17th Edition, third harmonics question

[Alessandro]

Hello, I will take 17th Edition soon and I was doing practice questions with a simulator. The only answer I got wrong was the third harmonics one, I read it on the Regs but I don't get it. Would you be able to help me with a demonstration of the calculatioins needed to find the answer?

Where a three phase circuit with a design load of 39 A has 20% third harmonics present, the design load after applying the rating factor for the harmonics would be:

a. 54 A
b. 45 A --> CORRECT ANSWER
c. 35 A
d. 4.5 A

Reference: Appendix 4, 5.5.2, page 335

The design load I get after applying the formula is 39.8 A but, according to the simulation, this is not the correct answer.

 

[roys]

Should you not be looking at at appendix 11 section 2.

20% harmonic has a rating factor of 0.86

39 / 0.86 = 45A

 

[roys]

PS well done on trying to answer by yourself first, it is the best way to get exam type questions answered by a forum.

 

[Alessandro]

Ahah, I only had to divide by the factor in the table then. I was reading 5.5.2. Thank you for your help roys.

For your information, Appendix 11 was deleted by Amendment 1 and moved to Appendix 4, secs 5.5 and 5.6.

 

[roys]

No probs, it was the original red 17th edition regs book I had handy with my coffee cup on top of it.

 

[Alessandro]

No probs, it was the original red 17th edition regs book I had handy with my coffee cup on top of it.

I think Amendment 3 introduced a regulation that prohibits that, I would be careful.

 

[kerching]

I think Amendment 3 introduced a regulation that prohibits that, I would be careful.

NOT if you bought the cup,at a Trumpton Open Day :innocent