Answers on a postcard please...

[JamesBrownLive]

I'm struggling to understand the answer to this question....

A ring circuit is 54 metres long and is wired in 2.5/1.5 Thermoplastic cable. The Ze for the installation is 0.24Ohms. Cable resistance is 19.51 mOhms per metre. Calculate the Zs for the circuit?

I'll reveal the 'book' answer, once I have had a couple of responses, but I just want to see if others come up with the same (wrong) answer I did. That way I'll know its less me getting it wrong, and more a poorly worded question.

Cheers

 

[La Poste]

R1+R2 = (19.51 * 54/1000) /4 = 0.26 Ohms

Zs = .24 + .26 = 0.5 Ohms

or
Zs = .24 + (.26*1.2) = 0.552 Ohms at 70C operating temperature.

 

[kenny7askew]

after a quick calc is it something like 2.8ohms my maths are not brilliant

 

[Octopus]

0.77?

Apparently my message was too short!

 

[wattsy]

0.5 ohms

 

[Adam W]

Well I got 1.29Ω

19.51 x 54 = 1053.54
1053.54÷1000 = 1.05354Ω
1.05354 + 0.24 = 1.29Ω

 

[Chr!s]

0.48 Ohms

Chris

 

[La Poste]

How do you get 0.48 Ohms?

 

[Chr!s]

How do you get 0.48 Ohms?

Cr

Chris

 

[JUD]

R1 + R2 = (r1 + r2) ÷ 4

r1 + r2 = (19.51 ÷ 1000) x 54 = 1.05Ω

R1 + R2 = 1.05 ÷ 4 = 0.26Ω

Zs = Ze + (R1 + R2)

Zs = 0.24 + 0.26 = 0.5Ω

 

[Adam W]

A ring circuit is 54 metres long and is wired in 2.5/1.5 Thermoplastic cable. The Ze for the installation is 0.24Ohms. Cable resistance is 19.51 mOhms per metre. Calculate the Zs for the circuit?

That's an ambiguous term IMO - is that the resistance of the line conductor, or the R1+R2 value, as, if I remember correctly, it's listed in the OSG?

Update:
According to the OSG, 19.51mΩ/m is the value for R1+R2 of 2.5/1.5.

 

[SirKit Breaker]

Its Friday and i'm off to the arms for a beer, a bit heavy these questions on a good day let alone a Fridayarty:

Cheers...........Howard

 

[MarkieSparkie]

Postcard in the mail!

 

[JamesBrownLive]

Ok, time for the book answer...

0.5 ohms

54 X 19.51 / 1000 = 1.05 ohms

1.05 ohms /4 = 0.26 ohms

Add 0.26 ohms to Ze of 0.24 = 0.5 ohms

Now the thing that threw me, even though I had read about it when studying Ring Final Circuit testing is the division by 4.
I understand the concept of half the length, double the CSA etc, but not how that applies in this context.

I understand that you divide by 4 because you have cross connected the ends of the ring, but as that has not been done in this case so why do you still divide by 4?

Sorry if this is blindingly obvious, and I am missing it. Maybe as someone has already said, it's a bit too deep for a Friday.

The book this was taken from is:

Chris Kitcher's Practical Guide to Inspection, Testing and Certification, Appendix C, Question 2.

Cheers

 

[Octopus]

Ok, time for the book answer...

0.5 ohms

54 X 19.51 / 1000 = 1.05 ohms

1.05 ohms /4 = 0.26 ohms

Add 0.26 ohms to Ze of 0.24 = 0.5 ohms

Now the thing that threw me, even though I had read about it when studying Ring Final Circuit testing is the division by 4.
I understand the concept of half the length, double the CSA etc, but not how that applies in this context.

I understand that you divide by 4 because you have cross connected the ends of the ring, but as that has not been done in this case so why do you still divide by 4?

Sorry if this is blindingly obvious, and I am missing it. Maybe as someone has already said, it's a bit too deep for a Friday.

The book this was taken from is:

Chris Kitcher's Practical Guide to Inspection, Testing and Certification, Appendix C, Question 2.

Cheers

Divide by 4 because at the furthest part of the ring you have "double" the copper back the start and hence why you divide by 4 and not by 2

Hope that makes sense!