Question on measured voltage on CT taps

[Kimbo Slice]

So this question I have been coming across on the practice tests. I just took a test and picked 500 Volts and it marked it wrong. I think I picked 0 Volts once and it marked it right. What is the correct answer to this question? Any help is appreciated. The difference I see in the question is measured in the first question applied in the second question.




This is the question I got and it was marked wrong.

Transformers
During a saturation test of a 1500/5 multi-ratio CT, 400 volts is measured across the X1 to X4 tap. The X1 to X4 tap is the 1200/5 ratio. What is the expected voltage across the X1 to X5 tap?
Your Answer: 500V

Here is a similar question?
During a saturation test of a 1500/5 multi-ratio CT, 400 volts is applied to the X1 to X4 tap. The X1 to X4 tap is the 1200/5 ratio. What is the expected voltage across the X1 to X5 tap?
Correct Answer: 500V
Your Answer: 400V

[joaogemal]

For your second question:

I will start with the following: 1200:5 = 240:1 and 1500:5 = 300:1.
Our question is that I will apply 400 Volts, X1 to X4, which give us (400/240 = 1.67 Volts/turn)
For the 300:1, X1 to X5, will be (1.67 V/turn x 300 turns = 500 Volts)

For the first question I'm still scratching my head.....

[SecondGen]

Clerical error perhaps, but it seems to me that these two answers are backwards. I am confused :confused: but here is my thought process when reading these questions:

The second question asks for the expected voltage based on 400 V being applied from X1 to X4. I'm thinking I would measure 0V from X1 to X5 because there is no ground reference on X5, it is on X4. Everything should still be the same potential from X1 to X5.

On the other hand, the first question to me is asking what the expected voltage would be based on the value that was obtained from my X1 to X4 measurement. This can be figured out using the example given by joaogemal.

[joaogemal]

Clerical error perhaps, but it seems to me that these two answers are backwards. I am confused :confused: but here is my thought process when reading these questions:

The second question asks for the expected voltage based on 400 V being applied from X1 to X4. I'm thinking I would measure 0V from X1 to X5 because there is no ground reference on X5, it is on X4. Everything should still be the same potential from X1 to X5.

On the other hand, the first question to me is asking what the expected voltage would be based on the value that was obtained from my X1 to X4 measurement. This can be figured out using the example given by joaogemal.



Not really. as a matter of fact, some times, and I would guess a lot o technician do that, to make sure of he number of turns are correct, we apply a voltage on any tap and see what voltage we will have on the others and this voltage measured follow the number of turns. A multi-tap CT with voltage on the secondary side is like a auto-transformer for the others tap. If it wasn't for that How will it work as a CT with multi tap? And more, if you use more than one tap of the CT you will have take in consideration that point where the current will split.
There's no ground on X4 as well.

[SecondGen]

A multi-tap CT with voltage on the secondary side is like a auto-transformer for the others tap. If it wasn't for that How will it work as a CT with multi tap?

:o should have known that one. thanks!

[husseycopper]

I thank you for your reply, my fruitless searches were leading to believe that this was the case.

I must be misunderstanding things an acquaintance told me, he said he was using a current transformer to monitor the mains and generator voltage in his equipment room, and I know the unit that monitors these reports them as voltages, ie. 240V on the mains.

It is possible that the voltage range he is getting on the secondary of his current transformers he is interpreting as a scale of 240V rather than a scale of the amperage in the primaries.