Question help needed - C&G 2391 - 52

[Jevans1996]

Hi - Just joined the forum as I'm making a career change and am training up.

I've come across some mock papers online and am concerned that either I'm way off or they're wrong, so would love some input. (Exams here)

Given that the resistance per metre of 2.5mm² cable is 7.41mΩ/m at 20°C , what would the approximate R1+R2 reading be at the at the extreme end of a 25m long 20A radial socket outlet circuit?

Possibile answers are

• 0.27
• 0.37
• .074
• 0.17

I came out with 0.37 using BS7671 and GN3 but the exam confirmed .17 as the answer. Could anyone take a look and tell me what I'm missing/where to find the missing info in either book? Thanks!

 

[westward10]

I concur with you. It is a badly written question but we must assume the cpc is also 2.5, it normally states this. Not sure where they got 0.17.

 

[DPG]

You would get 0.37 if the line and CPC were both 2.5mm

Are they referring to 2.5mm T&E where CPC is smaller?

Edit, beaten to it by Westie!
And also corrected my own mistake!

 

[Jevans1996]

I concur with you. It is a badly written question but we must assume the cpc is also 2.5, it normally states this. Not sure where they got 0.17.

Thanks for taking the time! It's really disconcerting when I'm trying to revise and the certainties are wrong!

The following question tried to state that polarity is to ensure that "The resistance is low enough to operate the protective device within the specified time"... Not sure how much I should trust this place.

 

[DPG]

Thanks for taking the time! It's really disconcerting when I'm trying to revise and the certainties are wrong!

The following question tried to state that polarity is to ensure that "The resistance is low enough to operate the protective device within the specified time"... Not sure how much I should trust this place.

It's amazing how many bad questions there are out there. Lots badly worded or vague, but some that are just plain wrong unfortunately.

 

[Jevans1996]

It's amazing how many bad questions there are out there. Lots badly worded or vague, but some that are just plain wrong unfortunately.

Good to know it's not just me. I'll almost certainly be back with more haha. Cheers!!

 

[westward10]

Have you paid money for this?

 

[Jevans1996]

Have you paid money for this?

It was a one time £19.99 fee. I'll be reaching out to them though to get that back. It was the only online extensive bank of questions that I could find, so it's quite frustrating