Troubleshooting reverse polarity issue


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Old 08-30-23, 07:35 AM
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Troubleshooting reverse polarity issue

I have an odd problem. I had some audio gear plugged into one outlet. My computer is plugged into another outlet. I tried connecting an audio cable between them and got shocked. It felt like 120v.

I know the computer outlet is wired correctly.

The problem outlet has no ground (it's an old house that was built without grounded outlets).

I plugged an extension cord into the good outlet and used a voltmeter to test between the good outlet and the problem outlet.

From the hot side of the extension cord to the neutral side of the problem I get 120v. To the hot side I get 240v.

From the neutral side of the cord to the neutral side I get 0v, and from neutral to hot I get 120v

Ground on the cord to hot I get 120v; ground to neutral is 0v

The breakers for the both outlets are on the same side of the panel.

 
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Old 08-30-23, 08:47 AM
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Breakers on same side of panel does not mean they are on same leg of incoming voltage. You verified different legs when you measured 240 vac between the 2 hots. When measuring voltages at the problem receptacle, did you measure to the wire or receptacle slot? Does the power cord for the audio equipment have a ground pin?
 
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Old 08-30-23, 10:24 AM
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The extension cord does have a ground pin.

I measured to the receptacle slot. It's a brand new receptacle if that matters. When I started investigating I saw the original receptacle was the cheapest kind and that it was wired via push-in back-wire slots so I put in a new commercial-grade outlet as a first step. The outlet is at the end of a run so there is only one pair of conductors in the box.

The audio gear that was connected to the ungrounded outlet does have a ground pin (three different devices); the other end is powered via USB-C. IDK if USB-C is connected to ground but I kind of think it is. I now have the gear plugged into a different (grounded) outlet via an extension cord and there is no problem with that, other than having a cord running across my room :-)
 
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Old 08-30-23, 11:01 AM
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How many outlets do you have with reversed polarity?

If it's only the one outlet you mentioned why not just pull the outlet and put the hot and neutral in their correct locations?
 
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Old 08-30-23, 08:45 PM
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The problem outlet has no ground
Having no ground causes problem with EMI filters. This causes grounded chases to become hot, usually about 1/2 of supply voltage. There is high impedance between ground and hot, so you will just get a little shock, but will not get electrocuted (similar to static discharge). However, this can damage electronic devices due to difference in reference ground. Basically, you can feed 120V into 5V signal momentarily.
So, never plug chassis grounded equipment on ungrounded outlet.

If your house has cable with metal sheathing (MC, AC, BX etc..), junction box may still have ground. Check voltage between hot and junction box or wallplate screw. If you get 120V, you may have ground. Also test if you have continuity between this junction box and ground of grounded outlet. If you have continuity, you have usable ground. Is it good ground is questionable in old houses, but there isn't good way to test this for home owners. (needs expensive tool). It should be good enough for EMI filters though.
Also, sometimes you have grounded cable in junction box, but just outlets are installed in 2 prong. In this case, it is easy to change over to 3 prong.

From the hot side of the extension cord to the neutral side of the problem I get 120v. To the hot side I get 240v.
That is perfectly normal reading of 2 outlets that are on different phase. If you had reversed polarity, you will get 120V between neutral of each outlet.


The breakers for the both outlets are on the same side of the panel.
The panel is made to have alternating phases on each side. So, odd numbers on one side will have one phase and even numbers on the same side will have another phase.
 
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Old 08-31-23, 12:24 AM
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Older un-grounded plugs could be turned upside down, and then you change Live and neutral.
Sometimes later the GFCI jack was connected without grounding, and you still had the outlet for the 3 prong plug.
If you are lucky the tubing to the outlet is metal with good enough connection to ground to eliminate your problem caused by the filter explained earlier in this thread. Metal water pipe may also be an OK ground for the low voltage.
The do not do this solution I have heard about and have to warn about is to connect ground to Neutral.
The Neutral is bonded to ground in the fuse box, it may be a considerable voltage drop in the wire to the outlet, or even switched polarity that may be extremely harmful. That is why this solution is forbidden.
 
 

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