Solution to 2391 Q26 (june 10 paper)

[pushrod]

and for last bit of question where it asked what you could do i put increase csa of conductor
head had gone completly by then just hope i did enough earlier on

nothing wrong with that other answers could be reduce the run length or reduce the load, but they might not be possible any way so i reckon you chose the best answer.

(England calls now)


edit : what a waste of time that was, shocking :-(

 

[Charles West]

here is my solution... and a definitive one too

cable is 80m @ 1.87 mΩ....
Resistance of line conductor= (80 x 1.87) / 1000 =0.15 Ω

Load is 45A. Expressed as a resistance 230/45 = 5.1 Ω

Resistance of Neutral conductor= (80 x 1.87) / 1000 =0.15 Ω

Total circuit resistance of 5.4 Ω


42.6A would flow in this circuit

So we have 3 resistors in parralel.

--------| 0.15Ω |-----| 5.1 Ω |-----| 0.15Ω|-------


Vdrop across Line conductor would be ----- 0.15 x 42.6 = 6.4V

Vdrop across load would be ------------------- 5.1 x 42.6 = 217.2V

Vdrop across neutral conductor would be --- 0.15 x 42.6 = 6.4V


So, Vrop in circuit conductors would be - 12.8 V

I did a slightly simpler solution. I took the 45A to mean the current which flows even with the cable length in the circuit, rather than thinking that 45A is the current which would flow if the fixed equipment was connected directly to the supply. The question described the current as part of the 'circuit characteristics' rather than a characteristic of the equipment. I ended up with a voltage drop of 13.464 which failed the max. permitted 11.5V.
I recorded a Code 2 observation i.e. 'Requires Improvement'.
I then suggested the only way to improve it was to replace the cable with a 16mm run. I can't think of any other way to reduce voltage drop. The equipment was fixed so cannot be moved. I suppose you could replace a section of the 10mm cable with 16mm, say 20m from the board to an FSU connected to the remaining 10mm cable, and that might reduce the VD enough so you don't have to replace the whole lot. Someone could work out exactly what this length should be.

 

[brucelee]

are you sure resistance of conductor was 1.87 according to fig 1 it was 1.83 as that was the info provided either way i naffed it up i think not sure how i answered dont know if i found resistance first ie m/ohm/M X l
divide 1000 ie 1.83 x 2 x 80 divide 1000 =292.8 divide by 1000 = 0.2928 and i may have rounded up to 0.3 or left it as it was and then x 45 if it was
0.2928 x 45 = 13.176 rounded up to 13.2v
or
0.3x 45 = 13.5 think i did it the first of these and ended up with vd=13.2v
which is not acceptable

all I know my head was mashed by then hope i maage to get a few marks from it even if i did get the answer wrong

 

[SafetyFirst]

Well i never even got to this question!

 

[Toonlad]

Well this question was really so fickle in the fact of one letter making the difference Conductor(s).So damn annoying LOL.

In all fairness city and guilds should not make a question like so, that it can be totally misunderstood.Proof of this alone is all the posts on this question on these forums.I felt a lot of questions in section A were poorly stated also.

We being electricians want to get it spot on in everything we do,so a question like this gets all the interpretations possible, which makes this a great forum.

On the whole the chief examiner needs to make comment on this in his wash up on this exams results.(maybe stick there hands up on this one) But it wont happen lol.

 

[T&Earth]

Hi Guys

Also did the 2391 exam on 10 June has anyone got a copy of the paper, as I think I might have FAILED looking at this post. Would love to get a copy to work through for my resit in October.

Many Thanks
Steve

 

[jreid67]

is there only 3 times in a year you can sit this exam? and how much does it cost if you need to re-take?

 

[CRAGS]

depends on centre etc some sit it more than others

 

[iceman]

i thought it said that the live conductors for 10mm2 was 1.83 mohms / m value

so it mite go something like this

2x mohms/m x Ib x L / 1000
2x 1.83 x 45 x 80 / 1000 = 13.1 v

 

[brucelee]

it did mate

 

[thorneellis]

eek things arnt looking good for me. i just waffled loads of figures. i found that one of the harder questions.

 

[brucegenerator]

get a life mate f u got it wrong move on the exam was 3 weeks ago

 

[GStueyXR]

get a life mate f u got it wrong move on the exam was 3 weeks ago

WTF ? Get a life ?

 

[Sintra]

get a life mate f u got it wrong move on the exam was 3 weeks ago

Keep it civil absolutely no need for that comment

 

[kmcgarry]

does anyone have a copy of the 2391 june paper they could e mail me please