2395 estimating r1 + r2

[Hogshaw]

Question in 2395 past paper need help please.

if someone can show me how they worked this out

a circuit wired in 2.5mm/1.5mm twin with CPC sheathed copper cable has an r1 + r2 reading of 1.33 ohms, the value of r2 measured separately would be

1. 0.83
2.0.5
3.1.0
4.0.33

all in ohms

thanks

 

[scott 123]

r2 will be approx 1.67 of r1

 

[Richard Burns]

1.33/2.67 = 0.498 = r1
0.498 * 1.67 = 0.832 = r2

 

[Silly Sausage]

1.33/2.67 = 0.498 = r1
0.498 * 1.67 = 0.832 = r2

Too complicated...

r2 = (5/8)*1.33 = 0.83

 

[Richard Burns]

So just using the 1.67 to 2.67 ratio in the former equation multiplied by 6 and divided by 2 to give a fraction of 5 divided by 8 to use as a factor on the given value of resistance to resolve the answer.

Sorry... just being silly!

 

[robsparky1975]

So as archy has said 1.33 x 0.625 = 0.83

 

[Hogshaw]

Too complicated...

r2 = (5/8)*1.33 = 0.83

Were does the 5 divided by 8 come out off?

thanks

 

[Richard Burns]

Because the earth is 1.5mm² and the line 2.5mm² the earth will have 1.67 times greater resistance than the line.

The total resistance is R1+R2, but because R2 will be 1.67 * R1 the total resistance will be 2.67 * R1 (1 R1 plus 1.67 R1).

So the resistance of the earth is the total resistance multiplied by (1.67 / 2.67) and this is equivalent to 5/8 (if you multiply both sides by 3).

Maths wise

R1 = R2 / 1.67
or
R2 = R1 * 1.67

Rt = R1 + R2

Substituting for R2
Rt = R1 + (1.67* R1)
Rt = 2.67 * R1

Since R1 = R2 / 1.67

Rt = 2.67 * (R2 / 1.67) = 2.67/1.67 * R2
So R2 = 1.67/2.67 * Rt
1.67 * 3 = 5
2.67 * 3 = 8

So R2 = 5/8 *Rt

 

[telectrix]

where 5/8 expressed as a decimal is 0.625, as in a previous post.

 

[cprfenom]

1.33/2.67 = 0.498 = r1
0.498 * 1.67 = 0.832 = r2

Where do you get the 2.67 from? Im confused

 

[Richard Burns]

Ignore my post number 5

If you cannot see where the 2.67 comes from by the use of post 8 then I am afraid I cannot assist.

total resistance is R1 + R2
R2 is 1.67 * R1 so you have

total resistance is R1 + (1.67 * R1)

therefore you have a single R1 plus 1.67 R1
1 + 1.67 = 2.67

so you have 2.67 R1