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?

 

[Phoenix]

7.41 + 12.1 = 19.51

 

[telectrix]

that figure of 19.51 comes from adding the res. of 2.5mm to that of 1.5mm, as given in the question ( 7.41 + 12.1 )

 

[PeterPan27]

Lmfao ya joking.. I've been looking at it for ages
Cheers mate im knackered for the exam looks like a August resit

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]

Thought b pretty hard to remember all copper resistances

 

[PeterPan27]

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

 

[mtfpuk2011]

mate you will be fine I got mine to tomorrow just rwmber the 3 strps to.it

 

[mtfpuk2011]

no they will give you the tabs if they want to do 80 % correction factor .trust I did 91 read the question carefully

 

[PeterPan27]

I no the 3 steps off by heart
Steps 1 identify the in going an outgoing
Then measure the line loop , neutral loop and cpc loop an record as r1 rn and r2. The line and N should b within 0.05 ohms of each other. The cpc be 1.67 greater.
Step 2 connect incoming line with outgoing neutral and vice versa. Then calculate the predicted reading I would expect to see at each socket using the formula r1 plus rn divide by 4
Step 3 then do same with line and cpc.. R1 plus r2 divide 4
Any higher reading means loose connections or spur off ring

While doing steps 2 and 3 take reading at every socket making sure within the 0.05 which each other..

 

[PeterPan27]

Heard loads diff answers but anyone know the pass mark?
It can't go on a national average no way

 

[mtfpuk2011]

thats all you got to rember fella

 

[Lou7]

There's a table in the on site guide.

 

[ruston]

All the figures you need to do the calculation will be in the question if it is still the same as the 2391 was. They will not expect you to to remember all the values, but they will expect you to be able to use the formula. I had a similar question and the instruction was to use the rule of thumb to see if the max ZS complied, the value of the breaker and type etc was included in the question.

 

[mtfpuk2011]

Heard loads diff answers but anyone know the pass mark?
It can't go on a national average no way

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!

 

[Jay Sparks]

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?

If this exam is as picky as the 2391 was, remember to write correct terminology...the r1 plus r2 you have written is wrong....R1 + R2 is what you should have put....r1 is the end to end resistance of the line conductor and r2 is the end to end resistance of the cpc. They can be very picky on this sort of thing and you may loose points.


Jay