More dodgy tat

[JohnW2]

Hypothetical question. A metal cased Class II ( double insulated ) fitting which manufacturer states "must not be earthed" had to be fitted to a piece of structural metal work which is connected to earth. ... Does one obtain and fit a mounting kit that isolates the fitting from the structural metal work ?

As I've said, in electrical terms that would really make no sense.

It may be that the manufacturers can see a situation where a failure in the double insulation would make the case live. If that happened and if the case was connected to "an earth wire " that was not earthed then all other metal work connected to that "earth wire" would become live.

For a start, "must not be earthed" would not really stop one connecting it to "and earth wire that was not earthed". However, more to the point, if they are envisaging a situation in which the double insulation 'fails' and results in a metal casing/whatever of the product becoming live, then I would (a) seriously question whether it really was Class II and, as a consequence, (b) would definitely want that metal casing earthed if they were telling me that it might become live!

Kind Regards, John

 

[SimonH2]

A metal cased Class II ( double insulated ) fitting which manufacturer states "must not be earthed" had to be fitted to a piece of structural metal work which is connected to earth.

Had to be?
Are you then not going against the manufacturers instruction and the principle of class 2 and the reason for it?
...

In this situation, what possible risk is there in earthing a piece of DI equipment by fixing it to a piece of steelwork which is already earthed ?
If the equipment really is DI, and that DI doesn't fail, then there is no problem created. If the DI does fail, then it should cause the OPD (and/or RCD) in the supply to trip removing what would be a hazardous situation.

If earthing the equipment casing creates any problem (related to the supply and safety*) then it cannot possibly have been truly DI.


* Now, I have actually heard of an issue - functional, not safety. The specific case was an installation of WiFi access points (APs) onboard a yacht - sadly not by me The APs were, naturally enough, simply fixed to bulkheads - but then nothing worked. The case of the APs was connected to the Power over Ethernet supply, and bolting them to the bulkhead shorted out the PoE as the cases were at -48V (ish).

 

[JohnW2]

I suspect that in a good few case we are simply talking about ignorance on the part of those who wrote the instructions.

The instructions are part of the equipment. So what does such ignorance say about the quality of engineering of the rest of the equipment ?

One might ask that. However, it seems that those responsible for the quality of the equipment may sometimes have little/no control over the instructions ...

... you will recall my conversation with a tech guy at MK about the 'current ratings' of their double sockets. He agreed with me that what their 'Technical Data Sheet' said made no real sense (certainly no clear sense) but explained that most such 'Technical Data Sheets" were written by the marketing department and, although he could 'make suggestions' to them, he and his colleagues had no direct control over what got written!

Kind Regards, John

 

[JohnW2]

A metal cased Class II ( double insulated ) fitting which manufacturer states "must not be earthed" had to be fitted to a piece of structural metal work which is connected to earth.

Had to be? Are you then not going against the manufacturers instruction and the principle of class 2 and the reason for it?

Does one obtain and fit a mounting kit that isolates the fitting from the structural metal work ?

I would say yes - or fit it somewhere else or use a class 1 product.

It seems that there is again confusion between 'need not be earthed' and 'must not be earthed'. If a product really is Class II, then it surely should/does not matter what any exposed metal casing/covering/parts are, or are not, connected to - be that ' earth'or anything else.

If it were in some way 'dangerous' for exposed metal parts of a Class II item to be earthed (which it's not, as far as the product is concerned), then it surely would not be safe to go anywhere near its exposed metal parts (in which case it surely could not be Class II)?

Does not the definition/specification simply mean that it is not necessary (but would no harm) to earth its exposed conductive parts in order to achieve the required degree of protection against the risk of electric shock?

Kind Regards, John

 

[EFLImpudence]

It seems that there is again confusion between 'need not be earthed' and 'must not be earthed'. If a product really is Class II, then it surely should/does not matter what any exposed metal casing/covering/parts are, or are not, connected to - be that ' earth'or anything else.

No, but you are defeating the object of making class 2 products.
They are deemed safe without earthing the exposed-c-ps.
Its e-c-ps will not become live through a fault at the item nor anywhere else.
Earthing is not a good thing in itself; it is a necessary evil.

If it were in some way 'dangerous' for exposed metal parts of a Class II item to be earthed (which it's not, as far as the product is concerned), then it surely would not be safe to go anywhere near its exposed metal parts (in which case it surely could not be Class II)?

I don't understand that.
It is no more dangerous to earth a class 2 item than a class 1.
It is safer not to just as it would be better if all metal were isolated.
It would be the same as earthing the proverbial spoon; it is not necessary.

Does not the definition/specification simply mean that it is not necessary (but would no harm) to earth its exposed conductive parts in order to achieve the required degree of protection against the risk of electric shock?

Have I answered that already?

 

[ericmark]

I am sure there is no problem with a light fitting. But I have seen some dangerous situations where class I items have been used in the same area as arc welding.

So if we look at that mag mount drill welding could cause earth wires to melt and these earth wires could then connect to one of the live wires.

If you get a live to earth fault in this case with a split 110 volt supply fed from a MCB in the transformer then it fails. But from a yellow brick it will likely not fail due to the volt drop in the extension leads. Until that is the extension lead melts.

At which point it fails but has likely done damage to earth wires galore and maybe more than one extension lead. With luck the leads are all cut up and binned but often one escapes and it is like a disease going through the building site.

It is no joke to open 100 extension leads and check of signs of over heating. Neither is it fun renewing all the earth wires in tray work or trunking which have been damaged in the first instance by a welder.

I will admit the first time I saw a class II drill I wanted to swap the cable and earth it. But be it a drill or hand lamp there are good reasons why it's not earthed. The problem is until it is too late and damage has resulted we tend not to realise the dangers.

I have table lamps made of metal which are not only class II but would fail to work if the metal was earthed. The touch control depends on there no being an earth.

I am not saying after careful consideration one should not fit earth cables but it does need careful consideration. For another example earthing the braid on a TV coax may seem a good idea until one considered this may encourage a lighting strike on the aerial and cause untold damage as a result.

As to German equipment one has to be super careful as items designated for export don't need to comply with German regulations we had this with a batching plant it would seem it was built to be exported to Africa but ended up in the UK. There was a whole list of things which did not comply with EEC regulations.

This plant had an IT supply to all control gear with an earth monitor. We did not like the idea but were stuck with it. The UK system would have also included an earth monitor with an array of lights to show how much earth leakage this system had just a single lamp.

Adding any earth wire would have removed the whole safety system it would have needed a full re-wire to change it.

I am sure many others can report where earthing systems did not seem right but it was a major problem to change them. We had around 50 portable flood lights with 110 volt outlets on them as well. I found the 110 was 0 - 110 not 55 - 0 - 55 the lights were fed with 110 - 0 - 110 as a result all the 110 volt outlets had to be blanked off. We clearly do get imported goods which do not comply. In that case they came from USA and the electrician came from New Zealand and had no idea the sets did not comply. Only when we went to help him was the problem found.

All I am saying is don't just jump in and start earthing items without considering why they are not earthed. If the answer is they are non compliant with British rules the big question is.

Should we modify?
Should we return as fault goods?

To me if there is a good reason like being designed for another country then may be we should modify. But if just cheap tat then unless we keep returning it the supplier will continue to supply cheap tat.

In the case of the lighting sets the supplier had no idea they did not comply with UK standards. It did result in no more being imported by that supplier. Not only was it 0 - 110 it was also 60 Hz.

 

[JohnW2]

No, but you are defeating the object of making class 2 products. ... They are deemed safe without earthing the exposed-c-ps. ... Its e-c-ps will not become live through a fault at the item nor anywhere else. ... Earthing is not a good thing in itself; it is a necessary evil.

All agreed - but in bernard's hypothetical example, the Class II item was going to get earthed 'by accident', and additional effort/hassle would have been required to avoid it getting earthed. Certainly not necessary (if it really is Class II), but nor something which should/must not be done (other than to reduce the amount of earthed metal around)

If it were in some way 'dangerous' for exposed metal parts of a Class II item to be earthed (which it's not, as far as the product is concerned), then it surely would not be safe to go anywhere near its exposed metal parts (in which case it surely could not be Class II)?

I don't understand that. It is no more dangerous to earth a class 2 item than a class 1.

Exactly. If the manufacturer's "must not earth" had some safety implication, I don't really know what it would be, and it would certainly not be Class II.

It is safer not to just as it would be better if all metal were isolated. It would be the same as earthing the proverbial spoon; it is not necessary.

Sure, as I've acknowledged above, and in my previous post, the only argument for 'not earthing' is simply to reduce the amount of earthed metal in the house. However, that is surely not a reason for a manufacturer saying their product "must not be earthed". If one were concerned about the amount of earthed metal in a house, one would have to start by addressing the major culprits - all the pipework, CH radiators etc..

Does not the definition/ specification simply mean that it is not necessary (but would no harm) to earth its exposed conductive parts in order to achieve the required degree of protection against the risk of electric shock?

Have I answered that already?

I'm not sure. As I've said, apart from the general desirability of reducing the amount of exposed earth metal in a house, an item being Class II means that it dos not need earthing, but that earthing would 'do no harm' - so "must not earth" seems totally inappropriate.

In practice, I can think of only two situations in which one would normally contemplate earthing exposed metal parts of a "Class II" item ... Firstly, as in bernard's hypothetical example, if the earthing was going to happen 'by accident', and one didn't see the point in expending effort to avoid it getting earthed. Secondly(per recent threads) if one didn't relieve believe/trust the fact that the item was satisfactorily 'Class II' (although one really ought to simply 'reject' such a product and get a proper Class II replacement).

Kind Regards, John

 

[winston1]

For another example earthing the braid on a TV coax may seem a good idea until one considered this may encourage a lighting strike on the aerial and cause untold damage as a result.

Aerials should be earthed. It does not encourage lightning strikes, it reduces the likelihood by discharging the ionised air. That is how lightning conductors work.

 

[bernardgreen]

I have invented Zh Impedance of a circuit that includes any part of a human body.

If the value of Zh is too high to ensure an RCD will trip in the required time then Zh is too high for safety.

A person touching the metal case of a class II item where the double insulation has failed ( moisture ingress for example ) will be connected to Live. If they are not touching anything grounded ( or earthed ) they will be Live but no current will flow through them so the RCD will not trip. Zh is very high close to infinity

But if they have a connection to something that has a path to ground or earth or CPC then some current will flow through them. If the current is higher than the trip current of the RCD it will ( should ) trip before they suffer irreversible injury. Zh is low enough to ensure the RCD trips.

But if Zh is too high to ensure the RCD will trip then this current will continue to flow through the person until they are somehow disconnected from the Live ( or from ground at the other end. ( birds on 11Kv cables are safe even though very Live )

Double insulation can fail and exposed metal can become Live. Two causes of failure are moisture ingress or melted insulation when overheated due to a poor contact or a lamp of the wrong wattage. A third cause is sheer bad cheap and thoughtless design.

 

[JohnW2]

I am sure there is no problem with a light fitting. But I have seen some dangerous situations where class I items have been used in the same area as arc welding. ... So if we look at that mag mount drill welding could cause earth wires to melt and these earth wires could then connect to one of the live wires.

There are obviously some situations in which one needs an 'earth-free area', but that is very different from what we are discussing here.

I am not saying after careful consideration one should not fit earth cables but it does need careful consideration. .... All I am saying is don't just jump in and start earthing items without considering why they are not earthed.

True - but, has been discussed, I think the first people who need to 'consider why they are not earthed' are the manufacturers who write these "must not be earthed" instructions. As discussed, in the vast majority of cases there is probably no electrical reason why they should not be earthed (although they do not need to be earthed). However, I agree that it will rarely be necessary to even consider earthing a Class II item. As I very recently wrote:

In practice, I can think of only two situations in which one would normally contemplate earthing exposed metal parts of a "Class II" item ... Firstly, as in bernard's hypothetical example, if the earthing was going to happen 'by accident', and one didn't see the point in expending effort to avoid it getting earthed. Secondly(per recent threads) if one didn't relieve believe/trust the fact that the item was satisfactorily 'Class II' (although one really ought to simply 'reject' such a product and get a proper Class II replacement).

Kind Regards, John

 

[JohnW2]

I have invented Zh Impedance of a circuit that includes any part of a human body. ... A person touching the metal case of a class II item where the double insulation has failed ( moisture ingress for example ) will be connected to Live. If they are not touching anything grounded ( or earthed ) they will be Live but no current will flow through them so the RCD will not trip. Zh is very high close to infinity ... But if they have a connection to something that has a path to ground or earth or CPC then some current will flow through them. If the current is higher than the trip current of the RCD it will ( should ) trip before they suffer irreversible injury. Zh is low enough to ensure the RCD trips. ... But if Zh is too high to ensure the RCD will trip then this current will continue to flow through the person until they are somehow disconnected from the Live ( or from ground at the other end. ( birds on 11Kv cables are safe even though very Live )

No argument with any of that - I think that most of us are familiar with all this pretty basic stuff.

Double insulation can fail and exposed metal can become Live.

Of course - anything can fail. However, I'm not sure what you're suggesting - that we should ban Class II items with metal exposed parts? .. that we should require that exposed metal parts of Class II items should be earthed? ... that we should simply ban all Class II items and require anything electrical to be enclosed in an earthed metal casing/enclosure? ... or what?

Kind Regards, John

 

[bernardgreen]

that we should simply ban all Class II items and require anything electrical to be enclosed in an earthed metal casing/enclosure? ... or what?

If there is no CPC to the location then ONLY Class II can be used.

If there is an effective CPC to the location then Class I and Class II can be used and that the exposed metal of the Class II can be connected to the CPC as an extra layer of safety.

 

[JohnW2]

that we should simply ban all Class II items and require anything electrical to be enclosed in an earthed metal casing/enclosure? ... or what?

If there is no CPC to the location then ONLY Class II can be used.

Well, that's a statement about practicalities - but I ask you again whether you think they should be 'banned' - since you seem to be concerned that double insulation can fail, leaving no protection in the absence of an earthed metalenclosure.

If there is an effective CPC to the location then Class I and Class II can be used and that the exposed metal of the Class II can be connected to the CPC as an extra layer of safety.

Well, yes, that's essentially my position - but, again, given your concerns about the failure of DI, do you think that it should be required that any exposed metal parts of a Class II item should be earthed??

The interesting thing is that discussion only makes sense in terms of Class II items which have exposed metal parts. Most Class II items do not have exposed metal parts - so I'm not sure where that leaves your concerns about the DI (the only possible protection against electric shock in that case) 'failing'. I suppose it takes me back to my first question above, asking whether you feel that Class II items should be banned and that anything electrical (including all accessories?) be within an earthed metal enclosure? ... but, there again, even that earthing could 'fail' ... so I'm not sure I understand exactly how far you would want your ultra-pessimistic (aka ultra-cautious) viewpoint to go.

Kind Regards, John

 

[bernardgreen]

Class II equipment purchased from reputable manufacturers and then properly installed and used is, in my opinion, safe without any external metal needing to be earthed. Earthing it does no harm.

So called "Class II" equipment from non reputable ( even criminally negligent ) manufacturers carries an un-acceptably high risk of failure of the insulation.

Perhaps all imported "Class II" equipment should be more stringently monitored and controlled. And UK manufacturers with less than perfect designs be made to improve.

 

[JohnW2]

Class II equipment purchased from reputable manufacturers and then properly installed and used is, in my opinion, safe without any external metal needing to be earthed. Earthing it does no harm.

Agreed,on both counts - for anything which really is Class II (i.e. compliant with requirements for Class II).

So called "Class II" equipment from non reputable ( even criminally negligent ) manufacturers carries an un-acceptably high risk of failure of the insulation.

Yes, but now you're just talking about poor quality (possibly illegal) products, and ones which don't comply with Class II requirements. Poor quality products can be dangerous, whether Class I, Class II or whatever. That is a totally different issue, and totally unrelated to the question being discussed about whether "must not be earthed" instructions are often, if ever, justified in relation to products which really are Class II.

Kind Regards, John