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Line Tester


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Consider a single winding of a three phase transformer or generator which is star connected and the neutral grounded. The polarities of the emf change every half cycle. But when we place a line tester at the phase terminal then it glows but if we place it on the neutral it doesnt glow even when the neutral has the positive polarity every half cycle. Can nayone explain the theory behind it. Its bit confusing for me. I consider that the neutral is grounded and we measure with tester with respect to ground so there is no potential difference to glow the tester whether it is positive half cycle or negative. Now what if we have a un grounded neutral or a single phase transformer with both the leads floating. Which will be positive and negative. will a tester glow if it is touched to a ny terminal?

Plese answer this.



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Hello Borges


The line tester will only glow if there is some voltage across it.

Typically, with a neon type tester, one side is connected to the terminal under test, and the other is effectively earthed.

The neon will glow if there is a voltage relative to earth.


In the case of a three phase transformer which has the star point earthed, there will not be any potential at the star point relative to earth because it is directly connected to earth.

Therefore, you will get an indication from each of the three phases, but not from the neutral point.


Best regards,


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