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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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 05 January 2013 09:46 PM
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leckie

Posts: 1863
Joined: 21 November 2008

Well keep ticking the boxes spin.

R&R is bang on and also Bod & DaveW (although because I'm brain dead now I don't fully understand all that he is saying).

I have taken the trouble to quote the regs that I think apply, I might be not totally correct because I am a humble sparks, but someone with a lot more knowledge might want to explain if I am right or wrong.

I'm not ticking!
 05 January 2013 11:03 PM
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Zs

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Ok, Here comes the non technical bit. I'm going to leave SELV out of this.

This matter is clearly so flimsy that I reckon all of us could persuade Judge Blodkins as to why we ticked the double insulated box on a form, or not. We could always use the tails as our fall-guy.

Were I to be a purist I think this would need some clarification from the IET. Not the NIC or the ESC, the IET. Since it is very clear that opinions are at play here. If I get a chance I will call the IET next week. However, I doubt if their reply will be conclusive and I expect that some of the tick-boxes are historical. As electrical installations have developed and methods of protection have improved (like they invented plastic) interpretation has been left open. I know of an example of an historical reg to which we abide concientiously and which is a made up number. So, I think that the IET will also need to take a look at how boxes are to be ticked. Oh buttocks, does that mean an amendment?

What I am seeing from this thread are two types of interpretation of double insulation; One which many of you think exists (and I don't) where a bit of cable has two layers of PVC and where 1+1= 2, and 2 means double.

Another which involves an appliance with the square inside a square symbol and which means something fundamentally different. Where no connection between the live parts and earth is required. I'd say the lighting transformer is our most common example of such.

However, talking of normal day to day installations, we all know that a cpc or earthing of some form is required as part of the wiring system right up to an accessory except for down to a lampholder from a ceiling rose (it is in section 411 somewhere) and that is what causes me to continue sticking to my guns and saying that I've never encountered an installation with no earthing on the wiring system.

Parsley's post of 8.47 on 2nd Jan provides an exerpt from GN5 which is very concise. It states, in paraphrase, that the protective measure of Double Insulation may only be used as the sole means of protection where the installation is under supervision. To me, that isn't a reference to tails with two layers.

As to double insulated fittings such as downlights or fancy laminated metal wall lights; Well I firmly believe that they are catered for on the schedule of inspections (for new stuff) as having been considered and correctly applied with a tick in the general section of particular protective measures for special installations and special locations. That's the bit where you can see that the installer took care to install the correct fittings. (my forms here, for this job, have n/a in that box which just made me smile). If not in a special location then the fact that they are double insulated isn't so important is it? But all well and good if they are.

So. My conclusion: It is unusual for me to stick my neck out with a firm decision and much of how I operate is guided by a core group of you guys to the extent of my following your ways in my modus operandi. And, knowing that my boss from my consultancy job looks in on here and this could cost me dearly. But this time no. I think quite a few of you are wrong on this. Unless I encounter an excessive amount of double insulated as per the two squares type of double insulated at this office block in the west end on the system itself, unless I decide that the system itself is protected by double insulation, which it won't be....

Then no, the boxes should not be ticked and I will observe such in my report. I will not tick the boxes on my own forms.

Boss, It is not rebellious, it is considered.

Three frets up and exactly the same pattern changes from major to minor.

Thank you. OMS gets back from his hols next week. I think I might have to hide for he will surely have something to say and is a difficult man to persuade. You have no idea how much I am hoping he falls onto the same side of the fence as I. Lighting blue touch paper.

Zs
 06 January 2013 12:08 AM
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rogersmith7671

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The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing. This is usually achieved at least in part by having two layers of insulating material (double insulated) surrounding live parts or by using reinforced insulation. one layer (eqiv, to a double layer).
It is surely not beyond reason to see that such a basic specification could be applied to an appliance or an accessory or anything that fits the description. why can't manufacturers say that their cables are double insulated if they have two layers of insulating material and no earthed metal casing? Why can't designers use such cable to provide basic and fault protection at the origin of, or at any part of the installation that they see fit, and correctly declare of statements of compliance? What's the problem with providing double insulated accessories in such a way that they become a integral part of the installation in the same installation and saying so on the same forma?
Regards
 06 January 2013 02:30 AM
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spinlondon

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I must admit, I do find it very odd.
Thirty odd years ago maybe less, we were installing lighting circuits with out a CPC.
We were using very similar cables to what are used today, in fact some are no different whatsoever.
Those lighting circuits were double insulated.
They used double insulated cables, double insulated accessories and fittings and double insulated metal back boxes.
Today we use cables that incorporate an uninsulated CPC and uninsulated back boxes.
However the cables that don't incorporate an uninsulated CPC are still the same as those used thirty odd years ago.
What has changed that those cables are now no longer double insulated?

Has there been some change in the laws of physics, has there been a new Regulation?
Not as far as I'm aware.
In fact as far as I'm aware BS7671 still considers those cables to be double insulated. Not only does BS7671 still consider insulated and sheathed cables to be double insulated, it also considers T&E to be double insulated.
At least that's my understanding of Regulation 412.2.4.1.
There's even a note about being at least eqivalent to reinforced insulation.
Reinforced insulation as we know being single insulation which provides the same degree of protection as double insulation.

As for not ticking the boxes, if you don't want to inspect the methods of protection that you have installed, or indicate that you have made such an inspection, that's up to you. Knock yourselves out.
 06 January 2013 08:02 AM
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leckie

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As an aside, I began my apprenticeship 41 years ago and never saw twin cable without a cpc and I worked on all sorts of jobs. Back boxes were nearly always metal and we used to bond to the back of socket boxes in a t&e installation. It was the 14th edition regs. 6sq.mm g/y bonding for pme was the order of the day. The only plastic back boxes I saw were made by Nettle, plaster depth only and with an earth terminal. You could also get metal plaster depth boxes with plastic lugs. I think these were intended to be used for retro fitting to older installation when there were no cpc's in the lighting circuit, but could be used in new installs with cpc's because they also had an earth terminal.

So I think lighting circuits with no cpc's were install pre 1970.
 06 January 2013 09:50 AM
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daveparry1

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So I think lighting circuits with no cpc's were install pre 1970
---------------------
Up until about 1966 Leckie,

Dave.
 06 January 2013 11:59 AM
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Zs

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Ah, therein lies the conundrum Spin. Ticking a box does not mean ' I have inspected this' it means 'I have inspected this and the result is satisfactory' otherwise we would be expected to tick all the boxes. this is not a Code 1-3 form so unsatisfactory is not an option.

It is in the interpretation of what the two boxes; double insulation and reinforced insulation are saying if they are ticked and I do not think they refer to counting the layers of pvc on a 25mm tail leading from the meter to the distribution board.

the form is for new installation work only. Anyone still installing lighting circuits with no CPC?

Until of course something more concrete comes along. I've some decent email addresses and perhaps I will prod one of the IET Gods for comment and clarification.

Zs
 06 January 2013 01:04 PM
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Jaymack

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Clue - reference to the book on NICEIC Inspection, Testing etc., version 6 P156.The "protective measures" of double insulation and reinforced insulation must not be applied to any circuit that includes a socket outlet, luminaire supporting coupler, a device for connecting a luminaire or cable coupler, where the user may change items of equipment without authorisation.

Perhaps these forms should include tick boxes for "Under the control of ordinary persons" etc.
 06 January 2013 01:09 PM
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rogersmith7671

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In the old 16th edition day's protection against electric shock could have been provided by; protection against direct contact (basic protection) and indirect contact (fault protection) there are two examples on the old schedule of inspections,(1) SELV and (2) limitation of discharge of energy. This bit used to cause confusion too, as some used to tick the second box when no electric fences where present in the installation. The veracity of the designer or inspector might then be called into question and arugments would ensue.
I agree with ZS this new arrangement under the 17th seems to be an area where individual interpretation could lead to more than one answer.
Perhaps this sort of problem is here to stay.
(copied off the internet);
Sample Questions - C&G 2382 17th Edition paper D
BS 7671 requires protection against electric shock to be provided , by
basic and fault protection. One method common to both is
a the use of RCDs
b the use of Class H equipment
c preventing current from passing through any person or livestock
d the use and implementation of equipotential bonding.
what do you think the answer is?

The Electrical Safety Council advise;
Quote. An installation uses Automatic Disconnection of Supply (ADS) as a protective measure against electric shock. One or more items of double insulated equipment are installed, such as luminaires. On the schedule of inspections, should the "double insulation" box for both basic and fault protection as a method of protection against electric shock be ticked?
No. That item in the Schedule of Inspections is intended for use when that protective measure is used as the sole protective measure, that is, where a whole installation or circuit is intended to consist entirely of equipment with double insulation. In such a case, the installation or circuit would be required to be under effective supervision in normal use.

Regulation number(s) 412.1.3 Appendix 6

And.
An installation uses Automatic Disconnection of Supply (ADS) as a protective measure against electric shock. One or more items of separated extra-low voltage (SELV) equipment are installed. On the schedule of inspections, should the "SELV" box for both basic and fault protection as a method of protection against electric shock be ticked?
No. That part of the Schedule of Inspections is intended for use when a part of an installation relies specifically on that method of protection.

Regulation number(s) Appendix 6

regards.

source; http://www.djtelectraining.co.uk (under fair use exemptions)
http://www.esc.org.uk

(edit for internet cut and paste, typo's ect)

Edited: 06 January 2013 at 02:34 PM by rogersmith7671
 06 January 2013 01:10 PM
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MrP

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Zs happy new year
I started to read this post and got half way before losing the will to live
But for my two ryals worth. The tick boxes in the method of protection for electric shock apply to the installation and the installation alone therefore unless it was a very specialist type of installation (I have never come across or know of anyone who has) then the boxes for double insulation and reinforced insulation would never be ticked on a box standard installation schedule.
Now its opinion and some have said so what if the wrong box is ticked, but it is my experience if tick boxes are wrongly marked then usually other more important bits of information are wrong
So if the contractor ticked these boxes then it would be my opinion that the integrity of the certification is in question.
My opinion

MrP Back in the sand pit
 06 January 2013 01:14 PM
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geov

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For what it's worth Zs, I'm with you on this one.

It's interesting that only by asking the question, we discover fundamental differences of opinion about a process which must get completed hundreds of times a day. Some will tick away (or not) without any real questioning of what they are doing, but it's threads like this that really help one to stop and think about practices that almost become routine.
 06 January 2013 04:11 PM
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spinlondon

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Does it really matter whether it was 1985 or 1965.
The fact still remains, that such cable were once recognised by all as being double insulated.
Something that today it would appear is only recognised by BS7671.

Yes the box is there for you to indicate that the item has been inspected, and by ticking the box, that it is satisfactory.

I cannot find, and it would appear no one else can either, anything in BS7671 which suggests that these particular boxes only refer to the installation as a whole.

Again I would refer you to Regulation 611.3:
"The inspection shall include at least the checking of the following items where relevant, including as appropriate all particular requirements for special installations or locations (Part7)."
The subsequent list includes SELV.

Section 701 list those protective measures which are not permitted in a location containing a bath or shower, it then lists the protective measures which are permitted.
One of the permitted methods is SELV and PELV circuits.
These methods of protection, do not apply to the whole installation, they apply to part of the installation, the part which is in a location containing a bath or shower.

Where exactly in BS7671 is it to be found that we are not required to inspect that the Requirements in relation to SELV when used as a protective measure in a location containing a bath or shower, have been met?
Which Regulation is it, that says we do not have to check that the safety source for a SELV fan is outside of any zones?

However, there is the fact that BS7671 considers that many wiring systems installed in accordance with Chapter 52, are considered to meet the requirements to be double insulated.
So if your installation utilises a wiring system that is considered as being double insulated, would it not mean that the installation as a whole is considered as being double insulated?

Edited: 06 January 2013 at 05:03 PM by spinlondon
 06 January 2013 05:16 PM
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perspicacious

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"The fact still remains, that such cable were once recognised by all as being double insulated"

Care to provide a non slang source of your proclamation Spin, as I've gone back to BS 6004:1975 together with the 14th and "double insulated cable" doesn't get a mention, only insulated and sheathed?

Regards

BOD

PS just finished hoovering and drafting this with my biro whilst illuminated with a sunflood..........
 06 January 2013 06:11 PM
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perspicacious

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Looking in my Filofax alongside a Tippexed out entry, I see that this time last year I spent some time opening a Jiffy bag with an Airfix model carefully Bubble Wrapped. It was a Dormobile (having Googled the best option and decided against a Jeep or JCB) but some of the parts needed trimming with the Stanley knife and then holding together with Sellotape after applying the glue with a Q-tip and wiping off the surplus with a Kleenex. Unfortunately I drew blood and needed a Band-Aid. The Polaroid photo I took really could have done with being Photoshopped as when Zeroxed, it didn't look too good, so mounted behind some Perspex over the Jacuzzi and Velcroed it to the wall......

Regards

BAD
 06 January 2013 06:37 PM
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spinlondon

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Originally posted by: perspicacious
"The fact still remains, that such cable were once recognised by all as being double insulated"

Care to provide a non slang source of your proclamation Spin, as I've gone back to BS 6004:1975 together with the 14th and "double insulated cable" doesn't get a mention, only insulated and sheathed?

Regards

BOD

PS just finished hoovering and drafting this with my biro whilst illuminated with a sunflood..........

As I've already pointed out, the current edition of BS7671 (Reg. No. 412.2.4.1) still considers such cables to be double insulated.
Note 1 of the above Regulation states: "Cable product standards do not specify impulse withstand capability. However, it is considered that the insulation of the cabling system is at least equinalent to the requirement in BS EN 61140 for reinforced insulation."
I would suggest that you stop looking for a reference to double insulated in your product standard, or perhaps look in another product standard such as BS EN 61140.
 06 January 2013 07:10 PM
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perspicacious

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"The fact still remains, that such cable were once recognised by all as being double insulated."

"I would suggest that you stop looking for a reference to double insulated in your product standard, or perhaps look in another product standard such as BS EN 61140."

As "meter tails" to BS 6004 are insulated and sheathed, I'd respectfully suggest that you find another historic source confirming that the sheath meets the requirements of insulation as per BS 6004 before wandering off at a tangent on other BSEN......

Regards

BOD

PS my Post-it note reminds me to microwave my missed lunch that the ladyfriend kindly put in a Tupperware bowl........
 06 January 2013 07:36 PM
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spinlondon

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What another historic source other than the current edition of BS7671?
Why?
 06 January 2013 07:58 PM
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perspicacious

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"What another historic source other than the current edition of BS7671?
Why?"


To back up your proclamation below that is historic....... There appears to be little factual support provided by anyone.......... Never mind, you'll get a reply from your manufacturers tomorrow that you can post for us all to read.....

Regards

BOD



"The fact still remains, that such cable were once recognised by all as being double insulated."

"I must admit, I do find it very odd. Thirty odd years ago maybe less, we were installing lighting circuits with out a CPC. We were using very similar cables to what are used today, in fact some are no different whatsoever. Those lighting circuits were double insulated. They used double insulated cables, double insulated accessories and fittings and double insulated metal back boxes. Today we use cables that incorporate an uninsulated CPC and uninsulated back boxes.
However the cables that don't incorporate an uninsulated CPC are still the same as those used thirty odd years ago. What has changed that those cables are now no longer double insulated?"
 06 January 2013 08:05 PM
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potential

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When is insulation on wire or cable not a sheath of some sort?
 06 January 2013 08:29 PM
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deapea

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For what its worth, 2 books which we've all got copies of, GN3 and OSG show double and reinforced insulation as N/A on there sample docs.
IET » Wiring and the regulations » Double and reinforced insulation

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