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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
    The sensing of an AC current, with ring cores as a current transformer and so forth is well established. how that core is scaled to handle waveforms that are full wave rectified bumps rather than sine waves is a detail of size of core, and perhaps change of magnetic material.

    The sensing of a DC that we must assume comes on slowly (perhaps ramped over several seconds) so not to look 'AC-like' though the transformer, has to be detected rather differently - we are really now looking to detect a static magnetic field.

    so the main part is like a normal RCD, plus a sensor for static magnetic fields.


    Two common ways to detect an un changing magnetic field.. I have not got  a B type in front of me, and if I did, I think given the price, I'd think hard about reaching for a hacksaw, but it will be based on one or other of these.


    Hall effect devices do this by deflecting the flow of a measurement current (much as the magnetic filed of the deflection coils of the old CRT telly moved the electron beam about, but the electrons are not a 'gas' of charges in vacuum, but instead diffuse  through  a doped semiconductor,and the amount the current is deflected more to one electrode than the other is measured in microns per mm.)

    Hall effect devices are used in  the modern car to detect notches in camshafts as they whizz  round, and are mass produced cheap items.


    The other method is the 'flux gate' where an AC signal, perhaps at a few kHz sweeps the magnetic core to saturation in both directions.

    When a DC generated magnetic field is present in addition to the test waveform, the clipping at the two extremes from the core saturation becomes asymmetric, and that can be detected. This is a surprisingly sensitive technique, and crossed pairs of such sensors are often  used in electronic compasses in smart-phones, where it costs a few pence.


    the  third and final component of the B type RCD must be a gold bar given the price, but I am at a loss to see what it does in there.

    Mike.
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  • 0
    The sensing of an AC current, with ring cores as a current transformer and so forth is well established. how that core is scaled to handle waveforms that are full wave rectified bumps rather than sine waves is a detail of size of core, and perhaps change of magnetic material.

    The sensing of a DC that we must assume comes on slowly (perhaps ramped over several seconds) so not to look 'AC-like' though the transformer, has to be detected rather differently - we are really now looking to detect a static magnetic field.

    so the main part is like a normal RCD, plus a sensor for static magnetic fields.


    Two common ways to detect an un changing magnetic field.. I have not got  a B type in front of me, and if I did, I think given the price, I'd think hard about reaching for a hacksaw, but it will be based on one or other of these.


    Hall effect devices do this by deflecting the flow of a measurement current (much as the magnetic filed of the deflection coils of the old CRT telly moved the electron beam about, but the electrons are not a 'gas' of charges in vacuum, but instead diffuse  through  a doped semiconductor,and the amount the current is deflected more to one electrode than the other is measured in microns per mm.)

    Hall effect devices are used in  the modern car to detect notches in camshafts as they whizz  round, and are mass produced cheap items.


    The other method is the 'flux gate' where an AC signal, perhaps at a few kHz sweeps the magnetic core to saturation in both directions.

    When a DC generated magnetic field is present in addition to the test waveform, the clipping at the two extremes from the core saturation becomes asymmetric, and that can be detected. This is a surprisingly sensitive technique, and crossed pairs of such sensors are often  used in electronic compasses in smart-phones, where it costs a few pence.


    the  third and final component of the B type RCD must be a gold bar given the price, but I am at a loss to see what it does in there.

    Mike.
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