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Topic Title: Double and reinforced insulation
Topic Summary: Definition
Created On: 31 December 2012 12:11 PM
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 31 December 2012 03:24 PM
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AJJewsbury

Posts: 11699
Joined: 13 August 2003

From a technical point of view UK installations are by law required to be 400/230V basic insulation and earth to the electrical outlet

Which law is that rock? I find it surprising that 110V systems are "illegal", not to mention 460/230V split phase supplies.

As I see it insulated & sheathed cables don't provide protection from electric shock from the cable by means of earthing - as the c.p.c. doesn't totally surround the live conductors, live parts could be accessed without the c.p.c. becoming energised, so defeating the whole purpose of earthing & ADS.

Where installation conditions are suitable (i.e. it's where the sheath isn't likely to be damaged) then the sheath & basic insulation can provide the equivalent protection to double/reinforced insulation. I'm sure there's a reg that says so - I think somewhere in 412 (sorry I don't have big green to hand at the moment).

We're required to provide protection from electric shock for all parts of the installation - not just accessories - Insulated & sheathed cabling isn't class 1 (ADS) - neither is PVC conduit or trunking. I'd tick double insulation.

- Andy.
 31 December 2012 03:35 PM
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Zs

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Joined: 20 July 2006

Thanks Andy,

At risk of becoming the class plonker, my instinct is really resisting this tails issue but I've got nothing to back it up with from text books. Nor the converse by the way. Such a strong feeling that two layers on a tail does not constitute double insulation. It is no different to a single thicker layer, just more colourful.

Zs
 31 December 2012 03:38 PM
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alanblaby

Posts: 394
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AJ, you are right:
412.2.1.1
Double or reinforced..... equipment shall be type tested and marked (square within a square).
 31 December 2012 03:42 PM
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daveparry1

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Maybe you're thinking too much Zs, just accept the fact that 2 = double! I have to say that i've learned over the years that in exams for instance a good thing to remember is "just answer the bleeding question! don't think unnecessarily!

Dave.
 31 December 2012 04:01 PM
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Zs

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Probably Dave but I'll stick to my guns and follow the instinct for a while. I have time to be sure about this before I go in...and possibly a great deal of my own certificates to adjust.

Going to see if the shops are open and to mess with some Clapton Blues for a while. To see if my mind opens on double insulated tails or not. Call it a blockage. I'll google later and see what comes up from here or there.

Zs
 31 December 2012 04:19 PM
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rogersmith7671

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It may be helpful to look at the SCHEDULE OF INSPECTION as a sort of Q&A session regarding the inspection that you have just finished.
The first question might be then;
What methods of protection exist to protect against electric shock?
For the purposes of compliance (BS:7671) at least twenty two possible answers are divided into seven main category's of which at least two may be regarded as most important;
1/ BASIC PROTECTION.
2/ FAULT PROTECTION
The methods under discussion here are categorized as providing both basic and fault protection therefore, during your inspection you will have observed and noted any measures taken, along the lines provided for you in the appropriate category. If the designer and/or responsible person has specified these methods as compliant with (BS:7671) and has provided/or not provided them.
Then it may be their responsibility to amend any part of the design or declaration of compliance.
I do not agree that you can force them to amend or delete an applicable electrical installation certificate simply to satisfy your own interpretation of what "insulation" means, however well intentioned. If you believe that the installation is unsatisfactory, then you should say so, for the record.The original designer/installer can then justify themselves to their client and/or rectify any errors as necessary.
If you believe the installation to be at risk or immediately dangerous then this should be pointed out to the person ordering the work, not to the person who did the work so they can alter the EIC.
regards
 31 December 2012 04:40 PM
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spinlondon

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I would suggest looking at the definitions in Part 2 of BS7671.
I would further suggest that the last sentence in the definition for Reinforced insulation describes sheathed singles and further indicates that testing of the sheath to meet BS6004 is not a requirement.
As to whether insulated and sheathed tails are double or reinforced, is perhaps another matter.
My opinion was that they are double insulated, an opinion that appears to be held by many manufacturers/wholesalers.
However after re-reading the definitions, I am now of the opinion that they are reinforced.
In either event, I would tick the box.
 31 December 2012 04:46 PM
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WiredScience

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I'd rather rely on the manufacturer's data sheet than a salesperson's literature.

Prysmian describe it as insulated and sheathed.

Prysmian 6181Y
 31 December 2012 04:49 PM
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rocknroll

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At the end of the day it does not matter what you stick in the boxes, a few smileys will do nobody looks at it anyway.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 31 December 2012 04:52 PM
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spinlondon

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Yes insulated and sheathed, which the definitions describe as being reinforced.
Reinforced offering the same degree of protection as double.
Eland cables: http://www.eland.co.uk/search....nsulated&button=Search
 31 December 2012 05:17 PM
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John Peckham

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I am not sure what legislation RnR is quoting from as this would make installations with other voltages such as 110V and 660V illegal. perhaps RnR could quote the relevant statute? Also what about those country installs on the end of a long run of poles with 230-0-230V supplies?

A for putting N/A in the boxes for SELV, PELV, Double and re-enforced insulation BS7671 does cover this type of protection probably why it is on the model forms.

If you have a look at the Part 2 definition in BS7671 for" electrical equipment" it lists, among other things, accessories, appliances, and luminaires. Then have a look at "electrical "installation" in the Part 2 definitions. Whilst there are separate standards for some electrical equipment Regulation 113.1 covers the selection and application of this equipment.

So the permanently connected Class 2 light fitting and items such as the Class 2 hand driers in the WCS are covered by BS7671.

There are tick boxes that require attention on the EIC inspection form for correct connection of accessories and equipment and if the requirements for special locations have been met. Some special locations require double insulation and there is a specific provision for outdoor lighting to have the whole installation protected by double or reinforced insulation.

As for the SELV box this should not automatically be ticked just because you have SELV transformers on the lighting as this has, or should have, basic protection. You might need to tick this box if you have one of those fancy lighting installation with a pair of exposed ELV terminals feeding a pair of wires spanning a room with light fittings tapped off between the wires.

So if you have Class 2 light fittings or Class 2 handriers for example in the bogs then I would tick the box.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 31 December 2012 05:24 PM
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John Peckham

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I would have said that "double insulated" tails are not double insulated but are insulated and sheathed. However as the man wearing a trus said, " I stand corrected".

Double insulated tails.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 31 December 2012 05:25 PM
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perspicacious

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I dare say that if I asked for a Biro, I would be given a Parker ball point pen, likewise, the counter staff on being asked for a Hoover would offer you a vacuum cleaner of another manufacturer such is the common use of slang at the counter and elsewhere in trade publications.......

However, "meter tails" (sic) or 6181Y are covered by BS 6004 and this makes reference to PVC insulated and PVC sheathed cable

From my copy of BS 6004:2000 Amended to 26 April 2005 (so it is not the latest, that being BS 6004:2012 published on 30/9/2012).....

The table has lost clarity due to copy and paste but for 25 mm2 "meter tails" (sic), the radial thickness of insulation is 1.2 mm and the radial thickness of sheath is 1.1 mm.


Table 7 - PVC insulated, PVC sheathed cable, 300/500 V, single core, flat twin and 3-core
National type
Construction:
Conductor - class 1 plain copper, solid, or class 2 plain copper, stranded, as shown below.
Insulation - PVC type TI 1.
Sheath - PVC type 6.
In twin and 3-core cables, the cores shall be laid parallel.
The sheath shall fit closely but not adhere to the core(s).
􀀟Colours for core identification:
single core - brown or blue.
twin - brown and blue, or, for 2 􀃕 1.0 and 2 􀃕 1.5 cables, brown and brown.
3-core - brown, black (centre core) and grey.
Colour of sheath:
single core - grey (other colours may be used by agreement between manufacturer
and customer).
flat twin and 3-core - grey.􀀠
Number and
nominal
cross-sectional
area of
conductors
Class of
conductor
Radial thickness
of insulation
Radial
thickness of
sheath
Mean overall dimensions Minimum
insulation
resistance
at 70 􀂮C
Lower limit Upper limit
mm2 mm mm mm mm M􀀵􀂵km
1 􀃕 1.0 1 0.6 0.8 3.8 4.5 0.011
1 􀃕 1.5 1 0.7 0.8 4.2 4.9 0.011
1 􀃕 2.5 1 0.8 0.8 4.8 5.8 0.010
1 􀃕 4 2 0.8 0.9 5.4 6.8 0.007 7
1 􀃕 6 2 0.8 0.9 6.0 7.4 0.006 5
1 􀃕 10 2 1.0 0.9 7.2 8.8 0.006 5
1 􀃕 16 2 1.0 1.0 8.4 10.5 0.005 2
1 􀃕 25 2 1.2 1.1 10.0 12.5 0.005 0

Table 8 - PVC insulated, PVC sheathed cable with circuit protective conductor, 300/500 V,
single core, flat twin and 3-core
National type
Construction:
Conductor - class 1 plain copper, solid, or class 2 plain copper, stranded, as shown below.
Insulation - PVC type TI 1.
Sheath - PVC type 6.
The core or cores shall be laid parallel with the uninsulated circuit protective conductor.
The sheath shall fit closely but not adhere to the cores.
􀀟Colours for core identification:
single core - brown or blue.
twin - brown and blue, or, for 2 × 1.0 and 2 × 1.5 cables, brown and brown.
3-core - brown, black (centre core), and grey.
Position of circuit protective conductor:
twin - centrally placed between cores in same plane.
3-core - centrally placed between black and grey cores in same plane.
Colour of sheath:
Grey.􀀠
Number and
nominal
cross-sectional
area of
conductors
Class of
conductor
(see 6.1)
Radial
thickness of
insulation
Radial
thickness of
sheath
Mean overall dimensions Circuit protective
conductor,
minimum nominal
cross-sectional
area
Minimum
insulation
resistance
at 70 􀂮C
Lower limit Upper limit
mm2 mm mm mm mm mm2 M􀀵􀂵km
1 􀃕 1.0 1 0.6 0.9 4.0 􀃕 5.1 5.2 􀃕 6.4 1.0 0.011
1 􀃕 1.5 1 0.7 0.9 4.4 􀃕 5.4 5.8 􀃕 7.0 1.0 0.011
2 􀃕 1.0 1 0.6 0.9 4.0 􀃕 7.2 4.7 􀃕 8.6 1.0 0.011
2 􀃕 1.5 1 0.7 0.9 4.4 􀃕 8.2 5.4 􀃕 9.6 1.0 0.011
2 􀃕 2.5 1 0.8 1.0 5.2 􀃕 9.8 6.2 􀃕 11.5 1.5 0.010
2 􀃕4 2 0.8 1.0 5.6 􀃕 10.5 7.2 􀃕 13.0 1.5 0.007 7
2 􀃕6 2 0.8 1.1 6.4 􀃕 12.5 8.0 􀃕 15.0 2.5 0.006 5


6.6 Sheath
6.6.1 Type of sheath
The sheath shall be PVC of one of the following types, as specified in the appropriate construction table:
a) 􀀟TM 2 or TM 5 conforming to BS 7655-4.1􀀠;
b) type 6 conforming to BS 7655-4.2.
6.6.2 Application
The sheath shall be extruded as an homogeneous layer, and shall be capable of being removed without
damage to the cores.
NOTE A separator consisting of a tape or film may be placed under the sheath.
6.6.3 Thickness
6.6.3.1 The radial thickness of the sheath, when determined by taking the average of a number of
measurements in accordance with D.2 or D.3 as applicable, shall be not less than the value given in the
appropriate construction table or, for cables with a non-preferred number of cores, the value tS2 calculated
in accordance with 6.6.3.2, and the smallest of the measured values shall not fall below the value by more
than 15 % + 0.1 mm.
6.6.3.2 For cables specified in Table 13, but with a non-preferred number of cores, the radial thickness, tS2,
in millimetres (mm) of the outer sheath shall be as calculated by the following equation:
tS2 = 0.08DS + 0.4
with a maximum value of 2.4 mm,
where
DS is the fictitious diameter over the screen, calculated by adding together the fictitious diameter
under the braid DB (calculated as specified in 6.5) and four times the relevant value for the
maximum diameter of the copper wires (as specified in 6.5).
6.6.4 Colour
The colour of the sheath shall be as specified in the appropriate construction table. The colour shall be
throughout the whole of the sheath or on its surface.
Where surface colouring is applied, the surface colour shall be of essentially the same material as the
underlying material and shall be applied as part of the extrusion process. The surface colour shall not be
separable from the underlying material and shall be durable.


Regards

BOD
 31 December 2012 05:33 PM
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perspicacious

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Just as a note, "double" or "double insulated" does not appear in my copy of BS 6004......

Regards

BAD
 31 December 2012 05:34 PM
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daveparry1

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but are insulated and sheathed.
---------------
But my way of interpreting it is that the sheathing is an insulator John so insulated and sheathed means there are two layers of insulation, therefore the insulation is double!

Dave.
 31 December 2012 05:35 PM
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alanblaby

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Regarding my earlier post, 412.2.1.1, further reading, on the next page, 412.2.4.1, Cables should not be identified by the 2 squares symbol.
I read that Reg. as saying that cables should be equivalent, in insulation, to BSEN61140 - Reinforced Insulation.

So they are not reinforced, unless type-tested, but give the same insulation as reinforced insulation.
 31 December 2012 05:48 PM
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spinlondon

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So is insulated and sheathed another term for double insulation, or reinforced insulation?
 31 December 2012 05:57 PM
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perspicacious

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Just as another note, "reinforced" does not appear in my copy of BS 6004......

Regards

BAD
 31 December 2012 06:06 PM
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GeoffBlackwell

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JP said
A for putting N/A in the boxes for SELV, PELV, Double and re-enforced insulation BS7671 does cover this type of protection probably why it is on the model forms.


We could argue all day about this . In my view - in general these boxes refer to the assembly of the installation and not to the individual parts as these are already covered by product standards.

JP said
As for the SELV box this should not automatically be ticked just because you have SELV transformers on the lighting as this has, or should have, basic protection. You might need to tick this box if you have one of those fancy lighting installation with a pair of exposed ELV terminals feeding a pair of wires spanning a room with light fittings tapped off between the wires.


I agree with this and it is a good example. You might also list an electroplating works.

If you need something for double and reinforced insulation consider Eatons Halyester system - see text below

"Fully insulated system
The principle of total insulation applied in accordance with EN-IEC 60364-4-41 and EN-IEC 60439-1 guarantees a high degree of safety for operating personnel. The plastic enclosures ensures that the energised as well as non-energised metal parts are protected against current bridging to earth. Furthermore, it is impossible for metal parts protruding from the enclosure to be energised. This is not only achieved by the use of double insulation for the operating shafts and buttons and levers fixed in the covers, but also by the use of plastic cable entries or glands. Furthermore, for increased operational safety, the internal components are mounted on plastic base plates. Metal parts within Halyester enclosures are not earthed.

Reduction of earthing problems
The "fully insulated" system considerably reduces the possibility of earthing problems during installation of Halyester distribution systems. For fully insulated systems, the maximum total earthing resistance may be derived from the rated current of the largest outgoing panel. This can result in savings with respect to earthing of the installation. For distribution systems that are not fully insulated, the rated current of the feeder is to be applied in this instance."

Or you might look at the original solution to 531.4.1 which was to fit an insulating kit to metalclad CUs on TT systems.

However, the need to do that has be largely removed by 412.2.4.1. - note that this regulation allows you to treat most wiring systems as providing compliance with 412.2.

The notes to this regulation rule out describing a wiring system as 'Class II' but it can be assumed to provide similar protection.

Regards

Geoff Blackwell
 31 December 2012 06:10 PM
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spinlondon

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Originally posted by: perspicacious
Just as another note, "reinforced" does not appear in my copy of BS 6004......

Regards

BAD


Does your copy of BS6004 refer to the sheath as a layer of insulation?
Perhaps even as a supplementary layer?

Is there such an animal as a SELV installation?
Should this box only be ticked, only if it applies to the whole installation?
What about the fact that SELV is on ly permitted as a method of protection, when it is used to supply one item of current using equipment?

Edited: 31 December 2012 at 06:29 PM by spinlondon
IET » Wiring and the regulations » Double and reinforced insulation

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