No earth cable on light fitting

But the cable is designed and tested for exactly that application.
Yes, but that's not what I was asking you. I am asking whether you personally really seriously believe that there is any Class II item which could not withstand 230V across its casing continuously. ... and, I suppose, in case you answer 'yes' to that, whether you personally believe that such an item would be safe to have in a domestic environment.

... or, to put it yet another way, do you believe that an item could qualify as Class II if all that existed between a line conductor within and the outside, touchable, world was no more than the equivalent of the insulation on a conductor in 1mm² T&E cable?

Kind Regards, John
 
Sponsored Links
What I believe is not important in this discussion. What is more important is the fact that connecting to earth any exposed metalwork that is protected by Class II construction is likely to be a breach of manufacturer's instructions, is likely to lead to an increased risk due to the presence of earthed metal, and might lead to an increased risk of failure of the item.
 
But there should be no paths to earth - it's a Class II item.
If that class II item is intended to be connected to other equipment, then even leaving aside accidental contact (pushing a metal case against an earthed radiator etc.) how do you think it possible to avoid it being connected to earth anyway in normal use?

I don't think I've ever seen something like a class II domestic VCR or DVD player in which the instructions tell people not to interconnect it with a class I amplifier, or to connect an earthed antenna.

Take a group of typical class II things like a TV, VCR, DVD player, audio amplifier, satellite receiver etc. all interconnected. Then grab a Belling-Lee coax plug or satellite F-connector which has earth potential on the shell and go to connect it to the appropriate unit.
You may not.
Why not? As above, do you really think that the manufacturers of all that double-insulated audio-visual equipment believe that nobody will ever connect the audio output to an amplifier which is earthed, or will never connect an antenna which is earthed? In many cases the external connection needs to be earthed for other reasons.

And once that connector is in place, the whole lot is then earthed through that external connection anyway
Not allowed to be.
According to what? Some standard for the class II equipment? Unfortunately, some of the lines which need to be connected to it in order for it to function are required to be earthed for other reasons. The coax bringing broadband cable services into the house, for example.
 
What is more important is the fact that connecting to earth any exposed metalwork that is protected by Class II construction is likely to be (a) a breach of manufacturer's instructions, (b) is likely to lead to an increased risk due to the presence of earthed metal, and (c) might lead to an increased risk of failure of the item.
(a) ... very possibly true, but that does not tell us the reason for that instruction.
(b)... I totally agree, and that is the point I have been making all along (for worktops, baths and window frames, as well as Class II electrical appliances.
(c)... I do find that hard to believe but, if it were true, I would be very nervous about having such an item (maybe any Class II item) in my home.

Kind Regards, John
 
Sponsored Links
John, I have some prescription medication, the instructions for which advise me not to take it within 6 hours of taking a particular non-prescription item that I would suspect is in 99% of domestic medicine cupboards in the UK. Do I need to know/understand the reason for that instruction? Do I need to know if the risk is that a reaction between the two might kill me, or that one drug might stop the other from functioning? Wouldn't you agree that I would be foolhardy if I didn't follow the instructions supplied?
Should I be nervous about taking such an item? After all, who knows what combinations the manufacturer might have overlooked?
 
If that class II item is intended to be connected to other equipment, then even leaving aside accidental contact (pushing a metal case against an earthed radiator etc.) how do you think it possible to avoid it being connected to earth anyway in normal use?
By not having any part of the signal connectors connected to any exposed conductive part(s)


I don't think I've ever seen something like a class II domestic VCR or DVD player in which the instructions tell people not to interconnect it with a class I amplifier, or to connect an earthed antenna.
Maybe no parts of the signal connectors are connected to any exposed conductive part.


Why not? As above, do you really think that the manufacturers of all that double-insulated audio-visual equipment believe that nobody will ever connect the audio output to an amplifier which is earthed, or will never connect an antenna which is earthed? In many cases the external connection needs to be earthed for other reasons.
It's really incredibly simple.

IF the maker decides that the case of his equipment MUST NOT be earthed then he must either forbid any earthed signal leads or he must isolate the I/O sockets from the case.


According to what?
Maybe I should have said "possibly not allowed". According to the MI's. IF they say "this item must not be earthed" then according to them it must not be earthed either via a cpc in the power cable or by earthed conductors in the signal cables.


Some standard for the class II equipment? Unfortunately, some of the lines which need to be connected to it in order for it to function are required to be earthed for other reasons. The coax bringing broadband cable services into the house, for example.
But does the outer part of the connector need to be connected to the case of the equipment?
 
John, I have some prescription medication, the instructions for which advise me not to take it within 6 hours of taking a particular non-prescription item that I would suspect is in 99% of domestic medicine cupboards in the UK. Do I need to know/understand the reason for that instruction? Do I need to know if the risk is that a reaction between the two might kill me, or that one drug might stop the other from functioning?
If you are happy and comfortable to just blindly follow those instructions and/or you do not feel that you have appropriate background knowledge and abilities which would enable you to make any reasonable judgements or decisions even if you did know the answers, then, no, you don't need to know any of those things.
Wouldn't you agree that I would be foolhardy if I didn't follow the instructions supplied?
If you did not have enough knowledge and/or ability to assimilate the answers to those questions and make reasonable judgements about how that instruction applied to yourself, then, yes, it would be foolhardy not to follow the instructions.
Should I be nervous about taking such an item? After all, who knows what combinations the manufacturer might have overlooked?
That analogy does not really work with the example you've given. However, if I had some knowledge of the subject and the instructions/warnings led me to feel that it was referring to a risk/danger comparable with the possibility that a Class II electrical item might be so 'electrically dodgy' that it might "fail" if its outer casing (outside of its double/reinforced insulation) were in contact with earth then, yes, I would undoubtedly be very 'nervous' about taking it, wouldn't you? It's not a realistic analogy, since such a medicinal product would not be allowed, just as I feel should be the case with an allegedly Class II item which couldn't be guaranteed to 'withstand' coming into contact with earth.

Kind Regards, John
 
I have several decades of experience in the design, testing and assessment of electrical and electronic products, in automotive, commercial, medical and domestic applications. However I would not have the arrogance to assume I could know more about how to use a product safely than the manufacturer of that product.

Have you never found, for example, a printed circuit that has tracked between line and neutral or between line and earth after some years' use?
 
By not having any part of the signal connectors connected to any exposed conductive part(s) {.....}

Maybe no parts of the signal connectors are connected to any exposed conductive part. {.....}

IF the maker decides that the case of his equipment MUST NOT be earthed then he must either forbid any earthed signal leads or he must isolate the I/O sockets from the case.
Some have no DC continuity between the connectors and the casing, but there are plenty of others which do just have BNC's, B-L coax sockets etc. just bolted to the chassis.

But even in the former case, there is some capacitive coupling between connectors and chassis, and this is the cumulative effect with multiple such devices connected together which Winston first mentioned above and which I've also been addressing.

Maybe I should have said "possibly not allowed". According to the MI's. IF they say "this item must not be earthed" then according to them it must not be earthed either via a cpc in the power cable or by earthed conductors in the signal cables.
I don't think I can recall ever seeing such instructions with the sort of AV equipment we're talking about here.

But does the outer part of the connector need to be connected to the case of the equipment?
As above - Even if not connected so as to provide DC continuity, there is always some capacitive connection which will allow AC.
 
I have several decades of experience in the design, testing and assessment of electrical and electronic products, in automotive, commercial, medical and domestic applications. However I would not have the arrogance to assume I could know more about how to use a product safely than the manufacturer of that product.
I don't think it's necessarily arrogance - MIs are necessarily very conservative, and assume no knowledge (and usually assume a high degree of risk-aversity), so as to cover all possible situations and users, but only you know the exact circumstances of your use, and your view about risk, particularly if (as you do) you have some relevant knowledge.

In all those decades of experience, have you never come across manufacturers' instructions which were either (a) wrong, or (b) there primarily in an attempt to 'cover' the manufacturer (sometimes in relation to theoretical risks which are of little/no relevance or concern to many users) or (c) as above, instructions which, in the interests of simplicity and widespread applicability, are blanket generic ones designed to apply to as wide a range of possible situations as possible (many of which will not be relevant to particular users) ??

Your medication example is quite a good one. Instructions/warnings for medications such as you mentioned are, in the interests of simplicity (and 'CYA'!) quite often 'fail safe' simplifications of a much more complicated reality. If one does know and understand the answers to the questions you were asking about (the reason for the instruction, and the possible consequences of not obeying it etc.) one will often discover, for example, that the instruction is only relevant to a subset of the population, and if one knows that one is not in that subset, then complying with the instruction is therefore unnecessary. In other cases, an understanding of the reason etc. will enable one to know that the two medications may be taken closer together than the 'blanket instruction' suggests provided that the dose of one or both is adjusted accordingly.

The documentation ('instructions') for a medication provided for prescribers (as opposed to patients/users) includes two sections, one 'Contraindications' and the other 'Warnings/Precautions'. Only the former contains "thou shalt not" (or "shalt") statements; there are usually few, if any, of these, and they relate to situations which things almost universally ill-advised/dangerous. The later section is invariably much longer, containing explanations of the nature of possible problems/risks and, where appropriate, information about the types of patient in which they may be relevant, so that the prescriber can make a judgement in relation to the situation concerned. However, many/most of those will appear as simple "thou shalt not" or "thou shalt" statements in the instructions for patients.
Have you never found, for example, a printed circuit that has tracked between line and neutral or between line and earth after some years' use?
I have, but I'm not sure of the relevance to this discussion. Are you perhaps suggesting that one cannot rely on a Class Ii item to remain safe indefinitely?

Kind Regards, John
 
412. Protective Measures: Double or reinforced insulation.

412.2.2.4 {actual}No conductive part enclosed in the insulating enclosure shall be connected to a protective conductor.
{precis} However, a pc may run through the enclosure to supply other equipment. It shall be treated as a live conductor.

{actual} No exposed-c-p or intermediate part shall be connected to any protective conductor unless specific provision for this is made in the specification for the equipment concerned.
 
No, I'm stating that one cannot rely on any item to remain safe if it is not used as specified by the manufacturer.
Oh, I see - but you didn't say anything about it not being used as specified by the manufacturer!

Kind Regards, John
 
I don't think it's necessarily arrogance - MIs are necessarily very conservative, and assume no knowledge (and usually assume a high degree of risk-aversity), so as to cover all possible situations and users, but only you know the exact circumstances of your use, and your view about risk, particularly if (as you do) you have some relevant knowledge.
Exactly. We all must have seen instructions which are clearly aimed at the person who has practically zero technical knowledge, and no doubt most of us have ignored certain manufacturer's instructions because we have the knowledge to assess the situation for ourselves and decide that it's acceptable to do so (or in some cases even desirable to do so).

I would not presume to ignore something like maximum dosage instructions on medicines without advice from a doctor because I am not sufficiently qualified to do so.
 
412.2.2.4 ... {actual} No exposed-c-p or intermediate part shall be connected to any protective conductor unless specific provision for this is made in the specification for the equipment concerned.
Yes, I know, but I still don't really understand the 'why' (at least, electrically) - I remain of the view that if, for some reason, it's not safe to earth any exposed CPs, then I would not consider the item to be safe in the first place! As I've said before, I wonder if this is perhaps about practicalities - the fact that attempts to earth an exposed-c-p which had not been provided with a suitable terminal might result in damage to the item and/or problems in relation to how one would get a CPC to the exposed-c-p.

Whatever, an interesting point about this regulation is that it does ("unless ....") acknowledge that the exposed-c-ps of a Class II item can, if 'allowed' by the manufacturer, be earthed. The regs therefore do not say that such the exposed-c-p of a Class II item must never be earthed.

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