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Topic Title: Office refurb
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Created On: 12 June 2017 09:22 AM
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 12 June 2017 09:22 AM
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CM0216

Posts: 4
Joined: 11 June 2017

I have recently started a new job and the company I work for are refurbishing an old maintenance facility.
I have been asked to look at the office power distribution and ensure each desk has a power supply. Currently there is a system left from the previous company which is the Ackermann Spider, used for raised floors in offices. There are around 25 'hubs', each hub is individually fed by SWA in tray work beneath the raised floor, from a local dist board. Each hub then has 8 16A MCB's with each MCB feeding a desk, so each hub feeds 8 desks. The system left behind has 4 core flex being used for each desk and uses 2 earths with a 4 pole wieland gst connecter on the end, from research i believe this was used to have a hight integrity earth on the circuit?
Idealy we could get the system that is left behind tested and use it but we are unable to find 4 gang extension blocks that take a 4 pole wieland connector anywhere, can only find those that take 3 pole.
So what I was considering was just running 3 core flex instead of the 4. I am just looking for some advice as I know if there is a lot of computer equipment on circuits in can cause issues but as each desk will be fed from its own MCB will that in effect solve that issue?

Hope that all makes sense

Thanks
 12 June 2017 10:28 AM
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davezawadi

Posts: 3806
Joined: 26 June 2002

Welcome.

The power to each desk can be entirely standard 3 core as long as there is not expected to be a high leakage appliance, or a number of lower leakage appliances where the total leakage exceeds 10mA. The reason for your distribution system is that 8 desks with computers is likely to exceed 10mA, typical computers will leak less than 3.5 mA each, but 8 may well exceed the 10mA figure. Clearly the high integrity earth cannot continue through the standard IEC connector, and equipment with any significant earth leakage is electronic in nature. See BYB section 543.7.
It is nothing to do with the MCB, that just provides overload and fault protection to each desk. A computer and printer and electronic desk light will be well within the 10mA limit unless faulty, so everything will entirely meet the regulations.
I will add a slight caveat, that is that RCD/RCBO protection could be required if any fixed wiring (not extension cables) is not SWA or buried from fixed surfaces by more than 50mm but from your description this is unlikely.

Regards

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 12 June 2017 10:48 AM
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mapj1

Posts: 9126
Joined: 22 July 2004

Do You mean these
connectors ?

I wonder if they might had a short 4 core lead from the right mating plug into the back of an RCD protected 13A socket per desk, if so from that point it becomes protected and a normal 3 core extension lead, and if anyone ever plugs anything in from home the chances of upsetting anyone but themselves is small - any faulty kit only cuts off the desk with a real problem. (the chances of so much stuff per desk that there is a RCD problem in normal operation is much less.)

(I'm imagining the kind that don't trip with a power cut)

Or of course I could have totally the wrong mental picture of what you are describing.

-------------------------
regards Mike
 12 June 2017 07:34 PM
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peteTLM

Posts: 3715
Joined: 31 March 2005

https://olsondirect.co.uk


Im sure they could sort you out with some pre-made leads etc.

No connection to them, just like their stuff when i have to deal with it.

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

Every man has to know his limitations- Dirty Harry
 14 June 2017 06:28 AM
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CM0216

Posts: 4
Joined: 11 June 2017

Thanks for your responses, to give a bit more info as I said each hub is individually fed from a 32A MCB. From each of the hubs 8 feeds then come off 8 16A MCBs to a plug and socket(Weiland gst 4 pole) then goes through 50mm flexibile conduit alongside data cables to each floor grommet, then there is about 1-1.5m coming out of the floor for it to plug in to each desks extension block. Each desk is likely to have a pc, phone charger etc
I would like to add an RCBO to the system but changing all 8 mcbs in each hub would prove costly so was going to change the 32A MCBs which feed the hubs to RCBOs but that would then mean each would have 8 pc's, would this likely cause nuisance tripping?

Yes mile those are the connectors currently used, looking to change to a 3 pole version

Thanks
 14 June 2017 09:07 AM
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mapj1

Posts: 9126
Joined: 22 July 2004

A PC, and any other piece of kit with a 3 core mains lead and a 13A plug, or European 16A plug for that matter, can claim CE compliance in that regard if it leaks 3.5mA or less to earth in normal use.

Actually more like 2 to 2.5 mA is more common. A 30mA RCD must fire before 30mA, and may fire as low as 15mA. I'd be a bit chary of a 30mA RCD per 8 desks, unless you know the model of PC, and what else may be plugged in - printers second monitors etc.
What would be the economic impact of losing 8 desks in mid workflow ? It may be acceptable to the business, it may not.
If not then you want the RCD at the desk, perhaps in the flex ?

-------------------------
regards Mike
 14 June 2017 09:21 AM
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Fm

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Joined: 24 August 2011

have a look at BS 6396 Electrical systems in office furniture
 14 June 2017 09:55 AM
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Ruddy

Posts: 89
Joined: 16 September 2014

surely all socket outlets upto 20amps will require RCD / RCBO protection regardless of the 50mm depth issue!!!
 14 June 2017 02:08 PM
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gkenyon

Posts: 4905
Joined: 06 May 2002

Originally posted by: Ruddy

surely all socket outlets upto 20amps will require RCD / RCBO protection regardless of the 50mm depth issue!!!
The current 17th Edition, BS 7671:2008+A3:2015 currently permits the 30 mA RCD to be omitted under certain conditions (Regulation 411.3.3 - see brief note on this here

The same regulation in Draft for Public Comment of the 18th Edition appears to require 30 mA RCD for all socket outlets up to and including 32 A, along with a requirement (531.3.2) for residual currents to be limited to 30 % of the rating of the RCD (9 mA in this case).

This means that the 18th Edition DPC in essence prohibits high protective conductor currents (> 10 mA) on socket outlets 32 A and below.

Of course, there's no saying that the final 18th Edition won't have different wording.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 16 June 2017 09:07 AM
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CM0216

Posts: 4
Joined: 11 June 2017

Thanks all for replies. The flexible conduit is 50mm that the cables run down, but the gap between the raised floor is 250mm from the floor.

We have found a solution to the problem now so thanks.

Another query though is that currently 3 meeting rooms are fed from 4mm single core cable as a ring circuit. The cables run in trunking down the corridor then through 25mm conduit into each room. They also want floor boxes added to each meeting room. I was going to modify the ring to add these floor boxes but concerned if i am extended the ring too much as seems a fairly big circuit already and can recall there being a maximum circuit length of 100sqm for 2.5 but unsure what it is for 4mm?
My background is more electrical maintenance rather than installation which is the reason for my posts
 16 June 2017 09:49 AM
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mapj1

Posts: 9126
Joined: 22 July 2004

Don't worry too much about the floor area calculation, it was only an 'advisory' figure. Your max circuit length will be limited by voltage drop and load- are you equipped to measure Zs at the point you want to add the new sockets ? (or R1+R2) you can convert this into an estimate of circuit resistance and so an upper bound on voltage drop. And what sort of loads are going in ? If it is stuff that used to be plugged in at the wall, and is moving to a floor socket for neatness, no change to total loading. If its a lot of new kit coming in then maybe more thought about total loads is needed.

-------------------------
regards Mike
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