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Old 11-14-2009, 02:34 PM   #1
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Voltage leak?


Hi all,
So I was going to do a simple outlet switch, and I happened to notice that one receptacle (on a 4-way switch) had voltage across it even when all the switches were off. I took apart the entire circuit, to find one stretch of 3-wire that, when I connect the black lead to the hot source, the red wire has about 9 volts (compared to ground). I verified that the other end of the 3-wire was disconnected from anything (anything I could access anyway, who knows what's in the walls), so I have no idea why this could happen. I'm sure the explanation is going to be impossible in a chat room, but is this something that I should worry about? Thanks for any help!

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Old 11-14-2009, 03:58 PM   #2
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Voltage leak?


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Originally Posted by brewster39 View Post
about 9 volts (compared to ground).
9v is kind of low for a
http://en.wikipedia.org/wiki/Phantom_voltage
unless it's being generated by a very short length of Romex or your tester has a very low input impedance.

If you put an incand. lamp across that connection and it still reads 9v you've got some real problems.


Last edited by Yoyizit; 11-14-2009 at 04:01 PM.
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Old 11-14-2009, 04:48 PM   #3
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Voltage leak?


Thanks a lot for your post. I did put a light across the connection and the voltage went to zero. Which I'm implying from your post I don't have some real problems . I'll have to do some research on this phanton voltage and try to understand what's going on.

One thing: I noticed the effect with a digital and an analogue meter. Is that reasonable?
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Old 11-14-2009, 06:11 PM   #4
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Voltage leak?


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Originally Posted by brewster39 View Post
Thanks a lot for your post. I did put a light across the connection and the voltage went to zero. Which I'm implying from your post I don't have some real problems . I'll have to do some research on this phanton voltage and try to understand what's going on.
>>Romex has about 100 pF per foot of length and the voltage is capacitively coupled from one conductor to another.

One thing: I noticed the effect with a digital and an analogue meter. Is that reasonable?
Yes, the voltage is really there but phantom sources cannot supply very much current. 900' of Romex may be able to supply about 5 mA into a milliammeter.
The cheapie analog meters have lower sensitivity so they will read lower and the reading you get may depend on the scale you are on, which really makes for some confusion while troubleshooting.
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Old 11-15-2009, 08:05 AM   #5
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Voltage leak?


The amount of "phantom" voltage as measured depends on the impedance of the meter. Two meters used one at a time on the same circuit can well show different voltages with the higher impedance meter registering the higher voltage.

On average, digital meters have higher impedances than analog meters.

The "voltage leak" is right throuigh the insulation of the wires in the Romex cable. In other words this plastic insulation is probably not defective but it is not "perfect". In reality the impedance is lower at 60 Hz compared with DC (and lower still for higher frequencies).

(copied from another foru )

There are 3 measurement points, the hot wire, the disconnected (dead) wire, and ground.

Hot wire to ground is 120 volts period.

Impedance #1 is the "resistance" of the insulation all along the route the wires were juxtaposed.

Impedance #2 is the meter.

When you make the measurement we now have a circuit, hot wire through impedance #1 to the "dead" wire through impedance #2 to ground.

(Roughly due to the oversimplifying needed to achieve a short English sentence) The voltage as measured by the meter is impedance #2 divided by the total impedance (#1 plus #2), times 120.
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Last edited by AllanJ; 11-15-2009 at 05:25 PM.
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Old 11-15-2009, 08:38 AM   #6
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Voltage leak?


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Originally Posted by AllanJ View Post
The "voltage leak" is right throuigh the insulation of the wires in the Romex cable.
You should measure insulation resistance with DC. If it's >10 megohms, with 12 vdc applied you'd measure <1.2 ĶA.

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