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I'm curious about this comment. It says what

... in the US or UK you should be able to go to a hardware store and buy an outlet tester for very little money. This is a little plastic housing which plugs into a wall socket and has 3 LEDs. The pattern of which LED lights up will tell you if your wall socket is properly wired.

But North American standard power connector isn't grounded, so 3 LEDs looks too many for me. Moreover, there is Japanese version of this connector which is polarity-agnostic even!

What is "mains outlet tester" (for proper wiring) in the context of US power connector standard? What extra LED indicates? Am I correct assuming what Japanese outlets are not testable at all?

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Here is a picture that shows what is tested with a USA outlet tester. Since about 1960, USA 120 V outlets have been required to have a third pin for earth ground. The neutral is also grounded but only at the distribution panel and at the service entrance.

This tester will not indicate a condition where the ground pin is connected to the neutral inside the outlet. I have seen an installation where all of the outlets were wired that way. The date of the installation was about 1962.

I believe that there was a time when outlet boxes were not required to be grounded in the USA. In that case, the cover-plate screw will not be grounded.

enter image description here

Here is a neon test light that can be used for two-prong receptacles. Note that the old extension cord is not polarized. I believe that there was a time when wall receptacles were not polarized in the USA.

enter image description here

Here is how a neon test light can be used to identify the hot slot when there is no ground available. The neon light draws only about 300 microamps with a solid connection and much less using a person's body as an antenna.

enter image description here

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  • \$\begingroup\$ So you no longer have to deal with non-grounded sockets? (which are mechanically incompatible with this tester) \$\endgroup\$
    – Free Consulting
    Commented Feb 1, 2016 at 1:38
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    \$\begingroup\$ There are still plenty of non-grounded receptacles around. There are adapters that allow 3-prong plugs to be used. They have a tab or lead that can be attached to the cover-plate screw that may or may not be grounded. \$\endgroup\$
    – user80875
    Commented Feb 1, 2016 at 3:16
  • \$\begingroup\$ Regarding your last addition with Japanese-style non-polarized socket: what can you tell about proper wiring of such socket as result of testing besides obvious wired or not wired? \$\endgroup\$
    – Free Consulting
    Commented Feb 11, 2016 at 5:22
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    \$\begingroup\$ Energized or not-energized is all you can tell. You could also check to see if the cover-plate screw is grounded, but not the quality of the ground connection. If you were to replace the receptacle with a polarized one, it would tell you which wire is hot. In the USA, any broken non-polarized receptacle would need to be replaced by a polarized one. The non-polarized socket pictured is an old USA socket, not Japanese, but they may be the same. \$\endgroup\$
    – user80875
    Commented Feb 11, 2016 at 20:42
  • \$\begingroup\$ Thank you, now its clear what can be tested about legacy sockets \$\endgroup\$
    – Free Consulting
    Commented Feb 11, 2016 at 22:04

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