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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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 04 March 2011 06:32 PM
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12216

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Joined: 22 August 2010

Hi All
i am at present doing a 2391-20 coarse and have to work out the assumed current of a small hotel based on Table 1B column 2 page 97 OSG.

please could someone give me advice on the following.

i have a 400v/230 TP-N-E DB. 3-off the final circuits from the DB have a 3 kw heater on each circuit

To work out the assumed current on that circuit is it 100% of each heater allowing no diversity as there is only 1 heater on each circuit. e.g 100% of largest appliance

Now if there was 3 of 3 kw heaters on 1 circuit then can i now apply diversity on that circuit.e.g 100% largest, 80% 2nd largest, 60% of remaining heater.

Or irrespective of how the heaters are installed on what circuit is it a simple case of i have 3 heaters in the installation and apply the diversity as Table 1B, column 2 in relation to the heaters..

Your help would be greatly appreciated on the above.

Many Thanks

Martin
 04 March 2011 06:49 PM
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OMS

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Originally posted by: 12216

Hi All

i am at present doing a 2391-20 coarse and have to work out the assumed current of a small hotel based on Table 1B column 2 page 97 OSG.

please could someone give me advice on the following.

i have a 400v/230 TP-N-E DB. 3-off the final circuits from the DB have a 3 kw heater on each circuit.

OK - so you have 9Kw demand on the main to that DB which could be balanced to give 3kW per phase before diversity or less if you apply diversity

To work out the assumed current on that circuit is it 100% of each heater allowing no diversity as there is only 1 heater on each circuit. e.g 100% of largest appliance./

The design current Ib for each circuit will be 100% of 3kW. The diversity applies to the supply to your distribution board. So each final circuit feeding a heater needs to be designed to carry 3000/230 = 13A. Say a 2.5mm2 radial on a 16A or 20A Type B MCB (other issues such as grouping etc notwithstanding


Now if there was 3 of 3 kw heaters on 1 circuit then can i now apply diversity on that circuit.e.g 100% largest, 80% 2nd largest, 60% of remaining heater.

You could, although you would be taking a hell of a risk unless you were very clear about when each heater could be used and understood the risk of three tired travellers all rocking up at about 7.00 pm on a cold March evening and turning on the heaters in thier respective rooms whilst they slumped on the bed and decided what to do for supper.


Or irrespective of how the heaters are installed on what circuit is it a simple case of i have 3 heaters in the installation and apply the diversity as Table 1B, column 2 in relation to the heaters..

No - you can apply the divesity upstream of the final circuits in this case - I would be wary of applying it at a final circuit level.


Your help would be greatly appreciated on the above.

Many Thanks

Martin


Regards

OMS

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 04 March 2011 07:10 PM
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kj scott

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Are you sure this is a 2391 course; calculation of diversity isn't 2391; or even 2382 subject matter.
The OSG is not relevant to 2391; it should centre around IEE GN3.

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 04 March 2011 08:10 PM
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Dale76uk

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Originally posted by: kj scott

Are you sure this is a 2391 course; calculation of diversity isn't 2391; or even 2382 subject matter.

The OSG is not relevant to 2391; it should centre around IEE GN3.


It's the 2391-20 (old 2400), it's the follow on from the 2391-10 which most electricians have done. This course really separates the boys from the men. For further information on this course visit www.electricaltraining.co.uk
 04 March 2011 08:21 PM
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Dale76uk

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The diversity allowances on page 97 of OSG are really for guidance only, and you should make your own judgement based on the installations utilisation.

The guidance given in table 1B allows :-

100% of fl of largest water heater (instantaneous type) + 100% fl of 2nd largest heater and 25% of remaining heaters.

However, water heaters that are thermostatically controlled have no diversity allowed.

Hope that helps
 04 March 2011 08:22 PM
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Dale76uk

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Might you be doing your exam on Tuesday evening?
 04 March 2011 08:24 PM
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peteTLM

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(yes dont i know it!)

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
 04 March 2011 08:39 PM
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Dale76uk

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Jump onto Www.electricaltraining.co.uk/forum there is a general discussion on the 2391-20. If you need help others and myself, also taking the exam on tuesday, will try to help.
 04 March 2011 08:59 PM
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neiljohnson52

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Probably won't be much use to you before Tuesday, but the IET Electrical Installation Design Guide has some useful information and calcs. Could be worth a look.
Good luck with the exam.
 04 March 2011 09:02 PM
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peteTLM

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yes it is an excellent book for sure.
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
 04 March 2011 09:50 PM
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kj scott

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Originally posted by: Dale76uk

Originally posted by: kj scott



Are you sure this is a 2391 course; calculation of diversity isn't 2391; or even 2382 subject matter.



The OSG is not relevant to 2391; it should centre around IEE GN3.




It's the 2391-20 (old 2400), it's the follow on from the 2391-10 which most electricians have done. This course really separates the boys from the men. For further information on this course visit www.electricaltraining.co.uk



I possibly stand corrected; C&G have changed the course numbers so many times; that you need to say what course it is; rather than use the numbers. Its been / is 100 /200 /300/ 10/ 20 who knows.
The practical is 301 at the moment. bring back 2400; giving clear distinction between qualifications .
Aside from that design should refer beyond OSG. Or everyone will need their own substation.
Why would I need to refer to the site quoted; when C&G give all information direct?

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 04 March 2011 10:05 PM
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Dale76uk

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Well the 2391-10 is the inspection and testing course and 2391-20 is the design course. Both courses comprise of modules, so 2391-10 has two modules 301 and 302. The 2391-20 comprises of three modules, 302, 303 and 304. Perhaps you didn't read the OP properly???
 04 March 2011 10:25 PM
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kj scott

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I did read it; but felt that OMS had already answered the question adequately. Just don't feel that OSG should be a basis for assessing diversity at level 3. C&G can't make up their minds as to what to call the course or its elements; as they keep changing it.

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 04 March 2011 10:33 PM
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Dale76uk

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You have to remember it's guidance only, real assessments can change depending on the utilisation of the system. OSG really is for most domestic situations, however, does cover some light/commercial situations. :-)
 04 March 2011 10:39 PM
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peteTLM

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We were told that we can make up our own minds regarding diversity, but whatever we do and what assumptions we make (its a pretty open ended project with bare information) we have to state what we are doing.
its the end of level 3. Long live level 3.

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
 04 March 2011 10:45 PM
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Dale76uk

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Yes pete, it's really down to you as the 'designer' but you need to justify your reasons. :-)
 04 March 2011 10:46 PM
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Dale76uk

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Take a look at 311.1
 05 March 2011 11:10 AM
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OMS

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As you will have concluded Martin - application of diversity in real world design practice is never easy and generally revolves around some degree of risk transfer between the client and the designer.

Essentially it's simply a financial tool to limit the installed cost of a design. Used wisely, diversity will give you a safe installation, with inherent flexibility at a competetive cost. Get it wrong and you may well have a few red lorries parked outside your former clients building and his insurers on the phone to you.

Regards

OMS

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Failure is always an option
 05 March 2011 12:33 PM
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markLEDS

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Going back to the OP, should any diversity be applied to electric heating in a hotel?

Is it just in the rooms or the common parts as well? How is it to be controlled? Is it controlled by the room contactor so that they can only be on when occupied? How many rooms are typically occupied at anyone time?

They arent questions that the OP needs to answer by the way, just an example of the questions that should be asked to arrive at an answer.

I hate diversity - as people have commented above it is a big risk and not something that can be assessed from a book but needs a good understanding of the building, how it is being used and the type of people using it are all key.

I dont really see how there can be a right or wrong answer and as such how can it be on an exam - unless it is designed to get you to show you understand the above and not just quote info out of the regs?
 05 March 2011 12:44 PM
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OMS

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Originally posted by: markLEDS

Going back to the OP, should any diversity be applied to electric heating in a hotel?

I would say yes - particularly as electric heating units are generally of fixed output but room heat losses vary and the whole system responds to diurnal swings so there is often a great mismatch between connected load and demand

Is it just in the rooms or the common parts as well? How is it to be controlled? Is it controlled by the room contactor so that they can only be on when occupied? How many rooms are typically occupied at anyone time?

They arent questions that the OP needs to answer by the way, just an example of the questions that should be asked to arrive at an answer.

Indeed

I hate diversity - as people have commented above it is a big risk and not something that can be assessed from a book but needs a good understanding of the building, how it is being used and the type of people using it are all key.

Nothing wrong with diversity - it just needs to be appropriate

I dont really see how there can be a right or wrong answer and as such how can it be on an exam - unless it is designed to get you to show you understand the above and not just quote info out of the regs?

I believe the award is based on examination of a submitted design project against a brief - I guess the outcome needs to be credible rather than a pass/fail criteria and is presumably "marked" by competent people


Regards

OMS

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Failure is always an option
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