Those things do not really have to necessarily be that conductive, just slightly conductive to provide a path lower in resistance than your body.
Cost-effectiveness of RCDs,SPDs, AFDDs etc., in perspective
- Thread starter JohnW2
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Those things do not really have to necessarily be that conductive, just slightly conductive to provide a path lower in resistance than your body.
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We clearly don't want to loose supply, specially to items like freezers, so RCD and AFDD use do have a down side, I was all in favour of RCD's until I went to work at Sizewell and had to try and find out why they tripped, man hours lost due to RCD tripping was high. I have never used AFDD and don't know how often they trip? But was not impressed when @flameport did the demo. What would happen if I used a welding set not sure.
The thing is with 100 mA RCD's I did not have much of a problem, it was only the 30 mA which caused problems, and the one and old 10 mA RCD socket by MK pointed out a design flaw when pressing the test button also took out the 100 mA and the 300 mA.
It did point out how silly the test button is, OK slight voltage variation so we need to allow for it to work at 200 to 260 volt, so 6.6kΩ should trip the unit without tripping next in line. I have seen a faultless RCD fail to trip with test button, but that was due to phase being down.
But the one item in list which does not cause problems is the SPD. When have you ever changed one of these
. I never have, we fitted filtered sockets when the PC came along, as we were told spikes could wreck the computer, the big question why was the filter not built into the computer?
So with many more electronic items being powered direct, not through a wire wound transformer, spikes can damage the components. But the component damaged needs to exceed £18.86 to be worth protecting with a filtered socket. Whole house some of the SPD's are down to £30 some over £100, so how many LED bulbs need saving to warrant the SPD?
So the question, how many times have all the lights switched on failed together? Think fair to say that is likely caused by a surge, but not seen it reported.
The thing is with 100 mA RCD's I did not have much of a problem, it was only the 30 mA which caused problems, and the one and old 10 mA RCD socket by MK pointed out a design flaw when pressing the test button also took out the 100 mA and the 300 mA.
It did point out how silly the test button is, OK slight voltage variation so we need to allow for it to work at 200 to 260 volt, so 6.6kΩ should trip the unit without tripping next in line. I have seen a faultless RCD fail to trip with test button, but that was due to phase being down.
But the one item in list which does not cause problems is the SPD. When have you ever changed one of these
. I never have, we fitted filtered sockets when the PC came along, as we were told spikes could wreck the computer, the big question why was the filter not built into the computer?So with many more electronic items being powered direct, not through a wire wound transformer, spikes can damage the components. But the component damaged needs to exceed £18.86 to be worth protecting with a filtered socket. Whole house some of the SPD's are down to £30 some over £100, so how many LED bulbs need saving to warrant the SPD?
So the question, how many times have all the lights switched on failed together? Think fair to say that is likely caused by a surge, but not seen it reported.
I use a MIG and a stick welder on my RCD protected workshop supply, I have never had it trip as a result of using either. I have though had it trip through my own stupidity, when modifying my garage /workshop wiring, where I have tripped the MCB, yet forgotten that a N to E short can also trip out the RCD.
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I was more thinking about AFDD's.
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I have no experience of those..
Loss of service until the damaged component is replaced could involve sums of money much greater that £18.86
It's actually a lot 'worse' than you suggest, since there can be a risk of significant electric shock by touching something with an impedance to earth much greater than body resistance/impedance whilst one is simultaneously touching something at 230V. If one is concerned, say, about shocks >5mA, the risk exists when the impedance to earth is less than about 46 kΩ, even if the 'body resistance' is zero.
However, that does not really alter what I wrote (and you quoted), which I could re-word as ...
"Off the top of my head, I really can't think of when I was last in simultaneous contact with two
Kind Regards, John
Then John, you are very wrong. Just one example of many I could think of - Do you never operate two separate taps at the same time, with two hands, I do often?
Long ago I mentioned in this forum a flooded banking hall and my making contact with a live cased industrial dehumidifier, on a soaking wet carpeted floor.
They undoubtedly "can" cause damage but, as I keep saying I haven't got a clue as to how frequentlky they "do".
As I always say, I've had countless electronic items powered by SMPSUs etc. for very many years, and what I would regard as 'premature failures' have been exceedingly unusual, almost unknown (unless one counts LED lamps/bulbs!).
As I also always say, it rather intrigues me that good few of those who now advocate (or stronger) SPDs are the same people who, not too long ago, ridiculed (and described as 'snake oil' or 'a waste of money') any mention of 'surge suppression'. Have these people perhaps suddenly strated experiencing lots of equipment failures which might possible be related to supply 'spikes' ... or what?
Kind Regards, John
I doubt that I personally do (and, in any event, most of my taps are either ceramic or plastic
- but, with respect, I think you are 'scraping barrels'.If you went around your house measuring the resistance to earth of everything 'touchable', I think you would find that in virtually every case, the resistance was either "extremely (possibly 'unmeasurably') high" or "very low" - neither of which (in the context we are discussing) would represent a problem/risk.
That surely moves the goalposts out of the stadium, doesn't it
One has to wonder why the electricity supply to "a flooded hall with a soaking wet carpeted floor" had not been well-and-truly isolated!Kind Regards, John
I'm not sure that "loss of service" would often have any significant financial consequences in the domestic settings we're talking about.
In any event, what when tens of millions of £18.86 components are installed - in comparison with the costs which possibly {and very rarely} might result from not installing them?
Kind Regards, John
Thanks for your detailed reply.
OK - so you're talking about leakage 'through' a PSU. I am actually fairly surprised that there is any acceptable level of such 'leakage' in a normal Class II item 'under normal conditions' (beyond a tiny amount attributable to inevitable 'stray capacitance') - and, as for 'single fault' conditions, unless that situation is 'qualified' that 'single fault' could result in direct connection between input and output of the PSU, couldn't it.
I'm familiar with requirements in relation to medical devices, but that is a very specialised/different situation which probably has little relevance to consumer Class II items.
However, I continue to be confused about what actually does (and does not) qualify as 'Class II' (see below).
Maybe true in relation to most of the appliances I mentioned, but I know from bitter experience that parts of hot air guns get extremely hot
Anything totally encased in insulating material. Things like wallwarts (e.g. 'USB chargers' etc.) are generally marked as Class II, but my question relates to the situation if they were not so (designed and) marked - would they then be 'not allowed'? .....
Indeed. BS7671 also recognises Classes I, II and III - and there certainly seems to be a general belief that Class 0 is 'not allowed'. Class '0I' seems a little odd, in as much as it is essentially Class I. However, if that is also 'not allowed' then that would presumably be a problem in a situation in which someone had run a separate CPC in order to install a Class I item on a circuit which had no CPC?
However, this brings me back to my point/question. We're not interested in Class 0 or Class III, and I will ignore Class 01, so we only have Class I and Class II to talk about. If an item is totally plastic encased, then it cannot be Class I per BS7671 definition, since it has no exposed-c-ps to earth. If id doesn't have double or re-inforced insulation, then nor can it be Class II - so, as I asked "What is it?", and is it 'allowed'? Is the answer perhaps ...
It would seem that the items I'm postulating would not qualify as any of the 'Classes' - so, I wonder, would they be 'allowed'?
In passing, I see that some of my oldest (plastic encased) wallwarts, although marked as Class II, have a metal earth pin - which begs the question as to what (if anything) that is connected to.
Going back to the earlier comments about leakage through PSUs, BS7671 seems to regard the purpose of 'double or reinforced insulation' as being to provide a barrier between 'accessible' and 'live' parts, and I've always thought (perhaps wrongly) that it was talking about parts which are 'live' under 'normal (i.e. no fault) conditions'. If that were the case, then it presumably would not apply to leakages through PSUs under (even 'single') fault conditions?
Kind Regards, John
I have a considerable amount of electronics here comparable to many other homes added together and I have never experienced such a failure in all the time I have lived here. Most of the modern and not so modern stuff I get involved with seems quite well designed to protect itself against mains spikes internally, so why might I need to pay more for extra protection? So my opinion is that one money stream has come to an end, so they have another one to convince people it is a 'must have'.
I doubt that I personally do (and, in any event, most of my taps are either ceramic or plastic
John it is always 'scraping barrels' so far as you are concerned - always it would never happen. Accidents happen when the dangerous circumstances conspire together and happen. Accidents are almost always a series of things going wrong and could have been avoided by changing just one thing in the series. Are you saying you have never had a belt - if you say no, I will not believe you. If you had been the one walking in that flooded banking hall, I am certain that would have changed your mind. That had to have a whole series of improbable things to go wrong and a fault condition to exist, plus a certain level of incompetence, carelessness stupidity in bypassing obvious and clearly signed safety equipment, plus me to walk in, in the dark and already soaking wet from the rain outside. Any one of those improbable steps not lining up and it would not have happened. but there you go.
The problem is the stray capacitance interacts with the switching operation of the converter, so create what is essentially a radio transmitter, to prevent unacceptable radio emmisions SMPSUs have to deliberately add more capacitance between input and output.
When designing such a PSU you have to make sure that the insulation of said capacitors is both reliable enough to meet the insulation safety standards, and make sure that the inevitable mains-frequency leakage current through the capaitors is low enough to comply with the product standards
A class 1 design can divert the leakage away from the user and towards the protective conductor. A class 2 PSU has no such luxuries.
There are clearly limits on what faults can be considered "single faults".
Sadly I suspect though that there is also a lot of equipment out there that fraudulently claims class 2 status.
My understanding is that the principles are broadly similar, medical devices just tend to have stricter rules.
I think there may be an issue that BS7671 is over-simplifying and/or translating imperfectly from "appliance designer terminology" to "electrician terminology".
It is very much possible for a protective earth conductor to be part of the protection and leakage management strategy even if it is not exposed directly to touch by the user and AIUI such an appliance is still considered class 1. This is common with laptop power bricks.
If the user is protected only by basic insulation it would be class 0 and hence "not allowed".
Exactly. My experience is the same as yours, and I totally agree with what you say.
Quite apart from commercially-manufactured items, I have many items of electronic equipment which I built 30-40 years ago, using very old technology, and certainly no explicit protection against 'spikes/surges', which remain fine after those decades in service, in at least some cases in virtually continuous powered-up service for all those years.
One can 'understand' manufacturers wanting to find new ways of making money. What I understand far less is why individuals, organisations and those who write regulations should succumb to the belief that there is a reasonable justification for needing, or even 'requiring', that these bits of new-fangled technology should be used. I can but presume that an awful lot of gullibility (and far too little ration thought) is at work
Kind Regards, John
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