R1 and r2 values

[Hogshaw]

If I have an overall resistance can some help me figure out how I would work out just the r2 value??

e.g

a ring final circuit is installed with 4.mm live conductors and a 1.5mm c.p.c if the line loop measures 0.44 ohms, what is expected c.p.c value?

 

[Tiny Spark]

Am I missing something? Why not just measure it? Also why have you got a 1.5mm cpc on a 4mm ring? Shouldnt it be 2.5?

 

[Hogshaw]

Am I missing something? Why not just measure it? Also why have you got a 1.5mm cpc on a 4mm ring? Shouldnt it be 2.5?

This is a 2395 exam question, sorry should have said

 

[Tiny Spark]

Well you could use the resistance per meter table to work out the length of run for the 4mm cable, then use the same table to work out the resistance of the 1.5mm.

Of course that is purely hyperthetical and not taking into account anything other than the loop.

 

[Hogshaw]

They don't give any more resistances only the one I've already stated..

thanks

 

[Tiny Spark]

The table is in the OSG, Not framiliar with the 2395 but I assuem you are still allowed to use your books?

 

[Hogshaw]

The table is in the OSG, Not framiliar with the 2395 but I assuem you are still allowed to use your books?

No books allowed

 

[Tiny Spark]

Ohhh......

Just to check something here though as you are talking about a ring do you need to work out the R2 value or the CPC loop?

 

[Hogshaw]

I need to work out the r2 value of the circuit

 

[HappyHippyDad]

Are you saying that r1+r2=0.44Ω?

Also this is definitly a ring? and definitely 4mm?

 

[Tiny Spark]

I dont see how you can work this out tbh :s

You would need atleast 2 resistances to work this out in my mind.

To get R1+R2 you have to do L Loop + E loop/4.

You could work out R2 only if you had R1 and the total resistance only but I cant see how you can calculate it with only 1 variable?

Hopfully one of the others can help coz ime stumped, But ile read with intrest

 

[HappyHippyDad]

You have the line size 4mm and the cpc size 1.5mm, so you know the ratio is 4:1.5, so r2 will be 72.72% of r1+r2... (I think!!)

I'm just not sure what the chap is saying initially, are you saying r1=0.44 or r1+r2=0.44

 

[Tiny Spark]

Think hes saying R1 is 0.44, So going with your idea it would be around 1.17?
This is one of them times if i was in the exam without a calc or a book ide just times it by 3 and pick the nearest number if its multi choice LOL

 

[telectrix]

think his r1 is 0.44, so r2 would be 0.44 x 4.0/1.5 ...i.e. 1.173 ohms.

 

[jamie-spark]

think his r1 is 0.44, so r2 would be 0.44 x 4.0/1.5 ...i.e. 1.173 ohms.

Bingo tel

 

[Hogshaw]

Bingo tel

0.44 is there value for r1 and I'm trying to find out what is the expected c.p.c value

the answer is 1.17 ohms but I don't no how this was got

 

[Richard Burns]

Tel has given you the answer!
The resistance of a 4mm² conductor is 0.44Ω
What is the resistance of a 1.5mm² conductor.

The resistance of the smaller (1.5mm²) conductor will be greater than the resistance of the larger (4mm²) conductor by a factor of 4 / 1.5 i.e 2 2/3.
So 0.44 * 2.66667 = 1.17.

 

[Tiny Spark]

4 divided by 1.5 to give you the ratio then times by 0.44.