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I live in EU and an outlet at a home has two holes (230VAC), no common ground.

I want to determine what hole is phase and neutral. Is it possible to do that by using a multimeter?

When I measure the AC voltage with the multimeter, it does not show the polarity, I used test stick screwdriver, but the led is turned on both holes. Do I need to use a different measuring device to determine it?

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    Why do you want to determine this? Most uses of this knowledge I can imagine would create a danger when the assumption turns out to be false!
    – Wouter van Ooijen
    Sep 28, 2014 at 11:25
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    Off-topic - nothing to do with electronic design.
    – Leon Heller
    Sep 28, 2014 at 11:48
  • One of TI's EVM uses a RC Power supply, its design guide says that I should pay attention to the polarity otherwise the MCU on the board would be fried. Therefore, I wanted to know how to determine which line is phase and which line is neutral in case I need to use the EVM somewhere else. I think keeping a question short is better for everybody if the details does not change the purpose (for this specific question).
    – angs
    Sep 28, 2014 at 12:07
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    this stackexchange is for electrical not just electronics
    – JonRB
    Sep 28, 2014 at 12:13
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    If you're not sure, add a transformer at the mains input of your setup. A fuse would be a good idea, too. As it stands, you could put a fuse on the Neutral, and when it blows, the whole circuit floats at active 230V ac.
    – Alan Campbell
    Sep 28, 2014 at 13:11

4 Answers 4

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Quickest way is to turn off the mains and unscrew the socket and determine the polarity by the colours (Blue being NEUTRAL and brown being LIVE).

Otherwise if you have an EARTH connection nearby (copper pipe for instance) use that to determine which one has the highest RMS voltage from EARTH. Neutral shouldn't be that far off EARTH while LIVE should be 230V (ish)

(this is also why I like the British mains plug)

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    I saw once that the technicians were cabled with the wrong color, it may not be reliable. I want to use this method on different places, therefore, I would appreciate to know if there is any way to do it by a multimeter or by using a different device?
    – angs
    Sep 28, 2014 at 10:55
  • well that technician should be stripped of his credentials (this is why the UK also has the IET Wiring Regulations which are legally binding). But as I stated in my reply, find an EARTH (copper pipe for instance)
    – JonRB
    Sep 28, 2014 at 10:57
  • I saw your edit after I posted the comment. The second method works fine. thank you
    – angs
    Sep 28, 2014 at 10:59
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    Voltage tester Pen can be used to determine the same.
    – Sanjeev Kumar
    Sep 28, 2014 at 11:04
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You don't necessarily need an active earth, but you do need some care if you don't.

Basically you are one big, large-surface capacitor to a distributed ground around you, in 9 out of 10 places, your body will work as an earth. Probably much more often even. Only in very old buildings with wire-based induction or wooden high-rise might your personal earth be too far from actual earth to make a difference.

This concept is used by the voltage tester pen, it has a 220kOhm to 510kOhm resistor and a Neon light and you touch the other side of the neon light. This way up to an absolute maximum of 1mA goes from phase through the neon light into your body, which then transmits that to the environment through your "personal capacitance". If you touch the neutral with the pen, no lights, because the neutral is much too close to the earth your body appears to have, and no current flows.

A floating 1mA into your hand is almost imperceptible and will not make it up to your chest at all, so it's safe as long as you don't use it under a running shower and know to always only touch the end that has the resistor and light between you and live power.


Now that I answered that to the best of my ability, I am very curious to the reason the EVM is curious about the phase. Does it interface to an outside world in some way? In principle AC circuits do not notice Phase and Neutral, because, as the term puts it: The current alternates. A circuit only connected to those two wires will always see the current shooting "left and right" at 50Hz, whether 'left' is Phase or Neutral.

The risk only becomes apparent when there's some interaction with the outside world that has no predictable connection to either wire. For example when there's a user doing something to the circuit or other electronics are connected of which the internal routing is unknown. But in this case I would strongly argue against RC-power reduction, on safety grounds.

(as a note: Triggering a Triac in the phase line, is still not unpredictable in this sense, as it is part of the exact same current-circuit).

Is it possible the Datasheet just means your safety? If the large resistor is in the phase line, you will not be as likely to kill yourself if you fiddle with something during experimentation?

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If you have an earth connection, you can easily determine the polarity by using a voltmeter. You will read 230V AC voltage between phase and earth, while reading a very small voltage around zero between neutral and earth.

The first lamp will be on in the schematic below, while the second one stays off.

schematic

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Note that if this is a grounded outlet, testers are available in most hardware stores which will check the relationship between the three contacts and tell you if they aren't arranged correctly. Just plug it in, check the lights, move on to the next outlet -- quick, easy, and no mucking about with probe leads.

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