What is "mains outlet tester" (for proper wiring) in the context of US power connector standard?

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?

Answer

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.

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.

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.