I'm not an electrician. Per, https://www.apc.com/us/en/faqs/FA158817/, I want to test the outlets in a room for the following:

  1. Overloaded neutral wire (>5 Volts AC measured between Neutral and Ground).
  2. Reversed polarity (hot and neutral wires are reversed).
  3. Missing ground wire. (Note: this also includes using a 3 prong to 2 prong adapter)

I've found simple plug-in devices on Amazon that I'm confident can test for #2 and #3, but what sort of tool do I need for #1, to measure the voltage between Neutral and Ground?

  • cheap 3-led testers should test for condition #1 by having the third LED come on.
    – dandavis
    Commented Feb 9, 2020 at 21:09

3 Answers 3


A simple multimeter can help you verify all three, as well as many other voltage related troubleshooting. It's a real must have for... anyone really.

To measure between neutral and ground, you will set the meter to AC voltage, 200V range, and stick one probe into the neutral slot of the socket (the wider slot for USA standard sockets, on the left if the ground hole is on the bottom) and one probe into the ground slot (the D shaped hole).

  • If it's not against the rules to extend the question here, how do I test for #2 and #3 using a voltmeter?
    – RML
    Commented Feb 9, 2020 at 20:05
  • 1
    you would need to test the voltage between the leg and ground to test #2. Once you've established zero volts between neutral and ground, check the resistance between the two: infinite means no ground, under an ohm or two means it's good.
    – dandavis
    Commented Feb 9, 2020 at 21:12

A common digital voltmeter will do everything you need. Quality should be reasonable since you will be sticking this thing in electric sockets.

You should be wary of the 3-light testers. While they do provide a simple pass/fail test for most outlets, the legend describing the likely faults for "fail" conditions are misleading and wrong. Many hours have been wasted because people believed the legend's interpretations of the fault. I just saw a labeled "hot-ground reverse" which was in fact a lost neutral. (night and day, like reporting a bad fuel injector when the problem is actually a flat tire).

Be doubly wary of anything from Amazon. Anything to do with mains AC power should be UL-listed (e.g. a responsible testing lab should approve it). Anything you buy at retail (e.g. Home Depot or Target) will meet this standard. Amazon will not. Amazon seems like a legit retailer, but they quietly opened the platform to third party sellers. As a result, it is completely flooded with the same junkstream you see on eBay, but in sheep's clothing: gussied up to look like a legit product.

Products which direct-ship from China, or are drop-shipped from China via Amazon's Fulfillment warehouses (e.g. Prime), do an end-run around all the product safety laws. That may be fine for fidget spinners, but anything that touches mains power can kill you.


You're going to need a volt meter with test leads and read between the ground point and the neutral point. Most home stores have inexpensive ones that will answer your question but if you're doing this for a living, you'll want to invest in a good/great one. Since you need the meter anyway, you can save yourself a few bucks and not get the plug-in device because the meter will be good for #2 and #3.

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