RCD wiki + Common Misconceptions about RCDs

[JohnW2]

I don't think so. I think they will balance. keep in mind that the N-E 5Volts (and the subsequent 5mA that you mention) is not from the phase.
It is from the Neutral via the sub-transformer so it passes through the Neutral side of the toroid (to earth) just as the phase current (5mA that you mention) passes through the phase side of the toroid (to earth) Both sides of the toroid see the same current.

As I've just conceded to echoes, I think that I (and you) have been overlooking the fact that, in your scenario, the N-E fault current is travelling the 'wrong' way through the N side of the RCD - so I think it actually is true that the fault currents will add, not subtract.

As for the TN-C-S situation I was thinking about throughout the early part of this discusion, I think the following diagram confirms that the fault currents again add (this time with current going the 'normal' direction through N sie of RCD):

Untitled

• JohnW2
• 9 Sep 2011
• 1

Kind Regards, John.

 

[echoes]

I think that's correct John. Your diagram looks exactly like the one I had to draw on the back of an envelope!... which was when I realised that the direction (or phase) of the currents through the RCD was crucial.
But I think we're there now finally!

Would make a good exam question.

Bedtime.

 

[JohnW2]

I think that's correct John. Your diagram looks exactly like the one I had to draw on the back of an envelope!... which was when I realised that the direction (or phase) of the currents through the RCD was crucial.
But I think we're there now finally!
Would make a good exam question.

Yes, I think we probably really are 'there' now. Thanks. As for an exam question, I don't know about you, but it's clear that sparkticus and I (and probably others) would still have had our confused heads spinning when the 'finish' bell sounded!

Kind Regards, John.

 

[JohnW2]

I think it is an excellent idea John. As we have discussed in the past there is great confusion/disinformation (amongst electricians as well) about what RCDs do and do not do. I think what you have written already plus the discussions that have taken place in this thread just about covers it as far as I can tell.

Thanks.

Kind Regards, John.

 

[sparkticus]

I don't think so. I think they will balance. keep in mind that the N-E 5Volts (and the subsequent 5mA that you mention) is not from the phase.
It is from the Neutral via the sub-transformer so it passes through the Neutral side of the toroid (to earth) just as the phase current (5mA that you mention) passes through the phase side of the toroid (to earth) Both sides of the toroid see the same current.

As I've just conceded to echoes, I think that I (and you) have been overlooking the fact that, in your scenario, the N-E fault current is travelling the 'wrong' way through the N side of the RCD - so I think it actually is true that the fault currents will add, not subtract.

As for the TN-C-S situation I was thinking about throughout the early part of this discusion, I think the following diagram confirms that the fault currents again add (this time with current going the 'normal' direction through N sie of RCD):

Untitled

• JohnW2
• 9 Sep 2011
• 1

Kind Regards, John.

B----r I knew that the phase issue would have got me in the end. Should have gone with my original thought about it which I immediately dismissed/retracted. Theres no substitute for sketching things out is there! But I still think there is still an open issue with TN-S/TT in a no load situation and will sketch it out later this evening.

 

[JohnW2]

[B----r I knew that the phase issue would have got me in the end.

Same here.It was only when I'd drawn both the TN-C-S and TN-S/TT diagrams, and 'noticed' that the directions of arrows at the RCD were different that I twigged.

[Should have gone with my original thought about it which I immediately dismissed/retracted. Theres no substitute for sketching things out is there!

Oh yes there is! The benchwork you scheduled for tomorrow is a much more fun substitute

But I still think there is still an open issue with TN-S/TT in a no load situation and will sketch it out later this evening.

Do you mean that you are not convinced by TN-S/TT diagram I posted to echoes last night? If so, what's the problem. As I wrote to echoes, I first thought that the diagram proved the opposite view, until I noticed that current was passing through the N side of the RCD the 'wrong' way. Hence, my 'imbalance current'of (IfL- IfN) should, if we are working with 'unsigned' numbers, really be written as (IfL- (- IfN)), aka (IfL+ IfN).

Kind Regards, John.

 

[JohnW2]

But I think we're there now finally!

Most answers to questions result in further questions ....
1...I am very thankful that we have been talking about 2-pole RCDs, I'm not sure that my mind, or drawing capababilities, could cope with the TPN situation, although I presume that the answers are conceptually the same!

2...I've yet to find a good explanation of the inner workings of an RCBO, but is is often said that the 'residual current' part of it is 'electronic', rather than electromagnetic as in the RCD. If that's true, what does that actually mean (I rather suspect that there must be some wound components involved), and how does it cope with the phase issues which we've realised are crucial to these recent discussions? Indeed, do the conclusions that we have eventually come to (about RCDs) actually apply to an RCBO?

Kind Regards, John.

 

[echoes]

But I think we're there now finally!

Most answers to questions result in further questions ....
1...I am very thankful that we have been talking about 2-pole RCDs, I'm not sure that my mind, or drawing capababilities, could cope with the TPN situation, although I presume that the answers are conceptually the same!

I've also not given that much thought, but I agree that there should be conceptually no difference.

2...I've yet to find a good explanation of the inner workings of an RCBO, but is is often said that the 'residual current' part of it is 'electronic', rather than electromagnetic as in the RCD. If that's true, what does that actually mean (I rather suspect that there must be some wound components involved), and how does it cope with the phase issues which we've realised are crucial to these recent discussions? Indeed, do the conclusions that we have eventually come to (about RCDs) actually apply to an RCBO?

No idea! Looks like we both have remedial RCBO homework now

[I do have a spare RCBO knocking about, but they are too expensive to submit to destructive investigation!]

 

[JohnW2]

No idea! Looks like we both have remedial RCBO homework now

I spent more time yesterday than I probably should hunting around for some good information about the mode of operation of RCBOs but found little, apart from constant references to the 'electronic' nature of the residual current sensing (which I guess is the reason RCBOs are said to be susceptible to 500V IR tests). There are photos of 'the works' around, but they don't really help me very much - beyond the fact that, as I suggested, there do appear to what probably are mini wound components involved. I'll keep looking.

I do have a spare RCBO knocking about, but they are too expensive to submit to destructive investigation!]

Same here. If the rumours are correct, there surely must be some people out there who have killed an RCBO with an IR test - if so, do they have retained material available for autopsy? !

Kind Regards, John.

 

[sparkticus]

I do have a spare RCBO knocking about, but they are too expensive to submit to destructive investigation!]

Same here. If the rumours are correct, there surely must be some people out there who have killed an RCBO with an IR test - if so, do they have retained material available for autopsy? !

Or simply murdered one in cold electrons - all in the name of science of course


//www.diynot.com/network/sparkticus/albums/9892

 

[JohnW2]

Or simply murdered one in cold electrons - all in the name of science of course

Many thanks - a very interesting autopsy!

I need to look more carefully, but at first sight there's nothing particularly 'electronic' about that - it looks like a conventional electromagnetic RCD mechanism, albeit there are a few capacitors knocking around which might not like 500V IR testing. Have you worked out what the few passive 'electronic' components are actually doing? And what's that grey thing that a white sheathed conductor passes through - is that some sort of current transformer/sensor?

Kind Regards, John.

 

[sparkticus]

Have you worked out what the few passive 'electronic' components are actually doing? And what's that grey thing that a white sheathed conductor passes through - is that some sort of current transformer/sensor?

That's what I initially thought until I found the sub-enclosure with the "RCD" board inside (posted additional pics a few minutes ago) It has an 8pin surface mount IC. Very hard to read it but will get a more powerful magnifying glass and have a look.

 

[sparkticus]

I always wondered if phase and neutral came into close contact in an RCBO (as a possible reason why there are reports of them blowing up)
They do indeed. They are both "squeezed" into the current transformer (toroid)

The flexible phase conductor is covered in what looks like silicone heat resistant sleeving while the neutral is solid lacquered copper wire.

 

[JohnW2]

Have you worked out what the few passive 'electronic' components are actually doing? And what's that grey thing that a white sheathed conductor passes through - is that some sort of current transformer/sensor?

That's what I initially thought until I found the sub-enclosure with the "RCD" board inside (posted additional pics a few minutes ago) It has an 8pin surface mount IC. Very hard to read it but will get a more powerful magnifying glass and have a look.

Thanks again. All that electronics was quite well hidden - at least we've now found plenty of things for an IR tester to blow uip I fear that your magnifying glass may well reveal 'non-informative' product markings, but we'll see!

However, I'm still confused as to what all this electronics is about. As I said before, it looks as if there is a fairly standard electromagnetic 'RCD' mechanism in the top half of that casing, so why the need for electronics? I don't think that 'electromagnetic' bit can be anything to do with overcurrent functionality, since it's not present in a standard MCB (which looks very much like the bottom half of your RBCO):

Untitled

• JohnW2
• 10 Sep 2011
• 1

I'm also interested to see the 3.9k resistor, which I presume was attached to the test button - which implies a 'test' current of about 60 mA.

Kind Regards, John.
Edit: typo corrected

 

[sparkticus]

EDIT