Yes, what you've done / are suggesting doesn't comply with BS7671:2008.
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Neutrals At Switch
- Thread starter jtizzard
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Yes, what you've done / are suggesting doesn't comply with BS7671:2008.
yes, I understand that and can correct it easily enough.... what I didn't follow was Ban and RF's comments...
RF pointed towards 526.5 which says that every termination and joint in a live conductor shall be made within one of the following or a combination thereof:
(i) A suitable accessory
(ii) An equipment enclosure
(iii) An enclosure partially formed or completed with building material which is non-combustible when tested to BS 476-4
I'm not savvy with BS 476-4 so couldn't say what's involved with the testing process though at a guess, wood wouldn't meet the standard.
BS 476-4:1970 specifies a test method for determining whether building materials, coated or non-coated, are non-combustible. This “non-combustibility test” determines whether materials are classified non-combustible or combustible. Reference is made to British Standard 4422, Glossary of terms associated with fire. Specimens (size and number, preparation, and conditioning), apparatus, test procedure, designation of non-combustibility and test report contents are explained. Internal diameter dimensions are given for electric furnace, draught shield, air-flow stabilizer, specimen holder and insertion device. Figure 3 shows the dimensions of the heat resisting steel wire to be used for the specimen holder. This standard replaces the combustibility test specified in BS 476-1.1:1953 and is issued under the authority of the Fire Standards Committee.
I would think it's easier to place the connections in an enclosure!
(i) A suitable accessory
(ii) An equipment enclosure
(iii) An enclosure partially formed or completed with building material which is non-combustible when tested to BS 476-4
I'm not savvy with BS 476-4 so couldn't say what's involved with the testing process though at a guess, wood wouldn't meet the standard.
BS 476-4:1970 specifies a test method for determining whether building materials, coated or non-coated, are non-combustible. This “non-combustibility test” determines whether materials are classified non-combustible or combustible. Reference is made to British Standard 4422, Glossary of terms associated with fire. Specimens (size and number, preparation, and conditioning), apparatus, test procedure, designation of non-combustibility and test report contents are explained. Internal diameter dimensions are given for electric furnace, draught shield, air-flow stabilizer, specimen holder and insertion device. Figure 3 shows the dimensions of the heat resisting steel wire to be used for the specimen holder. This standard replaces the combustibility test specified in BS 476-1.1:1953 and is issued under the authority of the Fire Standards Committee.
I would think it's easier to place the connections in an enclosure!
Wood can meet the criteria .. I believe that it is BS476-n that covers fire doors - for example a 42mm hardwood door meets this requirement I think....I have no idea how a 150x75 joist would be rated, although the area around the back of the backbox is entirely surrounded by rockwool which does meet 476-4.
In this particular rcase, I think that the construction of the partition could be shown to meet 526.5 iii ... but yes I think you are right, it would be easier to fit an enclosure, even though a judgement might say that in practice it would offer little additional benefit..
In this particular rcase, I think that the construction of the partition could be shown to meet 526.5 iii ... but yes I think you are right, it would be easier to fit an enclosure, even though a judgement might say that in practice it would offer little additional benefit..
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Rockwool, huh?
Have you applied the correct factors to the cable calcs?
Have you applied the correct factors to the cable calcs?
S
sparkyspike
Surely it can't be that difficult to terminate 6 circuits in a 2gang deep plasterboard box? Even with two way wiring there's surely no need to start going behind the box for terminating?
I knew that comment was coming !!!
I almost put a note in to pre-empt it... but yes almost every circuit in the entire house is buried in 75-150mm of Rockwool over significant length of their run.. includes every floor, wall and ceiling..
So yes account has been taken of need to apply derating factors..
In actual fact, in practice the system is very overspecced if anything.. ..
Its 150M2 total floor area with 18 circuits in total split between 2 CUs distributed to minimise lengths of each final circuit.
This includes a a ring final for each pair of rooms and most lighting either 11W GU10 CFLs of or a combination of 11W GU10s and upto 3x35w HIDs and 16 1W LEDs. No lighting circuit has more than 6 fittings in total..
As I am doing this under 'prior notification' to LABC, I am doing my best to be dilligent when it comes to planning / spec and documentation and have put together a spreadsheet giving all of the key data for each circuit.. the derating assumption will be shown on this....
Where I struggle a bit is with the detail of practical application (as seen by this thread).. as with many things, knowing the theory (or how to access the theory when it comes to 2382!) is one thing.. but its only part of the story.. fortunately sites/people like this are on had to provide some help..
I almost put a note in to pre-empt it... but yes almost every circuit in the entire house is buried in 75-150mm of Rockwool over significant length of their run.. includes every floor, wall and ceiling..
So yes account has been taken of need to apply derating factors..
In actual fact, in practice the system is very overspecced if anything.. ..
Its 150M2 total floor area with 18 circuits in total split between 2 CUs distributed to minimise lengths of each final circuit.
This includes a a ring final for each pair of rooms and most lighting either 11W GU10 CFLs of or a combination of 11W GU10s and upto 3x35w HIDs and 16 1W LEDs. No lighting circuit has more than 6 fittings in total..
As I am doing this under 'prior notification' to LABC, I am doing my best to be dilligent when it comes to planning / spec and documentation and have put together a spreadsheet giving all of the key data for each circuit.. the derating assumption will be shown on this....
Where I struggle a bit is with the detail of practical application (as seen by this thread).. as with many things, knowing the theory (or how to access the theory when it comes to 2382!) is one thing.. but its only part of the story.. fortunately sites/people like this are on had to provide some help..
Length of runs aren't the issue when derating factors are applied, that only relates to volt-drop.
For example, a 2.5mm cable in a stud wall with thermal insulation and not touching the plasterboard has a rating of 13.5A (table 4D5). You couldn't use 2.5mm cable in this instance for 32A ring final circuits, you would require 6mm.
For example, a 2.5mm cable in a stud wall with thermal insulation and not touching the plasterboard has a rating of 13.5A (table 4D5). You couldn't use 2.5mm cable in this instance for 32A ring final circuits, you would require 6mm.
OK, so I set my self up for that 
My heart sank when I read you post... not because the ring finals are a problem, but because I was rather less than circumspect in what I wrote last night...queue embarassing backtrack.....
"Buried' was a terrible choice of description.. I guess I was focused on emphasising the trouble I'd gone to to insulate the place, rather than the precision of to describe it....so there you go.. rather than trying to be a smart-arse, my post should have ready 'I don't think derating factors need to be applied because....'
In fact, the only parts of lighting circuits that are in direct contact with the inulation (on one side, other touching board) are the drops to the switches on the lighting circuits.
For ring finals the 70mm between the bottom of each socket and base of each partition is similarly covered on one side.
In both these cases this is installation method 102 I think.
Ring finals then continue from socket to socket clipped to the bottom of the floor joists, well clear of the insulation above and with 500mm of free air cavity below (its a very strange building...).
The 1 exception to this is that there are a couple of places where a ring final does unavoidably run through a section of insulation and is consequently surrounded (for around 10 cm), but my undestanding was that given diversity the .78 derating this required could be ignored.
The rest of the lighting circuits are clipped to the bottom of joists above a suspended plasterboard ceiling (50mm gap between bottom of joist and board) i.e. something like method 101 but without touching the plasterboard...
I split the CUs on advice from someone because to have not done so would have meant really long runs (maybe getting on for 80M for the ring finals) and it was more practical to do this..
I understood issue with voltage drop, but resistance is also a function of length isn't it?
My heart sank when I read you post... not because the ring finals are a problem, but because I was rather less than circumspect in what I wrote last night...queue embarassing backtrack.....
"Buried' was a terrible choice of description.. I guess I was focused on emphasising the trouble I'd gone to to insulate the place, rather than the precision of to describe it....so there you go.. rather than trying to be a smart-arse, my post should have ready 'I don't think derating factors need to be applied because....'
In fact, the only parts of lighting circuits that are in direct contact with the inulation (on one side, other touching board) are the drops to the switches on the lighting circuits.
For ring finals the 70mm between the bottom of each socket and base of each partition is similarly covered on one side.
In both these cases this is installation method 102 I think.
Ring finals then continue from socket to socket clipped to the bottom of the floor joists, well clear of the insulation above and with 500mm of free air cavity below (its a very strange building...).
The 1 exception to this is that there are a couple of places where a ring final does unavoidably run through a section of insulation and is consequently surrounded (for around 10 cm), but my undestanding was that given diversity the .78 derating this required could be ignored.
The rest of the lighting circuits are clipped to the bottom of joists above a suspended plasterboard ceiling (50mm gap between bottom of joist and board) i.e. something like method 101 but without touching the plasterboard...
I split the CUs on advice from someone because to have not done so would have meant really long runs (maybe getting on for 80M for the ring finals) and it was more practical to do this..
I understood issue with voltage drop, but resistance is also a function of length isn't it?
Yes, resistance could also be an issue as R1+R2 values combined with Ze should be within spec for the circuit to disconnect within the required time. Some here would argue that as long as Zs is lower than 1667 ohms and the circuit is protected by a 30mA RCD then disconnection times would be met meaning only volt drop would be an issue.
OK that makes sense.... and ('cause now I am REALLY keen not to make myself look more foolish twice in 1 day ), was my assumption re. diversity and insulation correction for the short runs through hte rockwool correct?
), was my assumption re. diversity and insulation correction for the short runs through hte rockwool correct?
I would say no.
According to Appendix 15 (Ring and radial final circuit arrangements), "Cables should, wherever practicable, be fixed in a position such that they will not be covered by thermal insulation. Where cable is partially or completely covered by thermal insulation, reference should be made to Regulation 523.6.6"
According to Appendix 15 (Ring and radial final circuit arrangements), "Cables should, wherever practicable, be fixed in a position such that they will not be covered by thermal insulation. Where cable is partially or completely covered by thermal insulation, reference should be made to Regulation 523.6.6"
Looking at table 4A2 in appendix 4 though, that note specifically referencing 523.7 is only present against insulation method 103 I think..(although a general note saying where practicable a cable is to be fixed in a positin such as it will not be covered with thermal insulation' is present at the bottom..
and.. my copy of the 17th doesn's have 5.23.6, but 5.23.7 which refers to appendix 4 (which then only refers back to 5.23.7 for type 103 installation methods...
... but Appendix 15 does!
... but Appendix 15 does!
That's one of the errors rectified by the July corrigendum
http://www.theiet.org/publishing/wiring-regulations/updates/
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