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In AC supply we have both positive and negative cycles then in positive half cycle current flows from phase to neutral and then in negative half cycle current flows from neutral to phase - how can current flow from neutral to phase?

mike65535
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pankaj prasad
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  • Who ever said it did not! – Oldfart Apr 30 '18 at 15:08
  • Possible duplicate of [bjt base connected to ground and still operates](https://electronics.stackexchange.com/questions/370488/bjt-base-connected-to-ground-and-still-operates)? – Hearth Apr 30 '18 at 15:13
  • Yea ofcourse ..... – Mitu Raj Apr 30 '18 at 15:14
  • then why tester glows only on phase only but not on neutral? – pankaj prasad Apr 30 '18 at 15:17
  • What tester? If your question is about some piece of hardware, please edit the original question by clicking on the "edit" button under the question, and add necessary information about that piece of hardware. You could add a picture or a link, or the manufacturer and model number. Otherwise we are not sure what tester you are talking about and the question might get closed. – user57037 Apr 30 '18 at 15:52
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    *"then why tester glows only on phase only but not on neutral?"* Because your feet are also connected to neutral. – Oldfart Apr 30 '18 at 15:56
  • tester will glow if you stand on phase line , instead of standing on ground............. – Mitu Raj Apr 30 '18 at 16:57

1 Answers1

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Voltage can be seen as the integral of the electrostatic field. As with any integral there is an arbitary constant of integration. In other words voltage is only defined with respect to a reference,

Importantly we can have voltages that are both positive and negative with respect to our arbitary reference point.

Things get a little more complex once we introduce varying electrical and magnetic fields, but the fundamental need to choose a reference remains.

So what reference to use? that depends on the application. Physicists working through hypothetical problems like to put their reference at infinity. Circuit designers will usually denote one node in thier circuit as "ground" and measure everything relative to that reference. Power grid designers will usually consider the general mass of the earth as their reference (hence the term "ground").

So your wall socket typically has two pins intended to carry current in normal use, "Phase" and "Neutral". Neutral is nearly always tied to ground somewhere, so it's voltage relative to the ground is normally very low. The "phase" connection on the other hand is alternating positive and negative with respect to the ground. When it is positive current flows from the phase to the neutral. When it is negative current flows from the neutral to the phase.

then why tester glows only on phase only but not on neutral?

Assuming by "tester" you mean something like a "neon screwdriver" or a "volt stick".

Capactitance is everywhere. So while in a DC sense things can be extremely well isolated in an AC sense there is a fundamental limit to how isolated two objects can be. Your body is hence weakly AC coupled to the general mass of the earth.

When you use a neon screwdriver or voltstick on the phase conductor a small amount of current flows to/from the phase conductor (remember it's AC), through the tester, through your body, through the stray capacitiance between your body and ground back to/from ground.

When you use it on the neutral conductor There is not enough voltage relative to ground to trigger the neon bulb in the neon screwdriver or the electronics in the voltstick.

Peter Green
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