecdotal evidence also shows that the trip doesn't always occur when the DC continuity test current is passed through the same device on the same day, shortly before or afterwards.

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  • 0
    I think at the most basic level, what is important is what the product is tested and certified to.


    I understand Zs asking the question about whether any particular product may or may not be blinded, to perhaps inform a decision, or even technical curiosity.


    However, from the point of view of answering whether an RCD may or may not protect someone [within the bounds of BS 7671 and the product standard of course] is it not more pertinent to understand what the product has been designed and tested for (e.g. Type A residual current detection), not what a product designed and tested for something else might or might not do when subjected to residual currents it outside the specification [standard]?


    I'm sure we've all come across even Type AC RCDs tripping during continuity tests, for example, which are carried out with DC test currents often in excess of 6 mA - but that sort of anecdotal evidence is at best very subjective and probably unreliable, because:
    1. When the test current is applied, it's not "steady DC" at the moment the current is applied.

    • Anecdotal evidence also shows that the trip doesn't always occur when the DC continuity test current is passed through the same device on the same day, shortly before or afterwards.

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