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PPI sample exam 1, #8 and #40


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Hello All!

I am having trouble understanding the solution for # 8 and # 40 in PPI sample exam.

Could someone please explain why the line current is being calculated differently (using the line-to-neutral voltage in #8) vs line-to-line voltage in #40)? 

Does the neutral ground in #8 matter?

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The first statement of both questions seems to be similar and so may confuse some but the other statements like asking neutral current in Q8 and not asking in Q40  and few other info, makes them different. If you read them carefully  you may find the difference that both loads are connected differently, one ckt has a neutral connection and the other load does not have a neutral connection. The Voltage  will be taken according to the connection of load. If the load is connected between Line and Neutral take V L-N and if connected between Lines take Line Voltage.   Ground does not matter, Neutral connection of load with source matters. If ground is the connection between the two, it is the same thing. If you let me know the exact point where you feel uncomfortable, may be I may be a help.

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Not the best worded questions. Took me a few minutes to figure out what they were saying. I agree with the above, although what confuses me is that the second question says 3 phase 4 wire but doesn't say if it's wye or delta. I guess wye is more common so we can assume it's wye.

Maybe this will help: On number 8, there is a load connected across phase A (AKA line to neutral), and also a load connected across phase C (AKA line to neutral). So you have to use LN voltage to find current.

Whereas on #40, there is one load connected between A and C (AKA line to line). So you have to use LL voltage to find current. It helps to draw these out.

This is what I gathered, feel free to correct me if i'm wrong.

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36 minutes ago, navixv said:

Not the best worded questions. Took me a few minutes to figure out what they were saying. I agree with the above, although what confuses me is that the second question says 3 phase 4 wire but doesn't say if it's wye or delta. I guess wye is more common so we can assume it's wye. Both Q are unbalanced loads  but in second Q ie. #40, it says a  single load between Phases A and C, (Though it is 4 wire, there is no connection to neutral at this load) that way, you may assume it as one leg of delta, else this is neither delta nor star.

Maybe this will help: On number 8, there is a load connected across phase A (AKA line to neutral), and also a load connected across phase C (AKA line to neutral). So you have to use LN voltage to find current.

Whereas on #40, there is one load connected between A and C (AKA line to line). So you have to use LL voltage to find current. It helps to draw these out.

This is what I gathered, feel free to correct me if i'm wrong.

 

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Thanks for the replies @rg1 and @navixv

The way I visualized the circuit  for #8 is incorrect then. Later I found another question similar to #8 where the question is more clear (#67 - Load is expected to be 150+75j between phase A and neutral and between phase B and neutral)

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