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I have a bunch of 2 prong outlets in my house. I am using a multimeter to see if they are grounded (1 in the hot slot and 1 on the screw). Almost all have read either zero (not grounded) or 120 (grounded). However, there are two outlets that read 50-60 volts when testing. Are these grounded or not grounded? Maybe they are grounded and the wiring on the box needs to be fixed?

Thanks

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Your house is grounded but doesn’t have an equipment ground. How a 2 wire system works (120v) is one side is actually grounded and the other is at 120v. A 240v receptacle has 2 each 120v legs that are 180 out of phase but single phase or split phase is the proper name for residential us power. So your maximum voltage to ground is 120v , leg to leg is 240. The common leg is grounded at the transformer that supplies your home and is called the neutral. If your receptacles that show a voltage are switched it is normal to measure a “phantom” voltage of 40 to 120v phantom voltage is induced into a wire that is not powered by running parallel to a live wire this is how transformers work. If you plug a load like a light into the receptacle the voltage will normally drop to zero. Phantom voltages can be found in just about any home. To have an equipment ground or a 3 prong outlet that is functional you can add a ground wire to that circuit this is a recent code change. The other way that you can add 3 prong receptacles is to add GFCI protection and label them GFCI protected no equipment ground. So your system has a grounded conductor by code the white or gray colored one but no grounding conductor the bare or green one. Hope this helps.

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60 volts is half of 120. It is almost certainly a "phantom" or "stray" voltage that results from harmless electromagnetic coupling from both hot and neutral (or ground), and it indicates that the metal you tested is not wired to anything.

The bad news is that your other test results could be the same situation, depending on the particular coupling at each box — if you read 120 volts between hot and box metal, you don't know whether the box is grounded or just happens to be coupled mostly to a neutral wire. A regular voltmeter, with a single test, cannot tell you whether there is a wire connection or just a weak coupling.

If you have a voltage tester of the sort that is not powered by batteries but has a lamp in it powered by the line, that can help, because it tests whether it is possible to draw power from the circuit, which can confirm a connection. Another tool is a multimeter with a "Low-Z" setting, which basically puts a resistor across the probes (thus drawing some current, if possible) and measures the voltage, giving you the same kind of answer as the lamp would but with an actual voltage reading instead of just glowing-or-not.

However, these tools will read zero for both "both probes on the same voltage" and "not connected". In order to confirm that something is connected, you need to make two tests, comparing the metal you're testing to two different other conductors (e.g. hot and neutral), and observe that one of the readings is zero and the other is 120 V.

There are better tests for integrity of ground, but they require more specialized equipment and safety considerations.

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  • Have you never heard of phantom voltage? Almost all switched receptacles will show a voltage and if the switch is illuminated Or a solid state switch without a neutral all of them will Because they are never fully turned off, next we can discuss multi wire branch circuits that may have a voltage on the neutral from the other side, so at least 3 major possibilities to explain a voltage with nothing wrong. – Ed Beal May 12 '20 at 17:47
  • @EdBeal You call it phantom voltage, I call it floating voltage. Same thing. My answer focuses on communicating that "open" circuits can read almost any voltage and what needs to be done on that. – Kevin Reid May 12 '20 at 17:49
  • I disagree a floating voltage is when the neutral is not solidly connected to earth , a phantom voltage is a voltage on a wire. Leakage voltage from solid state electronics and voltage return from a multi wire branch circuit are not floating but return to the source voltages. – Ed Beal May 12 '20 at 17:54
  • @EdBeal I was not aware there was such a specific usage. I've changed it. – Kevin Reid May 12 '20 at 18:55

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