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I have a 2-pin plug that was not label which is Live and which is Neutral. I do not want to damage the electrical device and was wondering what should I do in order to test and find which point is Live and which point is Neutral?

The 2-pin plug look something like the following: enter image description here

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  • \$\begingroup\$ In AC circuit there is not polarity as input is alternating voltage.So the plug can be connected either way. \$\endgroup\$ – Gopi Apr 3 '14 at 14:25
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A plug doesn't have polarity, it completely depends on the wiring of the socket. To check the socket all you need is this two dollar tester:

enter image description here

It has a neon light inside which connects via a high resistance to the contact at the end. If you insert in in the live pin and touch the metal dingus on the back of the tester, there will flow a very small, safe, leakage current to ground which is enough to light the neon light. If you try it on the neutral it won't light because the neutral is at the same potential as ground.

edit
Well, that's how the classical tester works. Cybergibbons points out that this one is probably a non-contact tester (because it doesn't go deep enough in the socket?). Anyway, those detect the electrical field emitted by the phase's voltage. They're battery operated to power the LED.

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  • \$\begingroup\$ Two things - the image looks like a non-contact voltage tester, and the question is for a plug, not a socket. \$\endgroup\$ – Cybergibbons Jun 15 '12 at 6:15
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    \$\begingroup\$ @Cybergibbons - Zut! I must have misread, probably because it makes no sense: a plug doesn't have a polarity, it relies completely on the socket. I'm non sure about non-contact; I don't think the field gives you enough energy to power the light. \$\endgroup\$ – stevenvh Jun 15 '12 at 6:20
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    \$\begingroup\$ I don't know about the rest of the world but I the UK it is common for lay people to call sockets plugs (which is confusing). It looks like a battery powered device like a Fluke Volt Alert fluke.com/fluke/uken/Electrical-Test-Tools/Electrical-Testers/… \$\endgroup\$ – Cybergibbons Jun 15 '12 at 6:32
  • \$\begingroup\$ @Cybergibbons - Yes, I also found some non-contact testers, but they indeed need a battery. I'll update my answer. Thanks for your reaction. \$\endgroup\$ – stevenvh Jun 15 '12 at 6:34
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    \$\begingroup\$ I guess "touch the end" should be "touch the metal plate on the back of the tester", otherwise it is ambiguous and someone might get electric shock. \$\endgroup\$ – sharptooth Jun 15 '12 at 8:11
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Assuming a US standard electrical system, polarized plugs/sockets have two different sizes in the plug blades. The neutral blade is the larger blade and the hot/live blade is narrower.

You can test it by measuring the voltage between the terminal in question and an earth ground using a voltmeter or voltage tester. If it is live, you'll measure the line voltage. If it is the neutral, you should not measure any voltage.

Note that these voltages can easily be deadly if you are not using proper measurement techniques. You can significantly reduce risk, by turning off the circuit you want to measure with a breaker, connecting your meter (set to the proper scale) and then closing the breaker. Read the meter without touching anything and then open the breaker to remove the meter probes.

Youtube video showing usage of electrical circuit testers

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  • \$\begingroup\$ May I know if you have any videos or pictures to illustrate on how to use the voltage tester to measure it? \$\endgroup\$ – Jack Jun 15 '12 at 4:59
  • \$\begingroup\$ @Jack added a video for you. \$\endgroup\$ – madrivereric Jun 15 '12 at 5:10
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    \$\begingroup\$ "Assuming a US standard electrical system" - The photograph shows round pins on the plug so this assumption fails. \$\endgroup\$ – uɐɪ Jun 15 '12 at 11:21
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    \$\begingroup\$ @Ian, I was not the only one thought that, so I looked at the edits to the post, 4 hours after this post the OP edited the question witha picture of a non-US plug. Not the answers fault, the OPs instead. The rest of the answer is still valid and the first paragraph might be helpful to someone else. \$\endgroup\$ – Kortuk Jun 15 '12 at 13:31
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The picture shows a european plug. The two pins are the same and it does not matter which way round it is used. If it did matter then there would be a third pin or socket to provide polarisation.

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  • \$\begingroup\$ For that device, but one of the two will be hot and one will be neutral. \$\endgroup\$ – Kortuk Jun 15 '12 at 19:21
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    \$\begingroup\$ As far as the device plugged in is concerned it sees 240V ac between the two wires. If the plug is connected the other way round then the device will still see 240V ac but with the phase inverted. I can't immediately think of anything that will complain about a 180 degree phase shift in its power supply. \$\endgroup\$ – uɐɪ Jun 18 '12 at 6:52

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