Thoughts on earth bonding on lights please

[EFLImpudence]

Have you not also been known to take that position in relation to similar issues?

Maybe, but in this case I realised that there may be no such thing as double insulated cable only and simply because we don't call it that.

Should a manufacturer start marking its T&E as double insulated then there will be - like electronic transformer and electric car.

 

[JohnW2]

Maybe, but in this case I realised that there may be no such thing as double insulated cable only and simply because we don't call it that. ... Should a manufacturer start marking its T&E as double insulated then there will be - like electronic transformer and electric car.

I don't know about the manufacturers but there is certainly such a thing as double insulated cable as far as many wholesalers and retailers are concerned - you can buy "double insulated singles" (or "double insulated meter tails") from TLC, CEF, QVS, Discount-Electrical, Edwardes, ElectricalDirect, RS and Screwfix, to name just the first few I found.

Kind Regards, John

 

[EFLImpudence]

Yes, I realise that but is it correct?

After all, they sell Low Voltage 12V lamps.

 

[JohnW2]

Yes, I realise that but is it correct? After all, they sell Low Voltage 12V lamps.

As I said before, it would seem to be 'correct' in terms of English. I think we can probably agree that the sheathing is made out of an 'insulating' (rather than 'conductive') material, so if there are two separate layers of 'insulating material', I would say that it's not unreasonable to call it "double insulated".

I have yet to see a technical or regulatory definition, anywhere, of a "double insulated cable" (the IEC Electrotechnical Vocabulary Dictionary does not have one). If there is not a technical/regulatory definition, then I suppose the question of whether a conductor covered with two layers of 'insulating material' can 'correctly' be described as "double insulated" really has to rely on English usage and dictionaries.

At a technical level, Risteard (and maybe others) seems to believe that the Standard for sheathing does not require it to pass any tests of its insulating properties. It would rather surprise me if that's true but, unless stillp (or someone) can help, we may never know. However, if Risteard is correct, then I suppose it might bring into question as to whether one can actually regard the sheath as 'insulating' - which, in turn, might be taken to mean that 'double insulated' was incorrect.

Kind Regards, John

 

[stillp]

I have yet to see a technical or regulatory definition, anywhere, of a "double insulated cable" (the IEC Electrotechnical Vocabulary Dictionary does not have one).

How about this? http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=195-06-08

I don't have many of the cable standards, but in the case of T & E, I believe Risteard is correct.

 

[JohnW2]

How about this? http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=195-06-08

Ah, thanks - I searched there for "double insulated" (rather than "double insulation"), and got no hits! That definition of 'double insulation' ("both basic insulation and supplementary insulation") doesn't really help very much. If the sheathing of cables counts as 'insulation', then I suppose "insulated and sheathed" conductors would qualify as having "double insulation", but we still don't know whether or not the sheathing qualifies/counts as 'insulation'.

I don't have many of the cable standards, but in the case of T & E, I believe Risteard is correct.

As I've said/implied more than once, I would be very surprised if it would be considered acceptable for sheathing of cables (that was not intended to be earthed) to be made out of a significantly conductive material, so I find it a little difficult to see how a Standard for such sheathing could not contain at least some requirements in terms of 'insulating properties'. If I'm wrong, I'll still be surprised

Kind Regards, John

 

[stillp]

I find it a little difficult to see how a Standard for such sheathing could not contain at least some requirements in terms of 'insulating properties'

Why would the standard include insulation requirements for something that is not specified to be an insulator? (If indeed it isn't, as Risteard and I believe).

 

[JohnW2]

Why would the standard include insulation requirements for something that is not specified to be an insulator? (If indeed it isn't, as Risteard and I believe).

IF you (and Risteard) believe that, if it were otherwise compliant, a sheath made of conductive material would be compliant with the relevant Standard for a cable sheath, then there obviously would not be a need for such a requirement. However ..... !!

Kind Regards, John

 

[stillp]

Well, the full title of BS6004 is "Electric cables. PVC insulated and PVC sheathed cables for voltages up to and including 300/500 V, for electric power and lighting" so we know the sheath is PVC, and that the standard refers to it as a sheath rather than a secondary layer of insulation. I suppose it could be loaded with carbon to make it slightly conductive, if that was thought necessary. However I suspect that the insulation uses a grade of PVC that meets the requirements for insulation, whereas the PVC used for the sheath is optimised for resistance to the foreseeable environmental influences, including abrasion during installation.

 

[EFLImpudence]

Do you think I was correct earlier when I wrote that the only requirement for conductors not in containment is that they be "insulated and sheathed" and there is nowhere any requirement for "double insulated" conductors?
"Double insulated" being a term reserved for Class II items.

I do think that -
"double insulation

insulation comprising both basic insulation and supplementary insulation"
is not really a definition but merely an obvious description.

Having said that, I must say that surely sheathing, however you look at it, is supplementary(additional) insulation and therefore the difference in terms is somewhat fatuous.

 

[JohnW2]

Well, the full title of BS6004 is "Electric cables. PVC insulated and PVC sheathed cables for voltages up to and including 300/500 V, for electric power and lighting" so we know the sheath is PVC, and that the standard refers to it as a sheath rather than a secondary layer of insulation.

Fair enough = but a I wrote early, it seems that the sheaths of what are widely sold as "double insulated meter tails" usually specify that the sheathing is to BS EN 50363-3.

I suppose it could be loaded with carbon to make it slightly conductive, if that was thought necessary.

Do you really think that would be regarded as acceptable? If it were 'equipment', rather than cable, I would think that an outer touchable surface which was only 'slightly conductive' would be still regarded as an exposed-c-p and would therefore have to be earthed unless there were 'double or reinforced insulation' between it and live conductors.

However I suspect that the insulation uses a grade of PVC that meets the requirements for insulation, whereas the PVC used for the sheath is optimised for resistance to the foreseeable environmental influences, including abrasion during installation.

Yes, I've suggested more-or-less that earlier in the discussion - except that I was suggesting that the relevant Standard might require sheathing to have specific mechanical properties in addition to a requirement for an adequate degree of electrical insulation.

Kind Regards, John

 

[securespark]

Just to throw a historic spanner in the works...

From my newly-acquired but very old 13th Edition of 1955, the definitions in the front carry this:

Insulation, Double

(1) of a conductor.
A conductor is said to have double insulation when insulating material intervenes not only between the conductor and its surrounding envelope (if a cable) or immediate support (if bare), but also between the envelope or support and earth.

Now, that is both tricky to interpret and ambiguous....

 

[stillp]

Having said that, I must say that surely sheathing, however you look at it, is supplementary(additional) insulation and therefore the difference in terms is somewhat fatuous.

I understand, but although the sheath is PVC, unless it meets specified requirements that make it suitable insulation for working voltages of 300/500V, then it isn't supplementary insulation. The definition makes more sense if akk the terms are replaced by their definitions, but cannot make complete sense without looking at the requirements for basic insulation and for supplementary insulation for the product in question.
Supplementary insulation is not just additional insulation, it is "independent insulation applied in addition to basic insulation, for fault protection"

 

[JohnW2]

Do you think I was correct earlier when I wrote that the only requirement for conductors not in containment is that they be "insulated and sheathed" and there is nowhere any requirement for "double insulated" conductors? "Double insulated" being a term reserved for Class II items.

Yep, but that in no way necessarily means that it is 'incorrect' to talk of "insulated and sheathed" conductors as being "double insulated"

I do think that - "double insulation insulation comprising both basic insulation and supplementary insulation" is not really a definition but merely an obvious description.

Quite so - and it seems unlikely that it was ever intended that that definition applied to cables.

Having said that, I must say that surely sheathing, however you look at it, is supplementary(additional) insulation and therefore the difference in terms is somewhat fatuous.

That's what I thought, and what I have been saying. However stillp has just suggested that it could even be the case that the material was deliberately made 'slightly conductive' yet still be acceptable as sheathing - if that were the case, I don't think one could call it "insulation", supplementary or otherwise, could one?

Kind Regards, John

 

[stillp]

"Double insulated" being a term reserved for Class II items.

No, double insulation is one means of achieving the Class II requirements.