"Earth to neutral fault" with TN-C-S supply ?

In a sound circuit if the test current is 30mA then if, say, 9mA is returning via this leakage then the RCD will not trip even though the 9mA may be unavoidable acceptable leakage.
I can't see that it makes any difference whether the N-E leakage is 'unavoidable' (due to connected equipment), in a 'sound circuit') or due to an N-E fault'. In either case, with the 'usual' (in my experience0 test button arrangement, that leakage will add to the residual current, so that , with 9 mA leakage, the device should trip with a test current of 21 mA or more. With the 'other' arrangement (per diagram you posted), what you say is right - in that case, with a 30 mA test current and 9 mA leakage (of any cause),the device might not trip.

BTW, I think that the test current is usually about 2.5 times the IΔn - hence about 75 mA for a "30 mA" device.
 
Sponsored Links
I can't see that it makes any difference whether the N-E leakage is 'unavoidable' (due to connected equipment), in a 'sound circuit') or due to an N-E fault'. In either case, with the 'usual' (in my experience0 test button arrangement, that leakage will add to the residual current, so that , with 9 mA leakage, the device should trip with a test current of 21 mA or more. With the 'other' arrangement (per diagram you posted), what you say is right - in that case, with a 30 mA test current and 9 mA leakage (of any cause),the device might not trip.

BTW, I think that the test current is usually about 2.5 times the IΔn - hence about 75 mA for a "30 mA" device.
Ok. "might" then but the leakage is reducing the current passing through the RCD sensor.

If the current is 75mA then that is not really determining whether the RCD is satisfactory as it might not trip until 74mA.
 
I don't think there is realy a 'wrong and right', in the sense that, in the absence of any faults in the connected circuit, the test button will work as intended with either arrangement. I need to think further but, off the top of my head, the only difference I can think of is behaviour in the presence of an N-E fault in the connected circuit.
If the test button circuit goes from supplyu L to load N, (which what the manufacture has designed it with) then a downstream N to E fault can stop it working by reducing the test current returning to the supply N through the sensing transformer. However, if the test was from load L to supply N the fraction of test current going to earth at the fault would pass through the transformer twice in the same direction possibly making it test OK even if it had reduced sensitivity.
 
Ok. "might" then but the leakage is reducing the current passing through the RCD sensor.
Sure - I was just point out that, with the nunmbers yiu suggested, failureto trip would not be n inevitabilty, which is what you seemed to imply (" ****")
If the current is 75mA then that is not really determining whether the RCD is satisfactory as it might not trip until 74mA.
Indeed. I may be wrong about the 2.5 x IΔn test current, but I've read that a good few times. Unfortunately, I can't remember what vakue resistor I've found in devices I've dissected - but I'll see if I have any 'carcasses' lying around to look at !

I think the test button is only intended as a fairly crude test, essentially of 'complete failure', and I don't think it is intended to be able to pick up situations in which the device is 'just a bit out of spec'.
 
Sponsored Links
If the test button circuit goes from supplyu L to load N, (which what the manufacture has designed it with) then a downstream N to E fault can stop it working by reducing the test current returning to the supply N through the sensing transformer.
Exactly. However, as I've explained, everything I've written (suggesting that downstream N-E faults should not stop the test button working) was based on the (seemingly incorrect) belief that all such devices had the test circuit going from load L to supply N. - which, as I said, has been the case with every one I've ever taken apart.
However, if the test was from load L to supply N the fraction of test current going to earth at the fault would pass through the transformer twice in the same direction possibly making it test OK even if it had reduced sensitivity.
As I wrote (when I believed that this was the arrangement was what we were dealing with) I would have thought that an N-E fault on the circuit would result in increased sensitivity' (more likely to trip), since the direction of flow of some of the test current through the N side of the device would, I believe, add to the residual current being sensed (per the most recent diagram I posted). Is that not correct?

Kind Regards, John
 
Exactly. However, as I've explained, everything I've written (suggesting that downstream N-E faults should not stop the test button working) was based on the (seemingly incorrect) belief that all such devices had the test circuit going from load L to supply N. - which, as I said, has been the case with every one I've ever taken apart.

As I wrote (when I believed that this was the arrangement was what we were dealing with) I would have thought that an N-E fault on the circuit would result in increased sensitivity' (more likely to trip), since the direction of flow of some of the test current through the N side of the device would, I believe, add to the residual current being sensed (per the most recent diagram I posted). Is that not correct?

Kind Regards, John
Like I said, to assume makes an ass out of u and me, hence my recommendation to contact the manufacturer which you didn't want to do.
 
Like I said, to assume makes an ass out of u and me,
I wouldn't personally call it an assumption but, rather, a 'reasonably-based deduction'. I had seen many diagrams of these devices, and had never noticed any showing this 'unusual' way of wiring a test button. Similarly,I've taken a good few of the devices apart in my time, and have never come across one wired in the 'unusual way'.
hence my recommendation to contact the manufacturer which you didn't want to do.
I "didn't want to" because it was really only of academic, rather than practical, interest to me, and I had better things to do with my time. However, when the discussion became protracted, I decided that I had nothing to lose, so wrote to them (but haven't yet hhad a reply). In view of your revelation today (about their diagram), I have written to then again, asking 'why' they decided to take this approach.

However, I don't really understand why you are pointing a finger only at me. Anyone who understood the principles will have realised that my argument was based on the 'assumption' that the diagram I drew and posted was correct, yet no-one (not even yourself) challenged what I'd drawn or pointed out that there was an alternative way in which a test button might be wired.
 
I wouldn't personally call it an assumption but, rather, a 'reasonably-based deduction'. I had seen many diagrams of these devices, and had never noticed any showing this 'unusual' way of wiring a test button. Similarly,I've taken a good few of the devices apart in my time, and have never come across one wired in the 'unusual way'.

I "didn't want to" because it was really only of academic, rather than practical, interest to me, and I had better things to do with my time. However, when the discussion became protracted, I decided that I had nothing to lose, so wrote to them (but haven't yet hhad a reply). In view of your revelation today (about their diagram), I have written to then again, asking 'why' they decided to take this approach.

However, I don't really understand why you are pointing a finger only at me. Anyone who understood the principles will have realised that my argument was based on the 'assumption' that the diagram I drew and posted was correct, yet no-one (not even yourself) challenged what I'd drawn or pointed out that there was an alternative way in which a test button might be wired.
I also have better things to do with my life as I am terminally ill. However, as I stated, the only way to find out why this happened was to contact the manufacturer.

You admit, you made the assumption your diagram was correct. Nobody else queried the action of the device other than you, and nobody persisted in refusing to contact the manufacturer other than you, hence the pointy finger.

I have given you the answers, so I have now finished with this thread.
 
I also have better things to do with my life as I am terminally ill.
I'm very sorry to hear that. I hope that things are not too bad for you.
However, as I stated, the only way to find out why this happened was to contact the manufacturer. You admit, you made the assumption your diagram was correct. Nobody else queried the action of the device ....
Indeed, and nor did anyone query my diagram, nor the conclusions I had drawn on the basis of my 'assumption' (or 'understanding', or whatever you care to call it).

There is any number of things which "are my understanding" but which may actually not be totally (or, in some cases, at all!) correct, and I would imagine that the same is true of you, and of everyone else - but that's how the world is. Any "understanding" (of anything) we have is inevitably based on what we have been told, what we have read or on personal experience - but in no case is that "understanding" guaranteed to be correct (totally or at all.

I have given you the answers, so I have now finished with this thread.
Fair enough.

Kind Regards, John
 
I will certainly concur that, in my limited experience, that the test button resistor is usually selected to draw well in excess of 30mA to operate the test regime as EFLI alludes to. That does seem reasonable to be sure that this "bang bang" test does have great merit as a "quick guess yardstick"
 
I will certainly concur that, in my limited experience, that the test button resistor is usually selected to draw well in excess of 30mA to operate the test regime as EFLI alludes to.
That's certainly been the case with all those I have dissected over the years., and I think that the resistors I've seen have all equated to a test current of roughly the 2.5 x IΔn that I have often read about.
That does seem reasonable to be sure that this "bang bang" test does have great merit as a "quick guess yardstick"
Maybe, but I've often thought that 2.5 x IΔn test current (if that's what it usually is) is a bit on the ('unnecessarily'?) high side . That will obviously reduce the number of 'false alarms' but, as I think EFLI was implying, if I were a person who believed that 30 mA RCD protection was essential to 'save my life', I might not be too reassured by knowing that the device 'protecting me' would trip at a current of say, 74 mA

Kind Regards, John
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top