Exam revision calculation

[mtfpuk2011]

Can someone please help with the following exam question revision. Have seen the same question twice now on two different exam papers and each answer supplied by them have been different. I am confused.

A radial socket outlet is supplied by a 2.5mm2/1.5mm2 cable that is 20m long. Given that the Ze of the installation is 0.43Ohms and the resitivity of the cable is 7.41mOhms/m/12.1mOhms/m at 20 deg C, what is the calculated Zs taking into account of operating temperature?

a 0.892 Ohms
b 0.8283 Ohms
c 0.3932 Ohms
d 1.2 Ohms

The first paper gave the answer as D. 1.2 Ohms, the second paper, A. 0.892 Ohms. I have figured out ways of getting both answers but don't know now which answer is right! Exam is tomorrow.

Thanks

 

[tony mc]

What is the operating temperature of the cable! I would guess that some information is missing from the question! I would calculate it this way 12.1 + 7.41 = 19.51 m ohms /m multiplied by 20 meters. 19.51 x 20 = 390.2 m ohms /m. Divide that by 1000 to bring it back to ohms. 390.2/1000 = 0.390 ohms. I would then multiply that by 1.2 to bring it up to 70 degrees as copper resistance will increase by 2% for each 5 degrees.(If that info was in the question ) 0.390 x 1.2 = 0.468 and then add the ZE of 0.430 = 0.898 ohms so even my answer is not on your list !

 

[mtfpuk2011]

What is the operating temperature of the cable! I would guess that some information is missing from the question! I would calculate it this way 12.1 + 7.41 = 19.51 m ohms /m multiplied by 20 meters. 19.51 x 20 = 390.2 m ohms /m. Divide that by 1000 to bring it back to ohms. 390.2/1000 = 0.390 ohms. I would then multiply that by 1.2 to bring it up to 70 degrees as copper resistance will increase by 2% for each 5 degrees.(If that info was in the question ) 0.390 x 1.2 = 0.468 and then add the ZE of 0.430 = 0.898 ohms so even my answer is not on your list !

I am sooo sorry. Your answer must be correct. I had missed out a digit on answer a which is 0.8982 Ohms.

Thank you. That clairifes it. Don't why the other paper worked it out to be 1.2 Ohms though. You can see my confusion! Thanks for explaining it so clearly

 

[zupos40]

What is the operating temperature of the cable! I would guess that some information is missing from the question! I would calculate it this way 12.1 + 7.41 = 19.51 m ohms /m multiplied by 20 meters. 19.51 x 20 = 390.2 m ohms /m. Divide that by 1000 to bring it back to ohms. 390.2/1000 = 0.390 ohms. I would then multiply that by 1.2 to bring it up to 70 degrees as copper resistance will increase by 2% for each 5 degrees.(If that info was in the question ) 0.390 x 1.2 = 0.468 and then add the ZE of 0.430 = 0.898 ohms so even my answer is not on your list !

I got the same answer

R1	R2	L		F	Ze	R1 + R2 x L/1000	R1 + R2 x L/1000 x F	Zs Design
7.41	12.1	20	1000	1.2	0.43	0.3902	0.46824	0.858

 

[mtfpuk2011]

I got the same answer

R1	R2	L		F	Ze	R1 + R2 x L/1000	R1 + R2 x L/1000 x F	Zs Design
7.41	12.1	20	1000	1.2	0.43	0.3902	0.46824	0.858

Thank you again. Is the correction factor of 1.2 always used or only at temperatures of 20 deg. C?

 

[Silly Sausage]

Thank you again. Is the correction factor of 1.2 always used or only at temperatures of 20 deg. C?

You need to look in your GN3 & OSG, explanation will be much better!
Basically, as temp rises, resistance rises, so you need to correct. See above references.

The 1.2 is the adjustment from 20 to 70degC

 

[mtfpuk2011]

You need to look in your GN3 & OSG, explanation will be much better!
Basically, as temp rises, resistance rises, so you need to correct. See above references.

The 1.2 is the adjustment from 20 to 70degC

Will do. Obviously confusion is setting in big time!

 

[zupos40]

Thank you again. Is the correction factor of 1.2 always used or only at temperatures of 20 deg. C?

F it is always 1.2 (PVC 70 Celsius bunched see table I3 OSG) when calculating Design Zs, if you use either r1 r2 values from OSG or GN3, if using LSF cable or MICC different correction Factor.
Only if installation is a specialist with either higher or lower temp would you need to use Table I2 (boilers houses would have an ambient say 40 Celsius) green house could be say 0 Celsius to 35 Celsius ambient. You would told this in an exam question. In the practical world if you were testing and installation and you need to wear extra clothing is probably lower then 20 Celsius, like wise if your sweating it greater then 20 Celsius. I used a digital thermometer similar to try you see in fridges, for measuring commercial fridge temperature.
So for your exam F will always be 1.2 (70 C PVC)