Copper resistance question

[PeterPan27]

Hi all, I've got the 2394 written tomoz. Passed the gola and practical. Just trying to do little brush up.
Got this question in a book..
A ring circuit is 54 metres long and wired in 2.5/1.5 thermoplastic cable 32 amp type c device. Ze is 0.24.
The resistance of 2.5mm copper is 7.41mohms per metre and 1.5mm copper is 12.1m ohms
Question is what's the zs of the circuit
Now I've got the answer of course in back of the book...
Basically 2.5/1.5 has a resistance of 19.51 per metre so it's
Length 54 metres x 19.51 divide by 1000 equals 1.05 and because a ring u divide that by 4 which equals 0.26 ohms making r1 plus r2 0.26 ohm then add that to the ze equals 0.5 which is the zs...
Using the regs type c 32a device is 0.75.
So I got 80 percent of the 0.75 by 0.75x0.8 equal 0.6 so the zs is within regs..
My question is the 19.51 resistance isn't in the question so how do I get that? Is there a page/table in regs I need to learn?

 

[PeterPan27]

U done the practical n gola yet mate?

 

[Lou7]

Easy way to get max zs
46 divided by mcb. B type
23 divided by mcb. C type
11.5 divided by mcb. D type
Cable resistances in osg.

 

[mtfpuk2011]

yes passed and got 85%

 

[telectrix]

0.75 is the 32 a type c max value

Do I have to learn all these values?
I know the 32 type b is 1.44 etc but do I need learn all as closed book exam

you can work the values out using ohm's law. R=V/I, where V=230 and I=5xIn for B type, 10xIn for C type, and 20xIn for D type.

 

[PeterPan27]

That 46,23,11.5 is close to spot on and don't even have to revalue the rule of thumb lol
Mint that

 

[PeterPan27]

Doing this question now and the Zs works out as 0.94
And the permissible value is 1.09x0.8= 0.87

Says this is acceptable??
I thought it has to be equal or higher for it to be ok??

 

[mtfpuk2011]

no its has to below the correct value

 

[PeterPan27]

Yeh that's what I ment. Comes out as o.94 and permissible value is 0.87 so I would say this would not be acceptable.
The answer in the book says its ok lol

 

[ruston]

Has the question asked you to use correction factors or the rule of thumb , you only answer the question.

 

[PeterPan27]

Yes it's says "using the rule of thumb for temperature correction the maximum permissible value is 1.09 so 1.09x0.8=0.87
It's strange for book to be wrong

 

[ruston]

Yes it's says "using the rule of thumb for temperature correction the maximum permissible value is 1.09 so 1.09x0.8=0.87
It's strange for book to be wrong

Depend where you got the book lol. .Are they stating they have used the rule of thumb , or for you to use the rule of thumb in the context of the question

 

[mtfpuk2011]

you dobt corrextion facter if your using on site gide as they have all ready be corrected.

 

[PeterPan27]

Practical guide to inspection testing and certificating by Christopher kitcher......
This is there exact answer

"Zs will be 0.34+0.6=0.94 ohms
So Zs=0.94 ohms
The maximum Zs for a 30A Bs 3036 fuse from table 41.2 inBS 7671 is given at 1.09 ohms.
Using the rule of thumb for temperature correction the maximum permissible value is : 1.09 x 0.8 = 0.87 ohms
This value means the gained value is acceptable"

 

[iamelectric]

tip for u mate know your inspection schedule , there are a lot of inspection questions in it

 

[bartkusal]

I don't think there is an average pass mark. I believe the passmark i dependant on how hard C&G think the paper is and on how many people have answered the questions right. I sat 2391 a couple of times and I was told the pass mark was different each time. Having said that I think we need to get at least 70% but don't hold me to that! Good luck to us all!

Correct, it fluctuates between 60% and 80%.they must fail someone, depends from how good the people sitting the exam doing, if good % rises