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Old 10-11-2011, 11:26 AM   #1
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Reverse Polarity? 120v on coaxial cable shield


120v on cable shield is what tipped me off to a problem. duh right?
My wife bought a new TV with a 3 prong plug. Our old TV had two prongs.
When plugged in it blew the HDMI board. I got to looking and found 120v from the coax shield to the outlet ground ONLY when a 3 prong plug was in the outlet. Huh?
From this outlet ground to another 120v ground to ground! What?
I took the outlet out and looked at the wiring, the colors are correct but someone bootlegged the ground lug to the neutral lug.
With power off I rang out the hot from one oulet to hot on my "bad" outlet
I get 22 ohms but from neutral on the other outlet to hot on the "bad" outlet I have 0. I assume 22 ohms is reading through a blower motor or something. I'm thinking although the colors look correct on both, somehwere the polarity is reversed but I havent swapped it yet. I did remove the bootleg ground wire.
If this is true (reversed) why does this not trip my breaker when I plug in a 3 prong plug which would short hot to neutral via the bootlegged wire?

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Old 10-11-2011, 11:30 AM   #2
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Reverse Polarity? 120v on coaxial cable shield


Did you measure the wiring from H -N and H -G? The bootleg may have been done to replace a missing neutral instead of fixing the neutral.

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Old 10-11-2011, 01:15 PM   #3
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Reverse Polarity? 120v on coaxial cable shield


the only difference between a neutral and a ground is one is intended to flow current at all times, and the other is not.

I really doubt having the neutral "shorted" to ground would cause your problem. they all go back to the same bar in the breaker box.
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Old 10-11-2011, 01:37 PM   #4
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Reverse Polarity? 120v on coaxial cable shield


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If this is true (reversed) why does this not trip my breaker when I plug in a 3 prong plug which would short hot to neutral via the bootlegged wire?
It would not short hot to neutral, it would just electrify the metal case of whatever appliance you plugged in. Neutral and ground must not be bonded together anywhere except in the main panel, so they remain separated in appliances. Why? Well, to prevent exactly the problem you've got - reverse polarity or an open neutral electrifies "grounded" metal objects if ground and neutral are not separated.

That's why it blew out the HDMI board. You didn't have 120V on the coax shield. You had 120V on the "grounded" metal of the TV (connected directly to the hot wire through the reversed polarity bootleg ground), and the coax was actually grounded. Hence the 120V between them.
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Old 10-11-2011, 02:47 PM   #5
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Reverse Polarity? 120v on coaxial cable shield


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That's why it blew out the HDMI board. You didn't have 120V on the coax shield. You had 120V on the "grounded" metal of the TV (connected directly to the hot wire through the reversed polarity bootleg ground), and the coax was actually grounded. Hence the 120V between them.
Right, the polarity is reversed on this outlet, and then the error is compounded by connecting the so-called neutral (actually hot) to the ground (which isn't really ground now)

Do you have one of those cheap plug in testers w/ the 3 lights? That will tell you if the polarity is wrong or not. This is more common then you'd think, I found at least half a dozen outlets wired with reversed polarity in the last rental I lived in. The funniest one being quadplex outlets (two duplex outlets in the same box... I guess maybe the hack who was doing it figured he'd be right for at least one of them? )

Last edited by bubbler; 10-11-2011 at 02:49 PM.
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Old 10-11-2011, 05:39 PM   #6
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Reverse Polarity? 120v on coaxial cable shield


The tester actually showed it as being good. I guess since the ground was tied to the neutral lug that was enough to fool it. It cant tell otherwise because it is AC
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Old 10-11-2011, 05:58 PM   #7
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Reverse Polarity? 120v on coaxial cable shield


A bootleg ground will show as correctly wired when tested with a three prong checker.
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Old 10-11-2011, 06:55 PM   #8
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Reverse Polarity? 120v on coaxial cable shield


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The tester actually showed it as being good. I guess since the ground was tied to the neutral lug that was enough to fool it. It cant tell otherwise because it is AC
You need a different type of tester, in order to show if it is a bootleg ground or not. This is what should be used to check http://www.licensedelectrician.com/S..._Analyzers.htm but for $265 and only going to use a couple of times, it can be cheaper to find an electrician that has one in their kit.
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Old 10-11-2011, 09:19 PM   #9
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Reverse Polarity? 120v on coaxial cable shield


A cheap non-contact voltage tester will also identify this problem correctly. It will show the neutral/ground as hot, and the hot as not.

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