I damaged my meter tails

[JohnW2]

Double insulated isn't the same as insulated and sheathed cable

The main issue is clearly a semantic/terminological one, since "double insulated" is very widely used to refer to cables which,strictly speaking, should be described as "insulated and sheathed".

However, as regards the "strictly speaking", I've never really got to the bottom of what this is all about. For starters:

. Is it really true that the (electrical) characteristics of the material used for the sheath of 'insulated and sheathed' cables
. means that it does not qualify as 'insulation?

. Do ('strictly speaking') "double insulated" cables actually exist and, if so, why does the outer layer of insulation not
. qualify as a 'sheath'?

Kind Regards, John

 

[RandomGrinch]

There's only one layer of insulation. The other layer is a sheath which is solely for mechanical protection, although it may be made of insulating material.

Ignoring the difference between 'double insulated' the definition, and having two layers of insulation...

Hypothetically, if we take T&E, wouldn't the outer 'sheath' be equally as insulating as the inner ones?
If not, then the earth conductor wouldn't be insulated?
Hence T&E can't by its nature be 'double insulated', but it could be 'insulated and sheathed'.

In the case of the meter tails, as with nearly every appliance lead, why can't they be 'double insulated', if the material of the 'sheath' is the same as the material of the 'insulation'?

I leave myself open to the wrath of the forum!

Edit: didn't see John's post before posting this - I'm absolutely sure his argument will be far more eloquent and informed than mine!

 

[JohnW2]

Hypothetically, if we take T&E, wouldn't the outer 'sheath' be equally as insulating as the inner ones?

IF the outer sheath were made of the identical material to the insulation of the inner cores, and it it were at least as thick as the insulation of the cores, then I cannot see how it could possibly be said that the conductors were not "double insulated" - hence my first question (about the material of the sheath).

Are we perhaps looking at this the wrong way around? Is it perhaps not disputed that an insulated and sheathed cable IS 'double insulated', but the material used for insulation of the inner cores may not provide adequate mechanical protection for use as an external sheath?

If not, then the earth conductor wouldn't be insulated?

True, but I'm not sure that there is a requirement for earth conductors to be insulated. In the distant past, they certainly were not necessarily insulated (I have some historical evidence in dusty corners of my house/roof!)

In the case of the meter tails, as with nearly every appliance lead, why can't they be 'double insulated', if the material of the 'sheath' is the same as the material of the 'insulation'?

Quite - so, as I said, this might all be down to a question of the materials - in particular (perhaps!) that a material providing adequate (electrical) insulation does not necessarily provided the required degree of 'mechanical protection' required for a 'sheath'?

I leave myself open to the wrath of the forum! Edit: didn't see John's post before posting this - I'm absolutely sure his argument will be far more eloquent and informed than mine!

I very much doubt that - but we are essentially presenting the same points and asking the same questions!

Kind Regards, John

 

[DetlefSchmitz]

In my experience, the term 'double insulation' arose from the specifications of classes of appliances. Class II was described as using double or reinforced insulation. In this case it does not (necessarily) mean that the appliance is wired with "double insulated" wire so much as that there is more insulation between the conductor and any metal parts than a single layer of insulation on a wire. It may be a big block of plastic or anything appropriate. People then started using the expression 'double insulated' to refer to cable with more than one layer of insulating material. It is not really correct because the term was coined to relate to an appliance and not to a piece of wire. So when people say it's not correct for a piece of wire that is the real reason, even though both layers are insulating. They just may not realise this is why they respond in this way.

 

[RandomGrinch]

Are we perhaps looking at this the wrong way around? Is it perhaps not disputed that an insulated and sheathed cable IS 'double insulated', but the material used for insulation of the inner cores may not provide adequate mechanical protection for use as an external sheath?

Thank you, good point, I had never considered the argument that way around

People then started using the expression 'double insulated' to refer to cable with more than one layer of insulating material. It is not really correct because the term was coined to relate to an appliance and not to a piece of wire

I'm afraid I am guilty of this. I know the meaning of the term, but wearing my PAT tinted specs, it didn't seem logical that a class II appliance wouldn't have at least a 'double insulated' cable - given that the cable was the part most likely to be damaged and that finding the inner cores exposed in the cable, would result in a PAT fail!

 

[JohnW2]

In my experience, the term 'double insulation' arose from the specifications of classes of appliances. Class II was described as using double or reinforced insulation... People then started using the expression 'double insulated' to refer to cable with more than one layer of insulating material. It is not really correct because the term was coined to relate to an appliance and not to a piece of wire. ...

I don't think the fact that the phrase was first "coined to relate to an appliance and not to a piece of wire" precludes it's use to also refer to other things that also have 'two layers of insulation'.

As I have implied, I find it rather hard to believe that the sheath of an 'insulated and sheathed' does not have the characteristics which qualify it as (electrical) "insulation" and if that is true, I see nothing 'incorrect' in describing such a cable as "double insulated".

As I've said, if that is the case (i.e. the cables are correctly describable as 'double insulated') then the only other thing I can think of is that the material used for electrical insulation is not deemed to be (mechanically) adequate to provide the degree of mechanical protection required of the 'outer layer'. However, that's highly speculative, and seems pretty improbable to me.

Kind Regards, John

 

[JohnW2]

Thank you, good point, I had never considered the argument that way around

Nor had I until just before I wrote it!

I'm afraid I am guilty of this. I know the meaning of the term, but wearing my PAT tinted specs, it didn't seem logical that a class II appliance wouldn't have at least a 'double insulated' cable - given that the cable was the part most likely to be damaged and that finding the inner cores exposed in the cable, would result in a PAT fail!

I probably should have added this to my recent response to Detlef .... I personally don't think it is sensible or appropriate for a discipline or industry to take an English phrase, ascribe a specific meaning to it and then claim that the 'everyday English use' is no longer 'correct'. In terms of everyday English, I would say it's fairly clear what is meant and understood by 'double insulated' (whether in relation to the cores of a cable or live parts within a Class II item) - just as it is clear what is meant by "double cooked" (as in chips ) - and I don't think that an 'industry definition' should over-ride that!

Having now thought of the possibility, the question which now interests me most is whether the material used as (electrical) insulation of cores of a cable has the properties required for it to give adequate mechanical protection when used as the 'outer layer' of a cable. IF it does, and IF the material of the sheath is usually the same as that of the core insulation, then I would suggest that we should stop being silly (and quibbling!) and accept that "double insulated" and "insulated and sheathed" are essentially synonymous! ... but that's just my view!!

Kind Regards, John

 

[RandomGrinch]

Interesting quote from a manufacturer of meter tails:

"As you can see from the cable construction table, these double insulated cables are more accurately insulated and sheathed cables."

The material of the sheath and insulation is the same, and perhaps your mechanical strength supposition is correct

6181Y (Meter Tails) Cable | Eland Cables

View Eland Cables' comprehensive range of 6181Y (meter tails / double insulated cables) manufactured in accordance with BS6004. Technical support, fast quote and fast delivery.

www.elandcables.com

 

[JohnW2]

The material of the sheath and insulation is the same, and perhaps your mechanical strength supposition is correct

I don't really get that, since if the materials are identical, my supposition would surely be incorrect, and "double insulated" and "insulated and sheathed" would be synonymous, wouldn't they?

Kind Regards, John

 

[Jackrae]

In the simple industrial world I lived in a 'sheath' was a means of protection, be it a condom, armour wire or braided material (metal or fabric). Double meant "two" and insulation meant "non-conducting". So a cable with an outer sheath was a cable with some form of mechanical protection that was not considered as part of the insulation properties. A cable that had two layers of insulation was considered double insulated.
A cable with two layers of insulation and a sheath was a 'double insulated, sheathed cable', etc....

 

[RandomGrinch]

I don't really get that, since if the materials are identical, my supposition would surely be incorrect, and "double insulated" and "insulated and sheathed" would be synonymous, wouldn't they?

Kind Regards, John

I've just realised, I think it is purely an issue of semantics; which is probably what has been the argument all along - you can insulate a conductor, but to insulate an insulator is nonsensical!
Hence (the manufacturer's description above) sheathing an insulator - even if the cable has two layers of insulation and the manufacture calls the cable 'double insulated'!

Edit: I think you were there first in #22

 

[EFLImpudence]

"A thermoplastic-sheathed cable (TPS) consists of a toughened outer sheath of polyvinyl chloride (PVC) thermoplastic, covering one or more individual annealed copper conductors, themselves insulated with PVC."

Thermoplastic-sheathed cable - Wikipedia

en.wikipedia.org

 

[DetlefSchmitz]

My point was that this thread seems to be a search for a definition of 'double insulated'. The only place you are going to find a definition is in a Technical Standard that uses the phrase. The only one I am aware of relates to appliances, so you would have to look there for the definition. I would suggest that part of the definition will involve breakdown voltage, though there may be more relating to single faults. Construction details such as air gaps in a pcb are probably also part of the definition. You can't look for an industry definition of 'double insulated' relating to a cable if the industry has not defined the term in a Standard for cables. IMHO.

 

[JohnW2]

In the simple industrial world I lived in a 'sheath' was a means of protection, be it a condom, armour wire or braided material (metal or fabric). Double meant "two" and insulation meant "non-conducting". So a cable with an outer sheath was a cable with some form of mechanical protection that was not considered as part of the insulation properties.

Yes, but as you go onto say, if that sheath is also 'non-conducting' (i.e. an 'insulator'), then it is also "double insulated".

A cable that had two layers of insulation was considered double insulated.

... but that's exactly what the cables are which (quibbling) people say should not be called "double insulated"!

A cable with two layers of insulation and a sheath was a 'double insulated, sheathed cable', etc....

Fair enough, but we're not talking about them. In fact, if the sheath was also made of insulating material, then I suppose one call call it "triple insulated'!!

Kind Regards, John

 

[JohnW2]

I've just realised, I think it is purely an issue of semantics; which is probably what has been the argument all along - you can insulate a conductor, but to insulate an insulator is nonsensical!

Maybe it sounds nonsensical to talk about 'insulating an insulator' but the concept of having to layers/stages of insulation (as 'belt and braces') is very well established and accepted - as you have said, in relation to Class II items.

Hence (the manufacturer's description above) sheathing an insulator - even if the cable has two layers of insulation and the manufacture calls the cable 'double insulated'!

As I said, if, in terms of everyday English, it's "double-insulated" (c.f. "double-cooked" chips), then I personally don't think it very reasonable to say that such a description is 'incorrect'!

Kind Regards, John