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Topic Title: 2391-20 help
Topic Summary: Working out diversity
Created On: 04 March 2011 06:32 PM
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 05 March 2011 06:19 PM
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12216

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Many thanks to you all for your replies.
Dale. no i wont be sitting the exam until june.
sorry kJ i should have said 2391-20 (design & verification. it is the 2391-20 (old 2400) was told that i should find this not as difficult as when i took the 2391-10 inspection & testing (which i managed to pass).

many thanks again for all your help

Regards
Martin
 06 March 2011 07:27 PM
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flashtestdummy

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Hopefully someone can advise me on this, im on this course as well and I am trying to work out the diversity for my socket circuits.
No information has been given as to approx loading per point etc, but it is a commercial installation comprising workshop, offices, and showroom

DB1 - 4No 32A Ring Finals
DB2 - 4No 32A Ring Finals
DB3 - 5No 32A Ring Finals
DB5 - 1No 20A Radial

How would I go about this if I dont know my current demand of largest point of utilisation and of every other point of utilistaion?

Many Thanks
 06 March 2011 07:44 PM
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Dale76uk

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Page 96 OSG might be a good start
 06 March 2011 07:48 PM
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flashtestdummy

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Originally posted by: Dale76uk

Page 96 OSG might be a good start


What a waste of a post.
 07 March 2011 07:53 AM
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OMS

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Originally posted by: flashtestdummy

Hopefully someone can advise me on this, im on this course as well and I am trying to work out the diversity for my socket circuits.

No information has been given as to approx loading per point etc, but it is a commercial installation comprising workshop, offices, and showroom



DB1 - 4No 32A Ring Finals

DB2 - 4No 32A Ring Finals

DB3 - 5No 32A Ring Finals

DB5 - 1No 20A Radial

How would I go about this if I dont know my current demand of largest point of utilisation and of every other point of utilistaion?

Many Thanks


Get into the habit of converting your circuit arrangements to a w/m2 basis.

For the offices you could use somewhere in the region of 30w/m2 based on occupancy of 1 person per 10m2

For the showroom probably about half that unless you expect to plug "display type lighting" into socket circuits - the demand will also be time stepped (generally cleaners of out of hours use) - if there are workstation in the showroom go for about 300W per workstation.

The workshop is really dependent on the function but somewhere around 50W/m2 would cover most applications.

When you have this data then you can decide what area a circuit covers (you can apply the same approach to lighting) you have a design current per circuit.

For your point loads (water heating, ventilation, cooling etc) you need to take a sensible view on what the load represents rather than just lumping nameplate ratings to them

You can then consider applying a bit of diversity to the sub mains feeding these boards so you end up with the size of your distribution.

Spreadsheets are ideal for this kind of work because with a bit of simple programming, you can experiment quickly with a variey of scenarios. There are a variety of formats floating about

If this is for C&G I would also try to use the methodolgy for diversity in OSG or GN1 as a comparator to the more grown up method above

Good luck

OMS

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 07 March 2011 07:55 AM
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OMS

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Originally posted by: Dale76uk

Page 96 OSG might be a good start


Personally, I would rip page 96 out of the book - it's about as much use as an ashtray on a motorbike for anything other than the classroom

Regards

OMS

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 07 March 2011 10:46 AM
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kj scott

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This is the problem; the classroom is usually limited by OSG; as it is easy reference for tutors with no actual experience of design. Which sends people out into the work place without a real starting point.

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 07 March 2011 12:03 PM
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OMS

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Well I guess there are tutors and then there are tutors - some I know do use several methods such as I described above and take the time to contrast them against OSG.

Personally, I used to set a simple homework exercise, for a school design, based on a real building and get the students to do an OSG exercise as homework. I then produced another method (or variety of methods) for discussion in class to get a comparison. Finally, I gave the students a copy of site recordings taken during the first year of operation for the whole site broken down with data for each DB, major plant item etc based on an enhanced Part L M&T strategy. That usually brought things into sharp focus

For sure though I still meet plenty of people who think OSG is the be all and end all of diversity and even a few who still insist on applying xyz Amperes per outlet as design criteria

Regards

OMS

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 07 March 2011 06:36 PM
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flashtestdummy

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Many thanks for your informative reply OMS. The figures you quote per m2, would they be something you have learned from trial and error or is there somewhere I could read further into that method?

The "real life" senario you set your class would definitltey be an interesting task, its a pity the C&G Course doesnt go into so much detail

As you suggest as its a C&G exam I may be best to go by the OSG allowances (even though the installation will be vastly over-rated) but at least I can state I have made reference to it
 07 March 2011 10:02 PM
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Scott, just wondered do you delver 2391-20 training then??? You seem to suggest the OSG is non runner?
 08 March 2011 07:51 AM
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kj scott

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Use of OSG will result in over sizing your installation design; and it is very limited in its range of application.
The methods suggested by OMS are much more use for real world application.
Surely practical methods should be taught, using realistic examples and the C&G projects and exams should also reflect this. You would then be training people to do the job instead of just to pass exams.

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 08 March 2011 10:02 AM
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OMS

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Originally posted by: flashtestdummy

Many thanks for your informative reply OMS. The figures you quote per m2, would they be something you have learned from trial and error or is there somewhere I could read further into that method?

You could take a look in such places as CIBSE Guide K, CIBSE Guide F and BSRIA Rules of Thumb. That said, most designers tend to keep a little record book of solutions for varying scenarios which might include measured data. In any event, the key is, the numbers are not cast in stone - they need to be evaluated and accepted as credible or not for a particular installation (and the approach taken to design risk) - for example, I won't design a school for less than 35W/m2 all up usually, regardless of how low we could go based on measured data.

In the example for the school I gave above, OSG suggested a minimum of a 280kVA supply (400A TP&N). A w/m2 approach brought that down to 140kVA (200A TP&N) - the measured data over the first year showed the school never went over 70kVA (100A TP&N) - so less than 25% of the original design estimate


The "real life" senario you set your class would definitltey be an interesting task, its a pity the C&G Course doesnt go into so much detail.

Well to be honest, I agree it's not actually on the "syllabus" but a little effort by the candidates and a little application by the tutor, a spare classroom and an hours overtime can usually sort this out.

I guess the key problem is it needs the willing to be led by the knowing - if either of those aspects is missing it ain't going to happen



As you suggest as its a C&G exam I may be best to go by the OSG allowances (even though the installation will be vastly over-rated) but at least I can state I have made reference to it


As I said, I would do both and state in your design assumptions why you have deviated from the OSG (principally because the result isn't credible in your project - it can't be marked wrong as BS 7671 requires an assesment of demand including diversity if applicable - it doesn't say how to get there however. Persoanally I would be aiming to award a better mark to a candidate who had tried to show some engineering judgement and had possibly gone a little adrift than I would to some unthinking candidate who had just replicated OSG without ever questioning the validity of the approach.

That said, it's a while since I taught in a formal situation so take the above advice with a degree of caution - being right doesn't always translate to being correct in an exam scenario



Spend a bit of time on documenting your Part 3 assessment and include a statement of design assumptions (including failure points such as max Ze you have designed for etc) - use a spreadsheet to collect together your circuit loads and manipulate them to get a bit of balance - apply a bit of diversity.

When you have this complete, then decide which circuits you are going to calculate - think about the idea of an index circuit approach - for example if you have 6 lighting circuits calculate the shortest, lightest load and longest, heaviest load - the remaining 4 circuits then must comply by default, no - and thus you save a bit of number crunching time you can then devote to product selection, drawings etc

Try and do volt drop first though, - if it doesn't comply, then usually you have an underlying problem anyway

Other than that - good luck

Regards

OMS

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 08 March 2011 10:21 AM
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OMS

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Originally posted by: Dale76uk

Scott, just wondered do you delver 2391-20 training then??? You seem to suggest the OSG is non runner?


I have done in the past - and for sure it needs to be discussed as it has some value, but any tutor who fails to point out the problems associated with the approach for real design problems is doing a gross dis-service to the candidates in my opinion.

If that means retiring to the local boozer for a "chat" after class then so be it .

Learning can take many forms, and in my experience a combination of formal syllabus instruction and a more informal "discussion amongst peers" pays dividends for the students.

Regards

OMS

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 08 March 2011 10:41 AM
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AJJewsbury

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It might be worth noticing that the conventional 30A ring covering the historical/rule of thumb 100 m2 equates to 69 W/m2 - and in the past that was expected to cover kitchen areas and an amount of space heating!
- Andy.
 08 March 2011 11:28 AM
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OMS

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And that equates nicely with mechanical design practice of 70w/m2 (or 40w/m3) for heating purposes in domestic properties just complying with the (then) building act.

For a domestic, for sure you may want a little more than say 15w/m2 for small power because at home people will max pack stuff into the space.

In commercial applications though, the power density is actually quite low - 15w/m2 is normal as a diversified load and say 25w/m2 capacity at the risers. Take a floor plate of 675m2 (9 x 7.5 structural grid, narrow plan(18m) glazed both sides and open plan, with a layout grid of say 1.5m2)

So deliverd over say 9 RFC's that would come to about 12 kW or about 16A per phase (say 6A per ring)

Contrast that with OSG which would tend towards 45A per phase - over double and would be about 45W/m2

Even at the riser level, using 25W/m2 the design would be about 25A/phase

Regards

OMS

Note the values above are for illustration - check them before using for design

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Failure is always an option
IET » Wiring and the regulations » 2391-20 help

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