Near enough, or actually stated?
It was perfectly clear to me, what was meant, even if words of more than one syllable were used.
Near enough, or actually stated?
I think you need to read more carefully what I have written, several times. What I have said is:This seems to be contradiction, first you say using a cpc on the classI appliance does not introduce a hazard then you say it is not safe. In fact you have stated it is not safe several times unless I'm totally missing your points.
I have been reading, and also considering the one and only time I have worked with an IT system. There was a step down transformer 660 volt to 220 volt delta it supplied a tunnel boring machine (TBM) made by Robin, in Hong Kong very wet iron laden water so very good conductor, it was very shocking, and a nightmare to work on, but no one died or had a belt bad enough to say anything more than ouch.As I said at the beginning, this isn't a request-for-advice thread. Its more of a Sunday afternoon discussion thats now run over into Monday. I understand the pros & cons of running an earth free system. Most of the suspects I expected to join-in have, with the exception of Eric who I'm sure will be along in due course.....
If this is the latest definition of classI then I have not kept up to date and neither have the first 3 definitions that have come up on our friend, none of them have come up with anything stating: 'The whole point of ... Class I item is so that a protective device will operate ...'. They, and my early learnings, indicate the reason for the earthed barrier is to prevent it becoming a electric shock safety hazard and AFAIC preventing harm to an operator is still paramount, whether that trips a protective device or not.I think you need to read more carefully what I have written, several times. What I have said is:
1... Using a Class I item when the supply is floating is, in itself, not safe in the first place, even if its exposed-c-ps are earthed. The whole point of having earthed exposed-c-ps in a Class I item is so that a protective device will operate if a 'live' (L not N) conductor comes into contact with the exposed-c-p. With a floating supply that will not happen. As I said, presumably because floating supplies are rare, MIs of Class I items don't seem to point out that they cannot be used safely with floating supplies.
"Class I equipment. Equipment in which protection against electric shock does not rely on basic insulation only, but which includes means for the connection of exposed-conductive-parts to a protective conductor in the fixed wiring of the installation (see BS2754)"
Seems nearly the same.BS 7671:2008 said:Class I equipment. Equipment in which protection against electric shock does not rely on basic insulation only, but which includes means for the connection of exposed-conductive-parts to a protective conductor in the fixed wiring of the installation (see BS EN 61140).
Oddly never bothered reading that, wonder what an artificial neutral is?BS 7671:2008 said:Protective conductor (PE). A conductor used for some measures of protection against electric shock and intended for connecting together any of the following parts:
(i) Exposed-conductive-parts
(ii) Extraneous-conductive-parts
(iii) The main earthing terminal
(iv) Earth electrode(s)
(v) The earthed point of the source. or an artificial neutral.
I've worked in a number situations with floating systems and totally agree we each carried neon & LED version screwdrivers and habitually tested everything before touching, even if it had been tested 5 minutes before.I have been reading, and also considering the one and only time I have worked with an IT system. There was a step down transformer 660 volt to 220 volt delta it supplied a tunnel boring machine (TBM) made by Robin, in Hong Kong very wet iron laden water so very good conductor, it was very shocking, and a nightmare to work on, but no one died or had a belt bad enough to say anything more than ouch.
OK all adults and we all used neon screwdrivers before touching anything.
My similar thoughts have been documented on here before....using yellow brick transformers.
The problem is not electric shock, it is fire, the yellow brick has a 10 amp overload on its supply, 10 x 230 / 55 = 41.8 amp and 1.5 mm² will not take 41.8 amp without melting.
So a risk assessment, which is higher risk, shock from 230 volt, or fire from 110 volt? If up to me I would ban yellow bricks unless overload fitted on outlet.
Are there any?OK now I stand back and see how you all pick holes in my thoughts.
The only times I've ever really read them, other than during 16th college course, is when put to task, and I must add it's usually been in relation to a consultants error.Seems nearly the same.
Oddly never bothered reading that, wonder what an artificial neutral is?
The whole point of earthing the exposed conductive parts is to provide protection against electric shock in the event of a failure of "basic insulation". That can be achieved either by tripping a protective device, or by limiting the touch voltage to an acceptable level.The whole point of having earthed exposed-c-ps in a Class I item is so that a protective device will operate if a 'live' (L not N) conductor comes into contact with the exposed-c-p. With a floating supply that will not happen.
Indeed, which is why floating supplies work best on a small scale.If one has a Class I item (with earthed exposed-c-ps) connected to a floating supply, and if one side of that floating supply somehow comes into contact with the exposed-c-ps, there will be no immediate hazard. However, that fault will have 'converted' the floating supply to an earth-referenced one so, in terms of any subsequent other fault, it will have lost the safety advantages of being floating.
What a brilliant summaryThe whole point of earthing the exposed conductive parts is to provide protection against electric shock in the event of a failure of "basic insulation". That can be achieved either by tripping a protective device, or by limiting the touch voltage to an acceptable level.
Indeed, which is why floating supplies work best on a small scale.
As a system gets larger, the probability of their being a fault somewhere in the system, and/or an accumulation of leakage currents to hazardous levels starts to approach 1. So the safety advantages of floating systems are lost and it is better to have a system with a well-defined relationship to earth.
Another alternative is to have a supply that is only weakly tied to earth, then implement fault monitoring by measuring the voltages between the supply conductors and earth. This is arguably the safest system for a small scale supply and I belive is used in some medical situations, but again it doesn't scale up well.
That's not materially different from what I wrote, is it? In the context we are discussing (a fully-floating supply), having earthed exposed-c-ps will not facilitate the operation of any protective device (OPD or RCD). As for "limiting touch voltage" (which I would have thought was technically bonding, rather than earthing), the only touch voltage it will limit will be that between the exposed-c-p and something else earthed and, again, in the context of a fully-floating supply a hazardous touch voltage cannot really arise (since any pd between the exposed-c-p and earth will be via stray capacitance etc., hence too high impedance a path to be hazardous).The whole point of earthing the exposed conductive parts is to provide protection against electric shock in the event of a failure of "basic insulation". That can be achieved either by tripping a protective device, or by limiting the touch voltage to an acceptable level.The whole point of having earthed exposed-c-ps in a Class I item is so that a protective device will operate if a 'live' (L not N) conductor comes into contact with the exposed-c-p. With a floating supply that will not happen.
Undoubtedly true, but I would think that all the scenarios we've been discussing here count as 'small scale'.Indeed, which is why floating supplies work best on a small scale. As a system gets larger, the probability of their being a fault somewhere in the system, and/or an accumulation of leakage currents to hazardous levels starts to approach 1. So the safety advantages of floating systems are lost and it is better to have a system with a well-defined relationship to earth.
Indeed, and I've seen that system used in some "medicakl situations". However, again, that goes way beyond what is being discussed in this thread.Another alternative is to have a supply that is only weakly tied to earth, then implement fault monitoring by measuring the voltages between the supply conductors and earth. This is arguably the safest system for a small scale supply and I belive is used in some medical situations, but again it doesn't scale up well.
You seem to be describing class II but then saying it doesn't qualify... Class I does not require an "additional insulation" so why introduce it into the description?The point I was making to Sunray was that if an item is designated as Class I, the implication is surely that the "basic insulation" (plus "additional insulation" if present) is not considered adequate to provide the required degree of protection against electric shock (i.e. does not qualify as "Class II") .
Kind Regards, John
Why does it not offer electric shock protection when the multiple faults, that have been described so many times, occur?If that is the case, and if (as above) earthing the exposed-c-p does not really afford any additional protection against electric shock,
Kind Regards, John
If it cannot be safely used as you insist on saying what makes it dangerous when there are no faults?then it surely follows that Class I items cannot be 'safely' used when the supply is floating, doesn't it? [Since the basic+additional insulation does not provide adequate protection , and the earthing of exposed-c-ps does not increase protection, IF the supply is fully floating].
Kind Regards, John
That's the bottom line of the reason for having the 'earthed barrier', but, as I've just written, it only achieves that with an earth-referenced supply..... They, and my early learnings, indicate the reason for the earthed barrier is to prevent it becoming a electric shock safety hazard and AFAIC preventing harm to an operator is still paramount, whether that trips a protective device or not.
Just to be sure: is there a typo?Hence, as I've said, if a Class I item is not considered safe (with an earth-referenced supply) if the exposed-c-p is not earthed, then this implies that the same item cannot be safe with a fully-floating supply, even if the exposed-c-ps are earthed - since that connection to earth affords to additional protection when the supply is floating.
I am saying that it is CXlass I (and hence only considered safe if its exposed-c-ps are earthed) BECAUSE it does not qualify as Class II.I believe you are now really confusing the issueYou seem to be describing class II but then saying it doesn't qualify... Class I does not require an "additional insulation" so why introduce it into the description?
If the first of the faults is such as to turn the floating supply into an earth-referenced one then, yes, one is then back to the situation which would have existed had the supply never been floating.Why does it not offer electric shock protection when the multiple faults, that have been described so many times, occur?
It's no different from Class I items in general. "Deemed to be 'safe' " relates to the situation if/when a fault does arise. A Class I item without its exposed-c-ps being earth is perfectly 'safe' so long as there is no fault, but that doesn't make it acceptable to omit the earthing.If it cannot be safely used as you insist on saying what makes it dangerous when there are no faults?
If it is not "deemed to be 'safe' " even when there are no faults, that question becomes moot.If it cannot be safely used as you insist on saying what makes it dangerous when there are faults?
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