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Topic Title: DSSOs in metal stud walls commercial situations
Topic Summary: RCD protection
Created On: 08 May 2013 07:20 PM
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 08 May 2013 07:20 PM
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tattyinengland

Posts: 787
Joined: 23 November 2006

A quick question:

How do most people install DSSOs in metal framed plasterboard stud walls in a commercial situation in such a way that you can leave off the RCD protection?

If, as I usually do, I allow to run a new circuit; I always loose the job or at least add (Unnecessary?) cost to the job - making me uncompetitive. Edited to add: I then add RCD protection to this circuit.

If I run the power from a nearby floor socket or available supply above the ceiling then no RCD protection is a common problem. A spur can't be fitted with RCD secondary side because of the same problems as for a socket on the primary side of the RCD. For a TV who would accept an RCD fused spur even if it fitted behind the TV.

When costing on a price per point document, I could not allow for an additional RCD fused spur anyway without looking like I was twice the price of my competitors.

Ideas?

Edited: 09 May 2013 at 10:44 AM by tattyinengland
 08 May 2013 07:37 PM
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OMS

Posts: 19861
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in a commercial situation


Skilled and instructed persons by virtue of EAWR - read 522.6.103

Personally, I would offer two prices - one with RCD protection, one without - explain the reason why - let the client decide how he's managing safety at his premises

Regards

OMS

-------------------------
Failure is always an option
 08 May 2013 08:03 PM
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tattyinengland

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Hi OMS

I want to offer a no RCD price.

How do I install the cables in such a way that this can be achieved. This is specifically a problem associated with plasterboard walls that have a metal frame.

In all other situations the no RCD installation is easily solved with metal conduit to a metal back box or if a wooden frame has been used, then just take the cable straight to the socket - if the depth is >50mm from the surface.

The regs insist I use RCD protection unless an earthed metal source of supply to the socket is used, but terminating this in a plastic plasterboard single or double socket box is a major - I don't believe it can be done unless you use that really, really expensive cable that is similar to FP but has a different BS number.

Edited to add: Switch drops pose a similar problem when office walls are moved around. I always try to install movement sensors, but sometimes this is not always allowed especially in meeting rooms where the client needs to switch off the lights for TV conferences etc.
Other ideas?
 08 May 2013 08:12 PM
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OMS

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Originally posted by: tattyinengland

Hi OMS

I want to offer a no RCD price.

OK - do that then


How do I install the cables in such a way that this can be achieved. This is specifically a problem associated with plasterboard walls that have a metal frame.

Anyway you like really - you aren't reading regulation 522.6.103 - the bit that says "in an installation not under the control"


In all other situations the no RCD installation is easily solved with metal conduit to a metal back box or if a wooden frame has been used, then just take the cable straight to the socket - if the depth is >50mm from the surface.

read the regulation - it's commercial, you expect there to be control of people and you won't have situations where people are unskilled or uninstructed.


The regs insist I use RCD protection unless an earthed metal source of supply to the socket is used, but terminating this in a plastic plasterboard single or double socket box is a major - I don't believe it can be done unless you use that really, really expensive cable that is similar to FP but has a different BS number.

Not it doesn't - read the regulation


Edited to add: Switch drops pose a similar problem when office walls are moved around. I always try to install movement sensors, but sometimes this is not always allowed especially in meeting rooms where the client needs to switch off the lights for TV conferences etc.

Other ideas?

Yes - take a deep breath and read the regulation (slowly) several times until your brain absorbs what it says in the context of a commercial environment.




Does that help

regards

OMS

-------------------------
Failure is always an option
 08 May 2013 09:17 PM
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peteTLM

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Tatty, you mean cable incorporating an earthed metallic foil, BS8436. FP400 complies.

Twice the price of T and E, but fantastic to install. The pirelli/ prysiamian product is huge cost, but BATT do a version which is friendly.
I wouldnt have thought the cost of that would be that prohibitive on a commercial job.

P

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 08 May 2013 09:33 PM
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dg66

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Originally posted by: peteTLM

Tatty, you mean cable incorporating an earthed metallic foil, BS8436. FP400 complies.



Twice the price of T and E, but fantastic to install. The pirelli/ prysiamian product is huge cost, but BATT do a version which is friendly.

I wouldnt have thought the cost of that would be that prohibitive on a commercial job.


P


FP400 is fire resistant armoured cable, standard SWA would be cheaper, unless Pete means FP200

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Regards

Dave(not Cockburn)
 08 May 2013 09:51 PM
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peteTLM

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yes it would be painfull! FP200 sadly dosent comply as the foil isnt thick enough, the armour helps fp400 as it does swa.

http://www.clevelandcable.com/...-1mm-10mm/Flexishield

8436 Isnt any good on anything other than Type B Mcb's though..............

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----------------------------------------
Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 09 May 2013 07:16 AM
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John Peckham

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You have to ask why this regulation is in the BGB? The bit about walls with with metal construction parts I can see as cables installed may be damaged by the sharp metal edges during construction and cause the metal framework to become live at some later date. So why the bit about the installation not intended to be under the supervision of a skilled or instructed person? How can this superior person influence the integrity of cables buried at any depth in a wall? Also I do not think that to many duty holders in buildings fit the BS7671 Part 2 definition of skilled or instructed persons.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 09 May 2013 10:40 AM
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tattyinengland

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I'm sorry for the delay to my reply - work - as usual got in the way.

I see the get out clause! I have never read it that way before (I do now) - My mind sort of skipped over that bit. Wacking in twin and earth through the top of these sorts of walls would be sincere folly - ne ce pas? (my limited French)

Why does it exist? I have to echo Johns question - why is the get out clause (Underlined below) there?

For those who do not have the BGB in front of them:

The Regulation to which I was referring is 522.6.103:

Irrespective of the depth of the cable from a surface of a wall or partition, in an installation not intended to be under the supervision of a skilled or instructed person, a cable concealed in a wall or partition , the internal construction of which includes metallic parts, other than metallic fixings such as nails, screws and the like shall: (summarised below)

(i) Earthed metallic covering
(ii) Earthed conduit
(iii) Earthed Trunking
(iv) Mechanically protected
(v) SELV or PELV
(vi) RCD complying with 415.1.1

For a cable concealed at a depth of <50mm 522.6.101 shall also apply.
 09 May 2013 11:16 AM
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Parsley

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I also understand John's concerns and have previously issued RFI's asking if the installation is under the control of skilled or instructed persons. This sort of question normally leaves the client scratching their head and me asking again at the next site meeting. PM's hate this sort of question, they think the contractor is trying to force them into paying for extras. I would also include a clarifying statement in the O&M.

Personally I think this is another good reason why T&E should not be used in commercial buildings, when I was an apprentice late 80's we never used it. Unfortunately all that matters now is money unless something goes wrong!

Regards
 09 May 2013 11:22 AM
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OMS

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The regulation exists, in my opinion due to the enquiry into the death of a plumber ina ceiling void where he had direct contact to the metal studs from the partition below and bonded metalwork.

Initially a cable had ben hit by a drywall screw and held in place but with no "fault". A second screw then pierced the "restrained" cable and put L direct onto the stud via the screw. The studs (as you would expect) are neither bonded nor earthed. The plumber touched the live stud.

Now testing should have found this in the first place ideally.

The regulation is focused on people fixing things to those partition systems and effictively hitting cables. Where unskilled and uninstructe people exist then there is no control over them drilling anything.

In a commercial environment EAWR applies, there is a duty holder and the people will be, by definition, either skilled or probably instructed. Instructed in this sense is to tell anyone who is fixing anything to identify first, any buried cables and to prhibit everyone else from bringing in thier own drill and fixing things to the employers walls.

It's not for you lot to decide if employees are skilled and instructed or otherwise - you just need to communicate with the duty hlder as to thier approach to managing safety in the context of EAWR.

Get it now ?

Regards

OMS

-------------------------
Failure is always an option
 09 May 2013 11:33 AM
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Parsley

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Originally posted by: OMS

The regulation exists, in my opinion due to the enquiry into the death of a plumber ina ceiling void where he had direct contact to the metal studs from the partition below and bonded metalwork.



Initially a cable had ben hit by a drywall screw and held in place but with no "fault". A second screw then pierced the "restrained" cable and put L direct onto the stud via the screw. The studs (as you would expect) are neither bonded nor earthed. The plumber touched the live stud.



Now testing should have found this in the first place ideally.



The regulation is focused on people fixing things to those partition systems and effictively hitting cables. Where unskilled and uninstructe people exist then there is no control over them drilling anything.



In a commercial environment EAWR applies, there is a duty holder and the people will be, by definition, either skilled or probably instructed. Instructed in this sense is to tell anyone who is fixing anything to identify first, any buried cables and to prhibit everyone else from bringing in thier own drill and fixing things to the employers walls.



It's not for you lot to decide if employees are skilled and instructed or otherwise - you just need to communicate with the duty hlder as to thier approach to managing safety in the context of EAWR.



Get it now ?



Regards



OMS


The IET obviously think organisations need to be educated and there's a market for this.

http://conferences.theiet.org/...ical-safety/index.cfm

Regards
 09 May 2013 11:41 AM
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Parsley

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I remember a block of live work apartments we did around 2004, the designer specified 4 or 6mm G/Y to be installed at various points to each metal partition wall and ceiling, I assume in an attempt to provide some sort of fault path under the same circumstance that OMS describes. Although I doubt the metal partition could have ever complied with the requirements of a CPC.

Regards
 09 May 2013 12:38 PM
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OMS

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More likley that the designer was a person who undertook his formative studies during the 15th Edition.

I worked on a number of construction types that were under the acronym "CLASP" and "SCOLA" in my early days as a designer - and many of them were all metal partion systems internally

To say that we were bonding crazy was an understatement of biblical proportions - it was just easier to do it than argue

regards

OMS

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 09 May 2013 12:43 PM
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OMS

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Originally posted by: Parsley

I also understand John's concerns and have previously issued RFI's asking if the installation is under the control of skilled or instructed persons. This sort of question normally leaves the client scratching their head and me asking again at the next site meeting. PM's hate this sort of question, they think the contractor is trying to force them into paying for extras. I would also include a clarifying statement in the O&M.


Personally I think this is another good reason why T&E should not be used in commercial buildings, when I was an apprentice late 80's we never used it. Unfortunately all that matters now is money unless something goes wrong!


Regards


We tend to do it the other way as designers. We issue a matrix of design assumptions, clarifications and failure modes with the design.

So if it's an office , then we say so, we state our assumptions wrt to RCD protection, implications of RCD protection on circuit integrity and for inrush, we state the maximum Ze at which the system or parts of a system fail.

We cover things like occupancy density, load allocation, diversity etc.

That way, if we've designed an office to say one person per 10m and the client max paks to one in 6 all on intensive PC use then we can manage the often hysterical claims about "design failure" etc.

regards

OMS

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Failure is always an option
 09 May 2013 01:26 PM
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Parsley

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Originally posted by: OMS

More likley that the designer was a person who undertook his formative studies during the 15th Edition.



I worked on a number of construction types that were under the acronym "CLASP" and "SCOLA" in my early days as a designer - and many of them were all metal partion systems internally



To say that we were bonding crazy was an understatement of biblical proportions - it was just easier to do it than argue



regards



OMS


He was a few years younger than me, I think he probably would have started early to mid 90's under the 16th. But I imagine the engineer who reviewed his design would have liked a bit G/Y.

Regards
 09 May 2013 01:42 PM
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Parsley

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Originally posted by: OMS

Originally posted by: Parsley



I also understand John's concerns and have previously issued RFI's asking if the installation is under the control of skilled or instructed persons. This sort of question normally leaves the client scratching their head and me asking again at the next site meeting. PM's hate this sort of question, they think the contractor is trying to force them into paying for extras. I would also include a clarifying statement in the O&M.





Personally I think this is another good reason why T&E should not be used in commercial buildings, when I was an apprentice late 80's we never used it. Unfortunately all that matters now is money unless something goes wrong!





Regards




We tend to do it the other way as designers. We issue a matrix of design assumptions, clarifications and failure modes with the design.



So if it's an office , then we say so, we state our assumptions wrt to RCD protection, implications of RCD protection on circuit integrity and for inrush, we state the maximum Ze at which the system or parts of a system fail.



We cover things like occupancy density, load allocation, diversity etc.



That way, if we've designed an office to say one person per 10m and the client max paks to one in 6 all on intensive PC use then we can manage the often hysterical claims about "design failure" etc.



regards



OMS


OMS, I know you do your homework and if you provide the same level of service to your clients as you do to the forum they will be very well looked after, unfortunately not all consultants seem to offer the same level of service.


In a meeting last week the clients consultant stated that a future growth of 25% must be allowed for to comply with BS7671. On this occasion, as it was the first meeting and the collaborative team was excited about the new project. I decided to keep my mouth shut.

Regards
 09 May 2013 02:14 PM
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OMS

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OMS, I know you do your homework and if you provide the same level of service to your clients as you do to the forum they will be very well looked after, unfortunately not all consultants seem to offer the same level of service.


Well, it's not all about level of service (although we all have a duty of care).

Some clients seeem to like to appoint thier consultant engineers in the same way they like to appoint thier contractors ie on a lowest cost first basis and employ practices that would make an arab in the souk blush.

So in a lot of cases, it's not neccessarily a level of service but one of rislk mitigation. as a design team lead, I know just how much it costs to have people running around fighting off ***** allegations by a client - I can allocate a set amount of time/money to myself or another sane engineer and prepare a designers statement.

If we then get into argument territory I just shove the document into the clients face, tell him to read it and then get back to me (in a nice way of course) - thus I manage the risk, and if it goes legal, it's a good basis for our legal guys to work from.

Like everyone else, there are clients, and there are clients - all deserve a service - some deserve more than others though.

In a meeting last week the clients consultant stated that a future growth of 25% must be allowed for to comply with BS7671. On this occasion, as it was the first meeting and the collaborative team was excited about the new project. I decided to keep my mouth shut.


I would have challenged it - on the basis that BS 7671 doesn't require it, it costs a lot of money to achieve and very often the client doesn't even know he's asked for it - his consultant has just "assumed". It also opens the door for the debate on diversity and everything else that a designer and certainly a D&B conractor needs to know about his probable solution based on his project cost plan.

Whilst I tend not to get too regimentally insane about terminology in a meeting where people have different backgrounds, that kind of comment is always worth a quick response as it has big (and sometimes very painful) implications for each party.

What I understand by an allowance of 25% for growth isn't always the same understanding by everybody - is it just in spare ways, is it 25% of the connected load or the diversified load - dos it apply also to the mech services plant in the sense that if the client can increase power consumption on a floor by 25% and can then do that on every floor then that has big implications of what eventually gets designed and installed from a chiller perspective - and those are electrically hungry loads so you get stuck in a loop as the total demand goes up and thus the 25% part also goes up.

Better to flush these things out early - the longer the go on, the more painful they become to resolve

"Collaborative Team" - of course, our post Egan culture makes us all collaborative teams

regards

OMS

-------------------------
Failure is always an option
 09 May 2013 04:17 PM
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jcm256

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As above posts would add, to avoid any comeback maybe you should consider complying either RCD protection or cables to BS-8436 as below. What lifetime does standard PVC cables have, was reading about that but wont give the site, (scary) according to temperature of course.


http://electricaltimes.co.uk/f...le.asp?articleid=6021

http://www.basec.org.uk/News/B...ments-Made-to-BS-8436

http://www.napitonline.com/dow...BS%208436%20Cables.pdf
 09 May 2013 05:05 PM
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Parsley

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Better to flush these things out early - the longer the go on, the more painful they become to resolve



"Collaborative Team" - of course, our post Egan culture makes us all collaborative teams

regards

It will also be ok once we all use B I M 2016
Don't worry I will formally ask the consultant for the regulation number he has basically produced a performance specification generic cut and . Paste job, I asked him if the existing 70KVA supply had enough capacity for the refurbishment? I think it will be close but it will be up to the D and B designer to assess was the response.

Regards
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