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Old 06-16-2014, 10:01 AM   #1
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Cabinet screws showing 30 Volts AC


We were changing a receptacle used for washing machine. Turns out this line is a 12/3 line run as a shared neutral for the refrigerator. That explains why the neutral was showing 120 VAC to ground, with the breaker off.

Last month my son was hanging a cabinet just above this receptacle. He was probably kneeling on, or had his arm resting on the dryer while driving in the 3" cabinet screws. I figured he hit a wire so we'd have to take the cabinet down, remove drywall and see what happened.

So, with this 120VAC neutral to ground, before I realized it was a shared neutral, I decided to connect a voltmeter to the neutral (disconnected from the receptacle and thus disconnected from the panel, I think) and this screw. It showed 30VAC. The other 2 cabinet screws, in 2 different studs, show about 20 VAC.

How could this be? It appears the screws are connected to earth ground ... ?

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Old 06-16-2014, 10:37 AM   #2
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Cabinet screws showing 30 Volts AC


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Originally Posted by jkk View Post
We were changing a receptacle used for washing machine. Turns out this line is a 12/3 line run as a shared neutral for the refrigerator. That explains why the neutral was showing 120 VAC to ground, with the breaker off.

Last month my son was hanging a cabinet just above this receptacle. He was probably kneeling on, or had his arm resting on the dryer while driving in the 3" cabinet screws. I figured he hit a wire so we'd have to take the cabinet down, remove drywall and see what happened.

So, with this 120VAC neutral to ground, before I realized it was a shared neutral, I decided to connect a voltmeter to the neutral (disconnected from the receptacle and thus disconnected from the panel, I think) and this screw. It showed 30VAC. The other 2 cabinet screws, in 2 different studs, show about 20 VAC.

How could this be? It appears the screws are connected to earth ground ... ?
It appears to me the pierced a cable.


Last edited by brric; 06-16-2014 at 10:51 AM.
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Old 06-16-2014, 10:49 AM   #3
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Cabinet screws showing 30 Volts AC


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It appears to me the pierced a cable.
Probably, but getting 3 screws, in 3 different wall studs, to show a path to ground from 1 screw that had maybe pierced but thdayen withdrawn and relocated ... seems very unlikely. I'll see later today when we remove drywall.
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Old 06-16-2014, 10:55 AM   #4
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Cabinet screws showing 30 Volts AC


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Probably, but getting 3 screws, in 3 different wall studs, to show a path to ground from 1 screw that had maybe pierced but thdayen withdrawn and relocated ... seems very unlikely. I'll see later today when we remove drywall.
Have you turned both breakers of the MWBC off?
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Old 06-16-2014, 11:29 AM   #5
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Cabinet screws showing 30 Volts AC


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Have you turned both breakers of the MWBC off?
Originally we only turned off one, until we discovered that shared neutral. However, I still need to learn why the screws-into-studs have some path to ground.
Btw, what is MWBC?
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Old 06-16-2014, 11:33 AM   #6
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Cabinet screws showing 30 Volts AC


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Originally we only turned off one, until we discovered that shared neutral. However, I still need to learn why the screws-into-studs have some path to ground.
Btw, what is MWBC?
Multi-wire Branch Circuit. 2 hots, one shared neutral, one grounding conductor.
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Old 06-16-2014, 12:14 PM   #7
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Cabinet screws showing 30 Volts AC


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I still need to learn why the screws-into-studs have some path to ground.
the screws don't, the wood does


google " pyrophoric carbonization "
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Old 06-16-2014, 12:21 PM   #8
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Cabinet screws showing 30 Volts AC


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the screws don't, the wood does


google " pyrophoric carbonization "
Well, the screws have to, albeit through the wood. Pyrophoric carbonizing does not seem to deal with electrical conductivity, just combustibility.
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Old 06-16-2014, 02:26 PM   #9
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Cabinet screws showing 30 Volts AC


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Well, the screws have to, albeit through the wood. Pyrophoric carbonizing does not seem to deal with electrical conductivity, just combustibility.
Just combustibility? You missed the point. The combustibility was do to conductivity.

It's the wood, not the screw. This phenomenon is the result of it's electrically conductive nature.

The screws and nails help it along, but it's in the wood.

The screw gave you a metal reference for measurement. It's not the screw.
The screw is a screw.
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Old 06-16-2014, 02:49 PM   #10
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Cabinet screws showing 30 Volts AC


I must say though, there should be no reason why you are reading 30v through a screw.

I don't think the wood would be that conductive.

Something else is going on which you may or may not find when you start the rip out.

Who know's, there may be a long nail plate across those studs that's hot.
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Old 06-16-2014, 02:54 PM   #11
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Cabinet screws showing 30 Volts AC


My first question would be what are you metering between? Ground and the screws? For all you know the voltage you are reading could be on the ground and not the cabinet screws.
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Old 06-16-2014, 05:35 PM   #12
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Cabinet screws showing 30 Volts AC


Before you start ripping drywall out.....use a different meter and see if you get the same reading...just saying.
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Old 06-16-2014, 05:41 PM   #13
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Cabinet screws showing 30 Volts AC


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My first question would be what are you metering between? Ground and the screws? For all you know the voltage you are reading could be on the ground and not the cabinet screws.
The measurements above were from the screw to a disconnected neutral that I think was coming from the fridge. That neutral measured 123 VAC to ground.

Now that I'm home, I measured other cabinet screws, to hot (black), in other parts of the kitchen, and I get anywhere from 2 to 15 VAC from screw to hot (black). In some studs, I have 2 screws ( top of cabinet and base of cabinet) - and I get 8.5 VAC at the base screw and 6.0 VAC at the top screw ( further from receptacle, if that matters but I don't see how why), measuring to hot (black). This happens on 2 different studs.

I'd like to see if other people get similar results from their kitchen cabinet screws to hot on receptacles.

The largest reading so far is 15 VAC from screw to hot.

Last edited by jkk; 06-16-2014 at 05:47 PM.
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Old 06-16-2014, 06:13 PM   #14
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Cabinet screws showing 30 Volts AC


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Originally Posted by DanS26
Before you start ripping drywall out.....use a different meter and see if you get the same reading...just saying.
Good suggestion!! Another meter shows 0.5 volts from various screws to hot, independent of where the screws are. The first meter showed 135 VAC across hot and neutral on a receptacle that has 1 125 AC home and 5Vdc USB ports. The 2nd meter reads 125VAC across those same points.

Not sure how I would get consistent bad readings from that 1st meter - how the readings would vary with distance from the hot wire ... Must be very sensitive to resistance changes in the wood ...
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Old 06-16-2014, 06:34 PM   #15
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Cabinet screws showing 30 Volts AC


Ok, the embarrassment factor gets worse. The 9V battery, on the meter giving bad readings, was really a 5.7V battery. Worse yet, the low battery symbol was on but I didn't see it. Wow. Good thing I didn't tell people here I've got a master's degree in electrical engineering from a well known engineering university.

Sorry to waste your time.

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