Shallow metal trunking


Postby Rinkerdink » Sat Sep 06, 2008 4:31 pm

Hi,

I am extending lighting cabling within a 25mm ceiling void (i am creating this void for this purpose). Having just checked the regs I need to encase the wiring in metal containment [i]to prevent penetration of the cables by screws and nails as it wont be 50mm deep in the ceiling[/i] I would like to restrict my ceiling void to 25mm and have seen some very shallow galvanised trunking installed but cannot find anything online?

Has anybody used this shallow trunking before and if so please could you tell me the manufacturer, I would olove to get some with recessed lid fixing screws but that may be expecting too much.

As ever hints and tips are greatly appreciated

Cheers
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Postby ericmark » Sat Sep 06, 2008 11:03 pm

BS7671:2008 says it must comply with BS 5467 (Aluminium wire armour) BS 6346 (Single wire armour) BS 6724 (Single wire armour) BS 7846 (FP100) BS EN 60702-1 (FP100) or BS 8436 (ali tube) also trunking to BS EN 50085 and conduit to BS EN 61386 which gives a lot more ways to comply with the regulations than you are implying. I would have thought the Ali tube option which includes FP100 is far easier to comply with? Maybe you have not got the BS7671:2008 book yourself and are using Chinese whispers method? With Ali tube complying with BS7671:2008 it is easy assuming that is there is some reason why you can’t use a 30ma RCD. I wonder if some one has given you selected information and has not said where a 30ma RCD is used there is no need for any special cables or conduit, trunking etc?
Eric
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Postby Rinkerdink » Mon Sep 08, 2008 7:57 pm

thanks for the response eric,

Very valid points although I should have explained myself a bit better. I have a concrete ceiling which I cannot chase and the existing wiring 1.5 t&e happens to be in the wrong place as I am changing the layout, I intended to extend the 1.5 t&e with the same using crimps to avoid the access for inspection issue associated with using maintained connections (junction boxes) and slap ceiling over the top. It is for this reason that I was looking for some containment. I would appreciate your thoughts on my thought process and also on a recommendation I was given today regarding using safe plates covering the cable routes through the 25mm joists only as used when protecting cables run through notches, perhaps i am going too far attempting to prevent cable piercing throughout the length of the run however shallow the ceiling space?

I honestly hadn't thought to cross refernce for RCD exemptions which is madness really considering the prevalence of them in the latest version of the regs. I am a qualified sparks but working in the industrial maintenance field so 3 phase industrial power and controls is more my forte hence my query regarding domestic niggles.

I enjoy what I do but have a new found respect for house bashing sparks as the amount of fiddling and niggles involved would drive me around the twist!

As ever all thoughts taken in the manner intended and I look forwrd to all comments and suggestions.

P.S. I am aware of part P and as much as it pains me I will be getting the work checked by a suitably P qualified friend - I regularly complete NICEIC inspection and test certs in industrial installations grrrrr
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Postby TOPSPARK » Mon Sep 08, 2008 8:43 pm

BS7671 says that if cables are buried in a wall less than a depth of 50mm then the circuit must be protected by an rcd. the smallest galvanised trunking i have worked with or come across is 40mmby 40 mm but cant remember where it was purchased. your best bet would be to check with one of the national electrical who;esalers like DENMANS OR WILTS OR REXEL SENATE.
regards
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Postby ericmark » Tue Sep 09, 2008 12:38 am

Rinkerdink if it is your house and you are a qualified spark in theory you don't need to comply with 50mm rule. See 522.6.7
It would be daft not to of course as you may some day want to sell the house to a non spark.
But between RCD for sockets under 20 amp and all bathroom requiring RCD protection one may as well RCD protect the lot and then you can use twin and earth.
A hilti nail will go through nearly anything so in real terms impossible to protect all one can do is ensure if it is screwed into it fails safe.
Rather surprised not to see SY cable included in the list of permitted cables I suppose because it is considered as a flex?
But if you want to avoid the RCD then the Ali-tube cable is the way to go there are a few now Flexishield is one Guardian is the other I have seen although no idea of price.
I would think you could get a really neat job like using mineral insulated which of course in another option which I didn't list earlier as not really a DIY option.
Big problem is people jump to conclusions rather than read the regs and there are a load of Chinese whispers going about and when you ask where it says it there is no answer.
The 17th Edition is causing some problems in the main as domestic has not followed the rules and regulations for years I am sure very few houses are checked every 10 years.
So they are now not only complying with 17th but also catching up with years of neglect.
But I would not like to tell a customer that?
I know when I was industrial there was one BS7671 between us all and the boss was not too keen to see us reading it and I wonder if you have access to a copy.
If so read 522.6.5 to 522.6.8 and you will see not quite as strict as you seem to think.
Also remember there is an error somewhere as the way it is put all LAN, Telephone, Door bell wiring would have to be surface this quite clearly is an oversight and I am sure there will be an amendment soon.
Eric
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Postby Rinkerdink » Tue Sep 09, 2008 8:14 am

Thx all,

I'll have a look at those regs eric, cheers again for the advice. I agree that the general standard of domestic sparking has probably been rough at best on the whole over the past (commercial/ industrial hasn't been much better!), this is what led to part P which as with most well meaning regulation changes has missed the mark and seems to be affecting those that have always operated in the correct manner.

Give me a control panel fault anyday!
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Postby ericmark » Tue Sep 09, 2008 11:36 am

I don't know about control panel faults get a PLC with Asii and one can get some real nasty faults where the inverter for a motor interferes with Asii sometimes. Can take weeks to find. Specially when you ban mobile phones only to find after the operator is hiding his in the control panel. You really think that is easier. More interesting may be but easy no.
And when you try to alter the SCADA program and the whole thing crashes time to pack bag and ask for cards!!! Didn't happen to me I was guy who had to put it right after.
HMI is no better when you forgot a internal relay was already in use etc.
I bet you have some stories to tell the domestic guys never see this side on industry.
You must have had "Tea break" "I'm losing £500 and minuet and you go on a tea break" The fact you have been working 24 hours does not seem to matter!
And it was the Push Push like that which caused my accident so not so keen to return.
Eric
ericmark

Postby Rinkerdink » Tue Sep 09, 2008 6:50 pm

Yup RCD it is, I had planned to stick the lights and general sockets, cooker etc on a 30mA RCD with the kithcen sockets on a seperate RCBO to offer a base level of protection to the food against nuisance light bulb RCD tripping, with some safety strips on the joist notches it will be fine and sweet.

Cheers eric it does help to use a sound board and get the benefit of your wealth if experience.

I have had a few hairy moments, apprentice with Ford Motor company for production line maintenance and repair and since worked in building services in business critical environments, data centre and huge buildings etc. Nothing like the prospect of thousands of people including traders sat in darkness while you try and work out what went wrong and switching 11KV to keep you on your toes. It's not necessarily easy but as you say it has it's moments and is certainly interesting.

Thanks again and get back into it soon, just don't let them get you down
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