This is more of an electrical general knowledge question rather than a Wiring Regulations one but they do pop up in the City & Guilds exam every so often.
“A twin and CPC, PVC cable has a 2.5 mm line conductor with a resistance of 0.46 ohms and 2.5 mm neutral conductor with a resistance of 0.45 ohms each. What is the expected resistance of the CPC ?”
The size of the protective conductor is 1.5 mm (It’s a twin and earth cable). So, with it being a smaller conductor the resistance is going to be higher. Therefore, it’s just a question of how much higher is the resistance going to be.
The difference in the sizes of the line or neutral conductor and the CPC or ‘ratio’ is 2.5/1.5 = 1.67 so if you multiply 0.46 by 1.67 = 0.7682 or if you multiply 0.45 by 1.67 you get 0.7515 so the expected value would be 0.76 ohms.
I think in an exam that is testing your ability to reference BS7671 they are out of order to include electrical general knowledge questions but I understand why they do it.
Cheers
Paul