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Topic Title: Block and beam floor/ceiling
Topic Summary: Safe zones
Created On: 29 January 2013 08:03 PM
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 29 January 2013 08:03 PM
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deleted_1_davidskelton1

Posts: 16
Joined: 10 December 2002

Good Evening
I'm wiring a new house with a concrete block and beam floor between the ground and first floors. 25mm battens are clipped to the underside of the beams to which the plasterboard ceiling will be fixed. My question is about the safe zone at the top of the downstairs walls. The zone is "150mm from the top of the wall" I've always thought of this as 150mm below the ceiling but can I go upwards as well?
NB. I am not proposing doing this in the middle of the room, where I can't get 50mm above the ceiling- only where there are walls.

Regards

David
 29 January 2013 08:59 PM
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mikejumper

Posts: 1824
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Originally posted by: davidskelton1
... The zone is "150mm from the top of the wall" I've always thought of this as 150mm below the ceiling but can I go upwards as well?
David

For practical purposes I'd say the zone dimensions start at the finished underside of the ceiling, ie, what you can see. The rest is void.
 29 January 2013 09:12 PM
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slittle

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As far as I'm aware, the area above the ceiling the 50mm rule doesn't apply so either run in the void between the battens or make use of the 150mm from the ceiling downwards


Stu
 29 January 2013 09:18 PM
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colinhaggett

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Seems a little strange to go to the expense of a concrete block and beam floor between the ground and first floors and not have a usable space between the ceiling and blocks. No downlighters and loads of fun for the plumber.
 29 January 2013 09:50 PM
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deleted_1_davidskelton1

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The heating is from an air source heat pump with UFH upstairs and downstairs hence the block and beam floor.
I thought the 50mm applied according to reg 522.6.100 (i)
 29 January 2013 10:13 PM
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AJJewsbury

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I thought the 50mm applied according to reg 522.6.100 (i)

I'd agree. You could argue that if you can run the cables between (i.e. not crossing) the battens, they'd be clear of the fixings, but that's going to be difficult to achieve at the edges of the ceiling as normally plasterboard is supported on all edges.

Personally, I'd go for BS 8436 cable. Other options include MICC or steel conduit, but they're much more work.

- Andy.
 29 January 2013 10:24 PM
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deleted_1_davidskelton1

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Thank Andy
I think I'll go for the BS 8436 cable.

Regards

David
 30 January 2013 09:18 AM
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OMS

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See if the battens can be counter battened with a further 25mm (in the opposite direction). That way you get a 50mm void, everyone is happy including the plumber who still has CWS and DHW to run - and there may well be MVHR ducting required as well - and it may well be cheaper than buying BS 8436 cable - battens and chippys are cheap compared to high spec cable and electricians

Regards

OMS

-------------------------
Failure is always an option
 30 January 2013 06:57 PM
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deleted_1_davidskelton1

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The plumber is putting his pipes underneath the insulation going under the upstairs underfloor heating. I could do the same with my cables (which would then be covered in insulation on one side) but it would mean drilling through the blocks of the block and beam floor and I am concerned that "others" might damage the cables before the floor is laid. I've got enough depth under the blocks so I am OK in one dimension but hardly enough depth under the beams - hence the original question. I wish I could draw a diagram!

Regards

David
 30 January 2013 07:18 PM
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OMS

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Originally posted by: davidskelton1

The plumber is putting his pipes underneath the insulation going under the upstairs underfloor heating. I could do the same with my cables (which would then be covered in insulation on one side) but it would mean drilling through the blocks of the block and beam floor and I am concerned that "others" might damage the cables before the floor is laid.

Bit of plastic conduit ?

I've got enough depth under the blocks so I am OK in one dimension but hardly enough depth under the beams - hence the original question. I wish I could draw a diagram!

I can picture it - the plasterboard will be almost touching the inverted "T" section of the beam - crossing at 90 degrees to the beam is the drama. Personally - I'd want 50mm between the MF ceiling and the lowest point of the beam - a bit of perforated band and away you go.

Would a top hat section in say 2.0mm steel act as a "cleat" and as mechanical protection. Fix them to the beams and wire thro' them ?

Good luck whichever way you go


Regards

David


OMS

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Failure is always an option
 30 January 2013 07:27 PM
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rocknroll

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A couple of months before Xmas we received some information from BASEC and speaking to a researcher his opinion was that BS8436 cables had still not satisfied the nail penetration test irrespective of what the manufacturers stated, in his opinion when applying the rules of BS7671 one needs to exercise caution and treat these cables as you would say T&E, the issue of they should only be used for low current applications because in high current applications they overheat very rapidly is still outstanding.

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!"
-------------------------
 30 January 2013 08:56 PM
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deleted_1_davidskelton1

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Would a top hat section in say 2.0mm steel act as a "cleat" and as mechanical protection. Fix them to the beams and wire thro' them ?



Good luck whichever way you go



OMS


Thats it! Thanks OMS

Regards

David
 30 January 2013 11:13 PM
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tillie

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Hi , just a quick question , hopefully related.

Can FP200 gold cable be located less than 50mm deep ?

Regards
 31 January 2013 10:00 AM
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OMS

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In theory Tillie, yes - but note RnR's comments re BASEC. If it helps, personally, I wouldn't accept the solution given the uncertainty that exists of the capability of that sheath to keep together under a penetration fault in all circumstances - it will in some cases, but not all.

I would currently not encourage the solution until there is some independant evidence on the issue - if BASEC are concerned, then so am I.

I'm not sure I'm talking to the same people that RnR is, but my contact with BASEC gives me the same vibe that all is not quite, perhaps as it should be with the manufacturers tests - not that the test is flawed, just that the test criteria are limited and dare I say installers won't verify within those parameters and just plough on with the "It's fine - the manufacturer says so" - - without fully understanding what they have to deal with - so MCB type selection or fault level at the point often get missed.

regards

OMS

-------------------------
Failure is always an option
 31 January 2013 10:59 PM
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TeesdaleSpark

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If BASEC are so concerned about BS8436 cables why are they still approving them?
 31 January 2013 11:47 PM
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stateit

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Is this just a single house build?

Are you the sole electrical firm working there?

Then why not liaise with the builders / other trades and agree mutually acceptable runs rather than over-engineer?

-------------------------
S George
http://www.sg-electrical.com
 01 February 2013 01:43 PM
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rocknroll

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If BASEC are so concerned about BS8436 cables why are they still approving them?


They are not concerned as these cables have been approved under the guise of being 300/500 V screened electric cable having low emission of smoke and corrosive gases when affected by fire, for use in walls, partitions and building voids and satisfy most of the EU directives on EMC.

BASEC have just advised caution when applying the rules of BS7671 to these cables with reqard to mechanical protection against nail penetration and have concerns regarding current carrying capacity.

As I said before I am not in a position to advise you on the use and installation of these cables but can only offer you the science, how you apply the science is your decision in conjunction with my opinion, the manufacturers opinion and the opinion of engineers like OMS who deal with these problems on a daily basis.

I will not reveal the test criteria or the results but without compromise it was noticed when the object penetrated the live conductor and foil the foil melted away from the object in microseconds and did in most cases not generate enough current in the time allowed to activate the appropriate protective devices (MCB's and RCD's), I have no more further to say other than that in my opinion these cables should be installed under the same conditions as T&E due to the uncertainty.

I am not with the group involved but a call to one of my colleagues at Cambridge has revealed that no further information regarding the issue of nail penetration has to date come to light since the last contact with BASEC.

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!"
-------------------------
 01 February 2013 02:26 PM
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AJJewsbury

Posts: 11772
Joined: 13 August 2003

They are not concerned as these cables have been approved under the guise of being 300/500 V screened electric cable having low emission of smoke and corrosive gases when affected by fire, for use in walls, partitions and building voids and satisfy most of the EU directives on EMC.

BASEC have just advised caution when applying the rules of BS7671 to these cables with reqard to mechanical protection against nail penetration and have concerns regarding current carrying capacity.

You seem to be suggesting that BS 8436 only has requirements for the screening of cable and the nail penetration test is BS 7671 invention, thus BASEC are only confirming the BS 8436 requirements. Is that correct? I was under the impression that the nail penetration test was a fundamental part of BS 8436 itself.

I must admit some anxiety that the tests seems to use a fairly modest fault current (in the region of 160A from memory), but I'm still a little surprised even at higher currents that protective devices apparently failed to reach the unlatching or arcing stage before sufficient aluminium was vapourised to isolate the nail from the screen. Being able to reset the protective device afterwards without it re-tripping I could much more easily believe.

Presumably there's less worry at lower fault levels, so in my case I could always TT things to keep L-PE faults more modest (there are no exposed-conducitve-parts shared with other installations).

- Andy.
 01 February 2013 02:29 PM
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OMS

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The nail penetration test is undertaken within very strict parameters - generally this means you can't use the vcale with anything other than a Type B MCB and for fault currents not exceeding 170A.

170A is pretty low

above this, as RnR suggested, the metal foil "burns back" from the nail, the fault "disapears" and the nail remains live.

Be carefiul also that there is an Irish standard that claims compliance for larger cables with 600/1000v grading - manufacturers often (perhaps mistakenly) aply that standard to UK cables - using them wouldn't comply in any respect with BS 7671.

the concern is not so much over the cable per se - just that the industry, perhaps unwittingly - , is not using them with regard to the limitations of teh installation criteria driven by the test methods.

RCD protection will help - but let's face it - most of this cable type is to get you out of having to put in RCD protection - so actually, it's a poor solution.

BASEC have been, and remain, concerned over this matter

Regards

OMS

-------------------------
Failure is always an option
 01 February 2013 02:41 PM
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AJJewsbury

Posts: 11772
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it's a poor solution

To some extent, I agree with that. In many ways I would have preferred to be able to use flexible metal conduit (not rigid as it's a pig to thread between joints etc in a domestic), but the wording of BS 7671 prevents that. Presumably flexible metal conduit is OK as an exposed-conductive-part and if solidly fixed wouldn't be liable to the vibration etc which was quoted as the reason for it not being reliable enough to be used as a c.p.c.
- Andy.
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