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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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 02 January 2013 03:07 PM
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GeoffBlackwell

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GB
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.


Andy said
So what about a door bell circuit? Transformer, lots of cable (typically not sheathed), Probably not IP rated bell push outside in all weathers? All put together on-site. Is the suggestion that the protection from electric shock is by earthing/ADS??


I cannot see what your point is here. A door bell circuit is an assembly of a part of the installation - it is not an individual item of equipment. It is usually a FELV circuit and as such would recorded as a tick in the FELV box under the heading Automatic disconnection of supply.

I was pointing out to JP that I would not apply the boxes to an item such as a single class II fitting as this has its own product standard.

GB
No the box marked SELV is only to be used when SELV provides both Basic and Fault protection. It does not apply when SELV is only providing Fault protection


Andy
I could argue that a SELV circuit always does. Just because you also have another means of basic protection as well doesn't detract from that. Indeed for SELV circuits over 25V a.c. or 60V d.c. additional basic protection is a requirement (reg 414.4.5) of the SELV system itself - so not relying on the extra-low/separated nature of the voltage alone. So saying if basic insulation is present, it isn't SELV doesn't seem to add up.


Again I fail to see your point - "So saying if basic insulation is present, it isn't SELV doesn't seem to add up"

I have not said that - I have specifically referred to the box marked SELV. The only box marked SELV on the model in BS 7671 is under the heading Both basic and fault protection

If you had quoted the full response you would see that:

GB

No the box marked SELV is only to be used when SELV provides both Basic and Fault protection. It does not apply when SELV is only providing Fault protection, as is the case with a 'SELV fan'.

Its all in the title above the box and in regulation group 414, in particular the last part of 414.4.5 specifically allows SELV as a sole means of basic protection if its conditions are met. This is the bare wire lighting system, etc.

You would need a SELV box in the schedule section Fault protection:. The only boxes there refer to electrical separation which is not SELV (see regulation group 413) - even given that SELV includes a form of 'electrical separation' within it.



Note that quoting from posts by different people in one block without attributing - complicates response.

Regards

Geoff Blackwell
 02 January 2013 04:20 PM
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rocknroll

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But which legislation Rock? I can't find any reference to 400V in the The Electricity Safety, Quality and Continuity Regulations - is there something else that I've overlooked?


I am not sure of the relevance of this as there are plenty of documents out there from the DTI, electricity supply regulations and even your contract with the supplier stating that they are mandated to supply you with;

Low Voltage Supply

Single phase 230V (AC) + 10% - 6% (frequency = 50Hz +/- 1%)
Three phase 400 / 230V (AC) + 10% - 6% (frequency = 50Hz +/- 1%)

High Voltage Supply

For high voltage customers (11,000V)

(Some documents may still quote 415/240V but that will eventually change).

Any other voltages or supplies such as 480V, 680V etc; are by special arrangement often referred to as by consideration of the Secretary of State, these inherited systems are being phased out rapidly and are now becoming few and far between for domestic LV supplies.

The rest regarding the filling in of the boxes was my opinion.

regards

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leave nothing but footprints!"
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 02 January 2013 04:21 PM
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AJJewsbury

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Note that quoting from posts by different people in one block without attributing - complicates response.

Apologies Geoff - I like to think of debating against points rather than arguing against people, so quote just the text. Maybe that was in error...

Again I fail to see your point - "So saying if basic insulation is present, it isn't SELV doesn't seem to add up"

I have not said that - I have specifically referred to the box marked SELV. The only box marked SELV on the model in BS 7671 is under the heading Both basic and fault protection

OK, I didn't explain that well either, please bear with me...

We sometimes (for convenience) do thing that exceed minimum requirements of the regs - e.g. insulated & sheathed cables inside conduit, or an RCD on a TN circuit with Zs well within limits - as a result one requirement can be covered simultaneously covered by two different approaches - and I don't see a problem with that as long as one doesn't undermine the other (e.g. earthing & separation).

when you said:
No the box marked SELV is only to be used when SELV provides both Basic and Fault protection. It does not apply when SELV is only providing Fault protection, as is the case with a 'SELV fan'. Its all in the title above the box and in regulation group 414, in particular the last part of 414.4.5 specifically allows SELV as a sole means of basic protection if its conditions are met. This is the bare wire lighting system, etc.

You would need a SELV box in the schedule section Fault protection:. The only boxes there refer to electrical separation which is not SELV (see regulation group 413) - even given that SELV includes a form of 'electrical separation' within it.


I understood you to mean that if there weren't bare wires (i.e. basic protection was provided by insulation/enclosure) then you couldn't regard it as SELV, as you wouldn't be relying on SELV to provide basic protection. I was suggesting that SELV could still be regarded as SELV (providing both fault and basic protection), even if basic protection happened also to be provided by other means as well.

I guess I've come at this from the opposite direction to most - I saw the schedule of inspections as just a list options that sections 411-414 provide, for convenience grouping together options that give the same protection, with the hint that by whatever combination (the designer) has chosen, both fault and basic protection must be provided. I.e. the headings simply reflects what facilities each protective measure can provide, as distinct from what they're being relied upon for (does that make sense?).

I cannot see what your point is here. A door bell circuit is an assembly of a part of the installation - it is not an individual item of equipment.

Agreed!
It is usually a FELV circuit and as such would recorded as a tick in the FELV box under the heading Automatic disconnection of supply.

Ah. Given that most bell wire isn't rated for 230V (the voltage of the primary circuit), and I've never seen the exposed conductive-parts (e.g. brass bell push) connected to the primary circuit c.p.c. - I was sort of hoping they would be classed as SELV.

- Andy.
 02 January 2013 04:28 PM
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GeoffBlackwell

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I understood you to mean that if there weren't bare wires (i.e. basic protection was provided by insulation/enclosure) then you couldn't regard it as SELV, as you wouldn't be relying on SELV to provide basic protection. I was suggesting that SELV could still be regarded as SELV (providing both fault and basic protection), even if basic protection happened also to be provided by other means as well.


On reflection I think I have an error in my logic here. I still would not apply this SELV box to a single item such as a fan - but I have failed to account for the fact that SELV in general does in fact provide both basic and fault protection.

In my defence I am suffering from man flu and most of us know what that is like .

Regards

Geoff Blackwell
 03 January 2013 09:53 AM
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AJJewsbury

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No worries Geoff. It's always good to examine other possibilities - a form of playing "devil's advocate" as it were, even if that wasn't the intended approach.

I think I can see the argument a bit better now. If I supply say one of those new all-in-one LED downlighters in a lounge with mains L/E/N I don't really care what it does with the current internally - it might use an isolating transformer to take it down to 12V, it might be 1.2V for the LED or it might be 51V. It might be an electronic converter which may or may not be SELV - as long as it's safe I don't really care - and the product standard deals with that so I don't have to worry about it.

On the other hand, if I've selected a device specifically because it is SELV - say because it's in zones 0 or 1 in a bathroom - it might be reasonable to recognise & record that on paper.

- Andy.
 03 January 2013 11:02 AM
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GeoffBlackwell

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Andy said
On the other hand, if I've selected a device specifically because it is SELV - say because it's in zones 0 or 1 in a bathroom - it might be reasonable to recognise & record that on paper.


My view is that it is still a single item made to a product standard - so I would not record it. I would put a tick in the special locations box provided all was correct.

If I installed say, SELV lighting consisting of a separate transformer and separate light fitting(s) there is then a need to indicate that all of the SELV installation rules have been complied with for cabling etc, so this is the case where I would use this box.

Regards

Geoff Blackwell
 03 January 2013 12:27 PM
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leckie

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I have been reading this thread and getting more and more confused

If I give some examples could I have some replies as to when the SELV box is or isnt ticked?

1/
12v SELV downlights lighting with individual Tx's above the ceiling of a bathroom, supplied from an RCD protected circuit

2/
As above but not in a special location

3/
A 12v SELV fan fitted in a bathroom with the Tx installed outide the bathroom, supplied via an RCD protected circuit.
 03 January 2013 12:44 PM
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GeoffBlackwell

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If the transformers are part of the light fitting I would not tick the box.

If the transformers are separate units that I have to cable to the light fittings - I now have cable I have installed and I must ensure that I have met the cabling requirements for SELV - in this case I would tick the box.

This is my opinion - others may disagree.

Regards

Geoff Blackwell
 03 January 2013 12:57 PM
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spinlondon

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Originally posted by: GeoffBlackwell
No the box marked SELV is only to be used when SELV provides both Basic and Fault protection. It does not apply when SELV is only providing Fault protection, as is the case with a 'SELV fan'. Its all in the title above the box and in regulation group 414, in particular the last part of 414.4.5 specifically allows SELV as a sole means of basic protection if its conditions are met. This is the bare wire lighting system, etc.

You would need a SELV box in the schedule section Fault protection:. The only boxes there refer to electrical separation which is not SELV (see regulation group 413) - even given that SELV includes a form of 'electrical separation' within it.

Regards

Geoff Blackwell


Originally posted by: GeoffBlackwell
If the transformers are part of the light fitting I would not tick the box.

If the transformers are separate units that I have to cable to the light fittings - I now have cable I have installed and I must ensure that I have met the cabling requirements for SELV - in this case I would tick the box.

This is my opinion - others may disagree.

Regards

Geoff Blackwell


Which one is it Geoff?
 03 January 2013 12:59 PM
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AJJewsbury

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I have been reading this thread and getting more and more confused

That's 'cos we're still debating it...

I think Geoff would tick the boxes if the parts of the installation covered directly by BS 7671 were SELV (i.e. outside of individual product standards) - e.g. "home made" connections between transformer and SELV appliance (rather than plug-together system supplied as part of the product).

On the other hand, I'd suggest that the selection of equipment is still a matter of BS 7671 even if the equipment itself is covered by other standard (and hence outside scope of BS 7671), so (at the moment), I'm still tempted to tick SELV if any SELV equipment is permanently connected to the installation - especially if it's a situation where BS 7671 requires SELV equipment (or similar) to be used.

I can't say that either is positively right or wrong though - I guess that why we have this forum! Maybe there's even a little room for individuality in the filling in of forms.

- Andy.
 03 January 2013 01:14 PM
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leckie

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Regarding the double insulated/reinforced insulation issue....

There was a thread recently , that I cant find now, regarding incoming supply cables to a TT system.

My view was, and still is, that the if the DB is metal that the RCD should be mounted in a moulded enclosure prior to the DB or using a manufacturers approved insulating incoming kit not a bit of grommet strip, etc.

Others, including the mightiest of forum members considered that meter tails were adequate protection directly into a metalclad DB because they were double insulated.

I would still personally mount an rcd in a moulded enclosure prior to the DB.

If carrying out an EICR though, would I dare to say meter tails directly into a DB via a brass bush carries any code? Hmm, you have to be very careful what you report as a non compliance
 03 January 2013 01:20 PM
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spinlondon

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If the TT tails are not Double/Reinforced insulation, then what method of protection is being used to protect them between the meter and the RCD?

Geoff seems to be of the opinion that we do not tick the box, if another method of protection is used.
However which box takes precedence?
If for instance both double/reinforced insulation is used along with ADS, should we just tick the double/reinforced insulation box, because it is first in the list?
 03 January 2013 01:27 PM
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AJJewsbury

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Geoff seems to be of the opinion that we do not tick the box, if another method of protection is used.
However which box takes precedence?

Spin - I think Geoff amended his stance on that - see his post of 02 January 2013 04:28 PM.
- Andy.
 03 January 2013 02:03 PM
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GeoffBlackwell

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One more go at trying to state my position .

If you fit something that comes in a box complete and all you do is connect it then, provided it is manufactured such that it complies with the relevant product standard, I would not tick a box.

If you fit items that together form say, a SELV system - and you provide the interconnections etc, then you have partly created the SELV system so you should certify that it is correctly implemented - so tick the box.

If a bathroom requires a SELV fan that comes complete in a box then you are not creating the SELV system - so don't tick the box. You may well have been responsible for the installation in the special location so you probably need to tick that box.

Now that is my view - its a free country - disagree if you want .

Regards

Geoff Blackwell
 03 January 2013 02:06 PM
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spinlondon

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Are you sure about that Andy?
His last post of 02/01/2013 at 05:28pm states:
"I still would not apply this SELV box to a single item such as a fan."

I do note however that opinions do seem to be lacking in relation to TT tails and double/reinforced insulation.
 03 January 2013 02:41 PM
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AJJewsbury

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Are you sure about that Andy?
His last post of 02/01/2013 at 05:28pm states:
"I still would not apply this SELV box to a single item such as a fan."

My understanding was the reasoning now was that it was a single piece of equipment covered by other standards, not because we weren't relying on both levels of protection.

I do note however that opinions do seem to be lacking in relation to TT tails and double/reinforced insulation.

I'd suggest that insulated & sheathed tails are indeed equivalent to double insulation - but that's not the whole story. The double insulation has to be suitable for the conditions to be effective. In the case of metal clad DB, if they're dragged through a rough hole, possibly with the sheath tripped back before connection and in an enclosure held together with sharp pointy screws, it would probably be sensible to doubt the protection given by the sheath. If the entry was properly bushed/glanded, the sheath left intact all the way to the incoming terminals and it was ensured that there wasn't anything sharp inside the enclosure that could damage the sheath, I don't see an objection. That said, a (separate) insulating enclosure, is probably an easier/more reliable solution.

- Andy.
 03 January 2013 04:18 PM
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rogersmith7671

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Dear ZS
In some installations double or reinforced insulation might be required even if in a particular location SELV is not provide but electrical separation is used.
For installations supplied under nominal voltages and currents.
Where basic and fault protection conditions are compliant (7671) if the protective measure electrical separation, is required and has been provided; For a single item of current using equipment i would tick the box in section 4 Electrical Separation (iet pro forma).
If electrical separation is provided as a means of protection for more than one items of current using equipment there is an appropriate box in that same section.
In the installation that you have outlined it is unlikely to find these measures, SELV would be most likely as this measure provides basic and fault protection. So i would tick that box if it complied. There are (as i am sure you know) electrical accessories that are considered double insulated simply because it does not have an earth wire fitted. They are designed in such a way that the electrical parts can never come into contact with the outer casing of the device. They can be ascertained either by mark or from the designers specification. I would tick the double insulated box, if i found such an item where required.
I realize that it sometimes might seem odd, but its just the nature of the the beast. If you want to be able to describe all the possible combinations of electrical installations, on just 3 pieces of A4. Then lots of things have to be included that most of us will never come across in the general run of things.

Regards

Edited: 03 January 2013 at 04:25 PM by rogersmith7671
 03 January 2013 04:24 PM
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spinlondon

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My point being:
That if we accept that insulated and shethed tails provide double insulation in TT installations, then it stands to reason that they provide the same with TN installations.
As such, which box do you tick?
Should the first box on the list be ticked, as it's first.
Should the second box be ticked, because it applies to a greater proportion of the installation?
Should we just toss a coin?
Should we tick both or all boxes that apply?
Should we take the stance, that it doesn't matter one way or the other?
 03 January 2013 04:30 PM
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AJJewsbury

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Should we tick both or all boxes that apply?

That would be my preference!

- Andy.
 03 January 2013 04:33 PM
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spinlondon

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Originally posted by: AJJewsbury
Should we tick both or all boxes that apply?

That would be my preference!

- Andy.


Mine also.
IET » Wiring and the regulations » Double and reinforced insulation

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