2396 Design exam help on processes

[chrisjw3330]

Hi
I am resitting my 2396 tomorrow and going through my past papers I have a question that I stumbled on last time out. The training provider I failed with are no longer in business, they weren't very good, so I am on my own.

The questions are ;

A 230v single phase circuit is wired using a multicore 70c thermoplastic cable having 1.5mm2 conductors and 49m in length.
It is protected by a 16a BS88c,PScc reading at the origin of the circuit is 110a.

Q. Determine if this circuit has adequate short circuit protection?

And -

Q. How do you determine if a circuit breaker will protect the live conductors in line with 434.5.2?

Not necessarily looking for answers but a breakdown of the process would be greatly appreciated.

Thanks

Chris

 

[Zdb]

Think the first one is calculating Zs and making sure that it meets disconnection time and the second one you need to use the adiabatic equation.

 

[chrisjw3330]

Think the first one is calculating Zs and making sure that it meets disconnection time and the second one you need to use the adiabatic equation.

Thanks for the response, didn't think I would get a reply as I have left it late. For the first question do you know how I might calculate the ZS without the Ze or even a clue of earthing arrangement?

 

[SparkyChick]

Read the first question again carefully... short circuit protection... you're not interested in Zs... you're interested in the line/neutral loop impedance at the origin (which you have information from which it can be derived) and the R1+Rn of the circuit.

 

[chrisjw3330]

Read the first question again carefully... short circuit protection... you're not interested in Zs... you're interested in the line/neutral loop impedance at the origin (which you have information from which it can be derived) and the R1+Rn of the circuit.

Thanks also. I did query this with the original course provider and he said just use the adiabatic equation, it came up and was worth 20 marks so knew he told me incorrectly.

So I am looking for the milliohms per metre for 2x 1.5mm conductors x LOR (49) x Cr (1.2)?

And then the adiabatic equation from there? Which is (?)

sqrt 110x110x0.2\115 ?

 

[SparkyChick]

For the first question about whether the circuit will be adequately protected from short circuit, I'd do this...

Calculate R1+Rn, calculate the LN loop impedance (R=V/I) and sum them, then compare that to the maximum loop impedance tables for the OCPD for the circuit. If the value you got is less than the table (adjusted as required) and the OCPD should meet the required disconnection times for the circuit.

 

[chrisjw3330]

For the first question about whether the circuit will be adequately protected from short circuit, I'd do this...

Calculate R1+Rn, calculate the LN loop impedance (R=V/I) and sum them, then compare that to the maximum loop impedance tables for the OCPD for the circuit. If the value you got is less than the table (adjusted as required) and the OCPD should meet the required disconnection times for the circuit.

I'll give that a go, thanks again.

I wish I hadn't let the gaffer talk me into this.

 

[Ian1981]

Adiabatic equation is I2t square root top answer divide by K
K in most cases is 115 70 degree thermoplastic conductors if conductor is not bunched or incorporated in a cable
143 for conductor incorporated into a cable or bunched.
For say steel like swa it’s 51 (using memory here) and so on.
So the fault current needs to be known either by direct measurement or by calculation.

Worth noting that for disconnection times quicker than 0.1 seconds for protective devices given in appendix 3 then the manufacturers let through energy is needed and can be used in the equation so that I2t is equal to or less than K2S2

 

[chrisjw3330]

Adiabatic equation is I2t square root top answer divide by K
K in most cases is 115 70 degree thermoplastic conductors if conductor is not bunched or incorporated in a cable
143 for conductor incorporated into a cable or bunched.
For say steel like swa it’s 51 (using memory here) and so on.
So the fault current needs to be known either by direct measurement or by calculation.

Hi Ian

Cheers for that. Does the fault current of 110a at the circuit origin need to be in the adiabatic or does the fault current at the end of the circuit need working out?

 

[Ian1981]

Hi Ian

Cheers for that. Does the fault current of 110a at the circuit origin need to be in the adiabatic or does the fault current at the end of the circuit need working out?

Surely the 110amps is at the end of the circuit?
And is your fault current for the circuit.
Sorry didn’t read earlier post about pssc.

Edit the fault current will be for the circuit at the most furthest point ie end of the circuit for a radial for example

 

[Richard Burns]

The questions appear to be very similar.
To determine short circuit protection for a 16A BS88-3c the modified adiabatic equation t= (k²S²)/I² should be used to determine the maximum disconnection time permitted and this should be compared to the actual disconnection time of the protective device at that fault current.
With 110A PSCC at origin then the resistance would be 2.09Ω.
Add the 1.42Ω of the circuit to give 3.51Ω, so a short circuit current at the extremity of the circuit of 65.5A.
t= (115²*1.5²)/65.5² =6.94s.
From graph 3A1 at 65.5A a 16A BS88-3c will disconnect in 2s which is less than the 6.94s above so the circuit has adequate short circuit protection.

The second question asks about circuit breakers and 434.5.2 which is the modified adiabatic equation t= (k²S²)/I². Circuit breakers are current limiting so the I²t for the circuit breaker needs to be obtained (35000 type B, 42000 type C) from the manufacturer or product standard and this value must be less than the k²S² (29756) for the circuit.

 

[chrisjw3330]

Surely the 110amps is at the end of the circuit?
And is your fault current for the circuit.

Edit the fault current will be for the circuit at the most furthest point ie end of the circuit for a radial for example

The question states at the origin of the circuit in question.

The questions appear to be very similar.
To determine short circuit protection for a 16A BS88-3c the modified adiabatic equation t= (k²S²)/I² should be used to determine the maximum disconnection time permitted and this should be compared to the actual disconnection time of the protective device at that fault current.
With 110A PSCC at origin then the resistance would be 2.09Ω.
Add the 1.42Ω of the circuit to give 3.51Ω, so a short circuit current at the extremity of the circuit of 65.5A.
t= (115²*1.5²)/65.5² =6.94s.
From graph 3A1 at 65.5A a 16A BS88-3c will disconnect in 2s which is less than the 6.94s above so the circuit has adequate short circuit protection.

The second question asks about circuit breakers and 434.5.2 which is the modified adiabatic equation t= (k²S²)/I². Circuit breakers are current limiting so the I²t for the circuit breaker needs to be obtained (35000 type B, 42000 type C) from the manufacturer or product standard and this value must be less than the k²S² (29756) for the circuit.

These were the methods I originally started working both questions out with but then started to doubt myself.
Thanks for the input, a lesson learned for me here is don't do home study and choose providers more carefully. Thanks.

 

[Richard Burns]

Good luck with the exam.

 

[Spottydogs]

Best of luck to all doing it!

I'm also doing the exam tonight, keeping my fingers crossed it isn't as bad as I'm expecting it to be! Long time since I've sat and done a 3 hour exam and my 2.5 year old kindly passed on his cold and cough to me!!

 

[Spottydogs]

How did you find it? I seemed to start off ok enough but time began to beat me and I'm not sure I answered some of the later questions as well as I might have. Not really sure if I did enough, blooming hope so though, I'd rather not have to do it all again!!

 

[Strima]

I remember doing mine. You may start, pens down....

Where did that three hours go and what just happened???

 

[Spottydogs]

Yep that middle hour just disappeared. Can you recall how long it took for results? I expect it will be well into January now?

 

[Strima]

Took about 10 weeks for the results to come through.

 

[Spottydogs]

Passed with Merit, pretty pleased with the result!

 

[Strima]

Well done, I was overjoyed with a simple pass. IIRC only 40% of my written test peers passed that part.

 

[Strima]

Have you started your project yet?

 

[Spottydogs]

Not sure on the pass mark this time round, maybe the report will be out soon. Didn’t like to ask tutor how others had got on.

I made a very brief start on the project, answers first couple of questions and began sourcing some suitable luminaries etc. But towards end of the course I decided it was best to focus on the exam, no point going too far with project if I hadn’t got through that!!

Just need to get my head down now between work and family and start chipping a few hours off.

 

[Zdb]

What type of building are you designing for your project ?

 

[Strima]

Not sure on the pass mark this time round, maybe the report will be out soon. Didn’t like to ask tutor how others had got on.

I made a very brief start on the project, answers first couple of questions and began sourcing some suitable luminaries etc. But towards end of the course I decided it was best to focus on the exam, no point going too far with project if I hadn’t got through that!!

Just need to get my head down now between work and family and start chipping a few hours off.

Take my advice, even if you dedicate a couple of hours a week stick to it. I aimed to complete mine within 6 months and it still took me a year.

Who's the company/college you taking it with?

 

[Spottydogs]

What type of building are you designing for your project ?

Electrical Training Centre!

 

[Spottydogs]

Hi all, beginning to make a start on the project now. Does anyone have any example projects they would be able to share? Obviously I know they’re all different, I’m just interested to get some ideas on how best to present it. For example how did you do your cable runs/lengths without it looking a mess. How did you present your calcs in electronic format (I’ve not really done much with excel). Any other time saving tips would also be welcome!

 

[Zdb]

I can't seem to upload any files other than images to this thread. P.M me your email address and I'll send a template over.

 

[Deleted account]

I did mine just over a year ago. I found the questions easy enough, but I just didn't have time. 3 hours goes by pretty quickly.
I got to 2 hours 45 mins and had only answered 7 questions, so I just scribbled what I thought was nonsense for the last 3 questions in the hope of grabbing a few more marks.

Amazingly I did pass. Guess I must have answered the first seven questions really well.

Good luck

 

[Spottydogs]

Sounds familiar Pete. I was frustrated after the exam that with a little more ‘real world’ time I could have done a better job of some of the questions. However I got through it, so I must have done well enough on the earlier questions!

Finding time around work and normal family life is definitely a challenge and I need to keep motivating myself not to just say ‘I’ll do some next weekend’.

 

[Strima]

I've attached a couple of examples of circuit sheets and one question I did for my project. This format may not be ideal for you but it worked for me. Please feel free to bastardise as you wish.

Make sure you answer each question, you may feel as if some of your answers repeat themselves but this is just the nature of the project.

DB1 Way 1-3 HVAC.pdf

DB1 Way 2L2-Coffee Machine.pdf

Max Demand.pdf

Edit: And as for getting time, I decided to dedicate one day a month to this, a couple of hours and you'll only just get into the swing of things. I manged to book a small conference room in work so I wouldn't be disturbed. However there might be a company in your local area that rents out hot desks etc. I gave in trying to do this at home due to too many distractions.

 

[Spottydogs]

Thanks for that Strima. How did you go about showing the wiring routes on the drawing, did you simply have line after line in a different colour showing the intended routes down to switches and then back to the luminaries? Or did you split lighting and power? I’m think I’m mainly a little unsure just how much details they’re looking for and I don’t want to spend time going ott or perhaps worse, having to go back over it again because I’ve not covered enough!

 

[Strima]

Thanks for that Strima. How did you go about showing the wiring routes on the drawing, did you simply have line after line in a different colour showing the intended routes down to switches and then back to the luminaries? Or did you split lighting and power? I’m think I’m mainly a little unsure just how much details they’re looking for and I don’t want to spend time going ott or perhaps worse, having to go back over it again because I’ve not covered enough!

Depends on what they are asking for. If they are only asking for wiring routes don't worry about cable drops etc. I went down my local stationary shop and got some of the drawings printed out on A3 and A2. I had quite a few A3 sheets used for rough drawings and routes.

I roughly sketched out my wiring routes then went with my circuit designs, so I could get away with buried in plaster others needed to be on perf tray for CCC etc. Once I was happy with the circuits I then went back to the wiring routes.

Don't show individual circuits on the sheets unless they have asked for it, just the wiring routes. Otherwise the whole thing will look like a plumbers scribble...

All I did was draw on the drawing where my tray/buried/clipped runs were using different colour pens, don't use crayons as plumbers like to eat them. I kept most of my installation run on tray wherever possible with drops buried in plaster. You do not have to specifically state, unless they really ask, what size tray you will be using. Just give them the generic 300, 200, 100, 50mm sizes.

Put in little comments on a cover sheet such as:

• Installers are to place tray in such a position as to allow access to cable drops whilst supporting the cable.
• Containment is to be tested for continuity and earthed as per manufacturers instructions.
• Cable drops buried in plaster are to have suitable mechanical protection.
• And any other -------s that sounds good etc etc etc

Also for the lighting I split the installation down into zones and just colour coded them on separate sheets.

I would stick a copy of mine up but I did these by hand and never made a copy of them.

 

[Zdb]

Managed to upload this. Someone may find it useful in the future.

 

[Leomax]

Hi
I am resitting my 2396 tomorrow and going through my past papers I have a question that I stumbled on last time out. The training provider I failed with are no longer in business, they weren't very good, so I am on my own.

The questions are ;

A 230v single phase circuit is wired using a multicore 70c thermoplastic cable having 1.5mm2 conductors and 49m in length.
It is protected by a 16a BS88c,PScc reading at the origin of the circuit is 110a.

Q. Determine if this circuit has adequate short circuit protection?

And -

Q. How do you determine if a circuit breaker will protect the live conductors in line with 434.5.2?

Not necessarily looking for answers but a breakdown of the process would be greatly appreciated.

Thanks

Chris

Do you have past exam papers

 

[Leomax]

Hi
I am resitting my 2396 tomorrow and going through my past papers I have a question that I stumbled on last time out. The training provider I failed with are no longer in business, they weren't very good, so I am on my own.

The questions are ;

A 230v single phase circuit is wired using a multicore 70c thermoplastic cable having 1.5mm2 conductors and 49m in length.
It is protected by a 16a BS88c,PScc reading at the origin of the circuit is 110a.

Q. Determine if this circuit has adequate short circuit protection?

And -

Q. How do you determine if a circuit breaker will protect the live conductors in line with 434.5.2?

Not necessarily looking for answers but a breakdown of the process would be greatly appreciated.

Thanks

Chris

Chris do you have past exam papers.

Leo

 

[westward10]

Chris do you have past exam papers.

Leo

Email address removed. The original poster has not been seen for nearly three years so you are not likely to get a response.