If you are a home inspector or an electrician and are using one of the low-cost, three-light outlet testers to check electrical outlets, you could be missing a wiring problem that you should be aware of. These three-light outlet testers will NOT indicate a hot/neutral reverse situation if the outlet being tested is not grounded.
If you are testing an ungrounded outlet, then all the tester can tell you is if there is a voltage differential between the narrow slot and the wide slot on the outlet. Without also having a ground wire connected, the tester does not know if the wide slot or the narrow slot is the one that is hot because it does not have a point of reference. Remember, with alternating current, the hot wire is at negative 120 volts as much as it is at positive 120 volts, so the tester has no way to know which of the two wires is actually the hot wire. It simply knows that there is a 120-volt difference between the two slots.
To test this theory, I set up my test rig with an outlet that had the hot and neutral wires reversed. I started with the outlet being grounded. I use my three testers plus my ideal Suretest to check the outlet. The three testers that I had available were a Sperry, a Southwire, and an Intertek. I plugged in each of the three testers in turn and got the correct response of a hot/neutral reverse on the outlet from all three testers. My Suretest also correctly showed that the outlet had a hot/neutral reverse. (I realize that it is not possible to see the flashing lights on the Suretest in the photo indicating a hot/neutral reverse, but the lights were flashing.)
Next, I disconnected the ground wire and again took turns plugging in my 3-light testers and the Suretest. This time, all three of the 3-light testers indicated the open ground, but none of them gave any indication that the outlet was also wired with reversed polarity. My Suretest did correctly show that it was a hot/neutral reverse situation – even though there was no ground wire connected. (On the photo of the Sperry, it appears that all three lights are on, but only the middle light is actually on.)
Whether you’re a home inspector, electrician, handyman, etc. and use one of these three-light testers in your work, it is important to know that they do have their limitations. Please keep in mind that while these simple, low-cost devices can be very helpful, they may not be completely accurate in what they are telling you. Maybe they are just telling you a part of the story, or worse yet maybe they are telling you a completely wrong story. I will have more situations to show you from my test rig in a few days.