2392 Question

[rustynails]

When testing continuity of a ring final circuit wired with 2.5/1.5 PVC/PVC conductors using 'Method 1'. The value of R1 is 0.8ohm, what should be the approximate value of R2:

A: 1.20
B: 2.00
C: 1.34
D: 0.32

 

[markthespark]

Is it C?

0.8 * 1.67 = 1.336

 

[jdd]

R1 size / R2 size = 1.666666667
1.6666667 x 0.8 = 1.3333333

Should be C, 1.34

 

[JUD]

I know it probably doesn't matter in the context of the question but the end to end values of a ring circuit are known as r1 (line), rn (neutral) and r2 (cpc) (notice the small r).

You would expect R1 (big R) to equal r1 ÷ 4 and R1+R2 to equal (r1+r2) ÷ 4

Just something to remember for the future.

 

[SirKit Breaker]

I know it probably doesn't matter in the context of the question but the end to end values of a ring circuit are known as r1 (line), rn (neutral) and r2 (cpc) (notice the small r).

You would expect R1 (big R) to equal r1 ÷ 4 and R1+R2 to equal (r1+r2) ÷ 4

Just something to remember for the future.

Very good and relevant point JUD, i know this is 2392, but make that small mistake in the 2394 or 2395 and marks will be lost.

Cheers.........Howard

 

[rustynails]

R1 size / R2 size = 1.666666667
1.6666667 x 0.8 = 1.3333333

Should be C, 1.34

The answer is C.

I see now. R1 size as in diameter of cable 2.5mm, doh! So it'll be - 2.5/1.5 = 1.666666667

then like you say 1.666666667 x the R1 value 0.8 = 1.3333333

Great, althought would it be different for a radial? 0.5?

 

[sedgy34]

radial would be R1+ R2 =answer

 

[rustynails]

radial would be R1+ R2 =answer

So In this book that I have it says

"A radial circuit is wired in 2.5mm2 phase and 1.5mm2 CPC. The test resistance of R1 + R2 is 0.37 ohm.

To calculate the resistance of the CPC on its own:

R2 = 0.37 x 2.5 / 2.5 + 1.5
R2 = 0.37 x 0.625 = 0.23 ohm

So using that with the original question I get:

R2 0.8 x 0.625 = 0.5 ?

 

[telectrix]

i get R2 = 0.23. R1 = 0.14.

 

[JUD]

The original question gives you the value of R1 on it's own so R2 is worked out as follows

R2 = R1 x (2.5 ÷ 1.5 or 1.67)
R2 = 0.8 x 1.67
R2 = 1.34

The question in post #8 gives you the combined value of R1+R2 so

R1 = R1+R2 x 1.5 ÷ (1.5 + 2.5)
R1 = 0.37 x 1.5 ÷ (1.5 + 2.5)
R1 = 0.14

R2 = R1+R2 x 2.5 ÷ (2.5 + 1.5)
R2 = 0.37 x 2.5 ÷ (2.5 + 1.5)
R2 = 0.23

This is the same for a ring or a radial

 

[Deleted member 26818]

I get R2 = 0.23125Ω, and R1 = 0.13875Ω.
Divide the value for R1+R2 by the combined CSAs.
Then multiply by the other CSA to the one you want to find the value for.
2.5 + 1.5 = 4.
0.37Ω / 4 = 0.0925.
R1 = 0.0925 x 1.5 = 0.13875.
R2 = 0.0925 x 2.5 = 0.23125.

 

[ackbarthestar]

So In this book that I have it says

"A radial circuit is wired in 2.5mm2 phase and 1.5mm2 CPC. The test resistance of R1 + R2 is 0.37 ohm.

To calculate the resistance of the CPC on its own:

As tel. has stated, its quite straight forward

If R1 + R2 = 0.37 Ohm and R2 is 1.67 times greater than R1

then 2.67x = 0.37
x = 0.37 / 2.67
x (R1) = 0.13 Ohm
and
R2 = 1.67x * 0.13 = 0.23 ohms or 0.37 - 0.13 = 0.24 ohms

You can find the theoretical resistance of r1 and r2 if you get excited

 

[Sparky Ninja]

I'm glad I arrived at this thread late, I'll leave you guys to it.


:beatnik:

 

[Duffman]

As tel. has stated, its quite straight forward

If R1 + R2 = 0.37 Ohm and R2 is 1.67 times greater than R1

then 2.67x = 0.37
x = 0.37 / 2.67
x (R1) = 0.13 Ohm
and
R2 = 1.67x * 0.13 = 0.23 ohms or 0.37 - 0.13 = 0.24 ohms

You can find the theoretical resistance of r1 and r2 if you get excited

Well explained. Simple solution that demystifies the problem.