Can I use a live or neutral coloured cable with earth sleeving?

[JohnW2]

... you should know that cables are not "double insulated" - the tails you refer to are insulated and sheathed.

Since you often raise this point, I've been thinking about it. If a bare live conductor was within an enclosure which didn't have an earthed conductive covering, there would be a requirement for it to be protected by 'double (or reinforced) insulation'. Are you therefore suggesting that the requirement for cables is less demanding than for enclosures - i.e. are you actually certain that there is not a requirement for the sheath to qualify as 'insulation'?

Kind Regards, John

 

[securespark]

There is a requirement for accessible live conductors (eg tails) to have two coverings of PVC. Tails that have the outer removed to reveal the inner are non-compliant.

You say they are insulated and sheathed, but to all intents and purposes, they are double insulated to make contact with the inner copper core extremely unlikely.

If the current arrangement was deemed insufficient for the role of insulation, tails would have to be constructed differently.

 

[ban-all-sheds]

I thought they were all banned?

Kind Regards, John

They won't listen to me.

 

[JohnW2]

You say they are insulated and sheathed, but to all intents and purposes, they are double insulated to make contact with the inner copper core extremely unlikely.

Indeed - you and I have said virtually the same thing.

If you recall, last time we tried (with stillp's help) to find the specification of the required insulating properties of the cable sheaths, we failed. However, I would suggest that it is fairly obvious that there must be a requirement for them to have 'insulating properties' and we simply don't know the required specification of those properties, or how it compares with the corresponding specification for the inner layer of insulation. Let's face it, there is no way that it would be permissible for the outer sheath to be made from an (unearthed) significantly conductive material.

Kind Regards, John

 

[ban-all-sheds]

AFAICT, the following are facts.

1. Some manufacturers use the term "double insulated" to describe 6181X or 6181Y cables.
2. Some manufacturers use the term "insulated and sheathed" to describe 6181X or 6181Y cables.
3. Some manufacturers use both terms to describe 6181X or 6181Y cables.
4. There is no formal definition of "double insulated" wrt cables.

 

[JohnW2]

AFAICT, the following are facts. ....

Yes, that all corresponds with my understanding, too.

As I've said, I think we can assume that the outer layer of material, whatever one chooses to call it, cannot be allowed to have significant conductivity - so the whole debate is about what degree of insulating properties (i.e. how low a conductivity, and how high a breakdown potential) it is required to have.

I have to say that I find it difficult to see why the 'insulating properties' requirements should really differ between the two. After all, I imagine that it would be difficult to produce anything made out of any sort of PVC (unless there were deliberate conductive additives!) that was not satisfactory for providing insulation at 230V or 400V!

Conversely, I could believe that, as well as having similar insulating properties to those of the inner insulation, there might be additional mechanical property requirements for the outer covering.

Kind Regards, John

 

[ban-all-sheds]

In terms of material properties, indeed not, but it would be easy to make something out of PVC which was too thin to provide satisfactory insulation at those voltages.

Polyethylene is a good electrical insulator, but when it's only 8-10μm thick I wouldn't fancy a layer of it as insulation around a LV cable....

 

[JohnW2]

In terms of material properties, indeed not, but it would be easy to make something out of PVC which was too thin to provide satisfactory insulation at those voltages.

I'm not certain what you are saying. Are you perhaps saying that PVC is such a good insulator that an incredibly thin layer would initially provide (electrically) adequate insulation, but that it would be so fragile that it could not be trusted to maintain its physical integrity (hence insulating properties) in use?

Kind Regards, John

 

[ban-all-sheds]

Basically I'm saying that even if the material is an insulator, there will be a minimum thickness required at any given voltage (e.g. I'd not like to clap hold of an HV cable with nothing but a covering of 1 layer of clingfilm), and as you point out, there is mechanical strength to consider.

Accordingly it is possible that the sheath on an insulated and sheathed cable could be too thin to be classed as an insulator for it.

IHNI if this is the case with 6181X or 6181Y, but if it is it might explain why some people claim that that cable is not DI.

 

[stillp]

it might explain why some people claim that that cable is not DI

A better explanation might be that there is no definition of DI for cables, as you wrote above, therefore it would be meaningless to claim that a cable is DI.
Remember we are not talking about 'pure' PVC, rather a mixture of PVC and various additives. The cores will be coated with PVC which has additives that are optimised for insulating properties, compatibility with copper(remember the green goo), and strippability, the outer sheath will be made of PVC with additives to enhance its mechanical, thermal and environmental properties.

 

[JohnW2]

Basically I'm saying that even if the material is an insulator, there will be a minimum thickness required at any given voltage (e.g. I'd not like to clap hold of an HV cable with nothing but a covering of 1 layer of clingfilm), and as you point out, there is mechanical strength to consider.

I would be hesitant to clap hold of it, too, but probably only because of my concerns about the mechanical properties - i.e. cling film is so fragile that I would not be confident that it was totally intact!

Following some very similar discussions, I actually 'did the very experiment' some time ago. I clamped a bit of cling film (as well as various other materials) between two copper plates and ID'd at 1000V - and the result was 'off the top of the scale' - so, provided the cling film is intact, it seems to be fine as indulation. I've just tried to repeat that experiment, just to be sure, but find that my MFT is apparently ill (see separate thread)!

Accordingly it is possible that the sheath on an insulated and sheathed cable could be too thin to be classed as an insulator for it.

Yes, that's a possibility, but is it not usually the case that the 'sheaths' (be it of T+E, 6181Y or whatever) is at least at thick as the insulation around the core?

Kind Regards, John

 

[JohnW2]

A better explanation might be that there is no definition of DI for cables, as you wrote above, therefore it would be meaningless to claim that a cable is DI.

Yes, that's a fair statement. Mind you, I'm not sure that I've ever seen a definition of DI (in terms of the properties of the materials used) in relation to enclosures (Class II equipment, etc.), either.

Remember we are not talking about 'pure' PVC, rather a mixture of PVC and various additives. The cores will be coated with PVC which has additives that are optimised for insulating properties, compatibility with copper(remember the green goo), and strippability, the outer sheath will be made of PVC with additives to enhance its mechanical, thermal and environmental properties.

All true, but as I keep saying, even though we have so far failed to find them, there surely must be requirements in relation to the insulating properties of the sheath - since it would make no sense for it to be permissible to use a significantly conductive material.

Kind Regards, John

 

[stillp]

Mind you, I'm not sure that I've ever seen a definition of DI (in terms of the properties of the materials used) in relation to enclosures (Class II equipment, etc.), either.

DI is defined in terms of the behaviour of the equipment, not the properties of the materials used.

there surely must be requirements in relation to the insulating properties of the sheath - since it would make no sense for it to be permissible to use a significantly conductive material

What about SWA?
If there are no requirements for the sheath to have insulating properties, than it would be permissible for the sheath to be conductive, provided that the cable met the overall requirements for insulation between the cores and the outside environment.
However, I can't think of any material distinctly different from PVC that would be suitable for use as the sheath, regardless of its conductivity. It has to be flexible, resistant to moisture, easily extruded, strippable yet meet the requirements for mechanical protection, compatible with the inner core insulation... sounds a lot like PVC to me!

 

[JohnW2]

DI is defined in terms of the behaviour of the equipment, not the properties of the materials used.

Could you perhaps expand on that a bit. In common sense terms, I think we understand what DI means in that context but, in a formal sense, it presumable depends upon a definition of the "I", doesn't it?

What about SWA?

Sorry, unlike the previous times I've written much the same in this thread, I omitted to include the qualifier "unearthed". I presume that a cable consisting of a 'single insulated' core surrounded just by an unearthed metallic 'sheath' would not be acceptable?

If there are no requirements for the sheath to have insulating properties ...

As I've said, I'm still doubtful that such is the case

.... than it would be permissible for the sheath to be conductive, provided that the cable met the overall requirements for insulation between the cores and the outside environment.

Are you perhaps suggesting that, analogous to the "reinforced" of "double or reinforced insulation", it would be acceptable to have just a 'single-insulated' conductor if the single layer of insulation was thick and strong (etc.) enough?

Kind Regards, John

 

[stillp]

Could you perhaps expand on that a bit. In common sense terms, I think we understand what DI means in that context but, in a formal sense, it presumable depends upon a definition of the "I", doesn't it?

Typically it would be an impulse voltage test and a power-frequency test between the conductors of the EUT and a layer of foil wrapped around the enclosure. No requirements at all for the materials used.

I'm still doubtful that such is the case

Well, go and find them then!

Are you perhaps suggesting that, analogous to the "reinforced" of "double or reinforced insulation", it would be acceptable to have just a 'single-insulated' conductor if the single layer of insulation was thick and strong (etc.) enough?

Acceptable to whom, and for what purpose?
In some documents a single layer is sufficient, if it meets the provisions of the relevant standard.