# Induced voltage in circuir that is shut off????

I was working last night on an electrical circuit that was shut off at the breaker.
Nevertheless, the voltage from neutral to ground and from hot to ground both measured about 30V. This was enough to cause my voltage probe to buzz and to even cause a compact flourescent to light dimly. The current across the circuit however measure just 0.7 mA.
Am I correct in assuming that this is probably just induced voltage from neighboring wires that run alongside it or is there potentially something more serious and sinister going on?
Thanks!
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I think 30v is more than induced curent, I have no idea on a fix but also check your ground connection to see if current is being wasted to ground, be carefull the ground could also be hot.
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It's voltage, not current, and easy to see on an open circuit next to a powered conductor... 120 V across 10 pF/foot over 100', ie 1000 pF, ie 1/(2Pi60C) = 2.6 meg in series with a 10 meg meter would make it show 95 V.
You seem very determined to show your ignorance :-)
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

At last! Somebody else who really understands what's happening in such cases.
Jeff
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Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)
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On 13 Mar 2005 14:02:55 -0500, snipped-for-privacy@ece.villanova.edu wrote:

Redo your problem Nick, your problem solving for was for a single conductor, and measuring voltage end to end. Not for two(or more) conductors running side by side, and measuring voltage at one end. The voltage induced in one conductor, would be exacly the same as the induced in the others, therefe for no potential would exist between the conductors(as in OP's story) where the voltage should be sensed, between the conductors and an immediate grounding source(not the ECG, since that is one of the parallel conductors).
I'll give you an A for effort, Faraday would be proud. :-P
BTW, the NEC covers taking steps to minimize induced voltages and inductive heating, even for common fokes, it gives enough information about how running conductors close together will reduce/elminate it. Just that I see often that digital meters show voltage, on dead legs, and I've chatted with others about how to ensure you have no juice left. One smart chap said, cind the ECG and Neutral are bonded, why not bond the ungrounded(hot) conductors, we cam up with an idea that you can bond all three with a shorted out three prong plug. Now voltage should be ZERO, but we figured come time to close the breaker, we would forget it in place, and melt it. ;)
later,
tom
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The Real Tom wrote:

Not the way I read it Tom. The voltage measured end to end on that isolated conductor would be zero.
I believe he was describing a single isolated conductor running near one with an ac voltage on it. The 95 V he mentioned would be measured relative to the other side of the ac source swinging that powered conductor (Which would of course be "neutral" and/or "ground" in a home.) That voltage would be the same anywhere along its length.
He really should have described the most likely case; that the isolated conductor in a home wiring system would be a black wire in a length of Romex and the white wire and ground leads in that Romex would be connected to the other side of the ac source (ground). So, there would be a fair amount of capacitance between the isolated conductor and the two grounded ones in the Romex, probably more than the amount per foot he estimated for a "nearby" powered conductor. That would create a capacitive voltage divider, so the voltage measured on the isolated conductor couldn't possibly reach the 95 V level he mentioned.
Not for two(or more)

Twisting the conductors really helps with the inductive heating stuff.

Jeff
--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)
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You fail to tell where you were working. If it was in your home then you definitely have some wiring problems. Neutral grounded at than the service comes to mind.
finding the problem you can try turning off circuits one at a time until the problem goes away. Then a physical inspection of every box on both circuits may lead you to a solution
If your at work then who knows. I have seen over 30 v when low voltage conductors were run in the same conduit as medium voltage conductors. Took us for ever to find it. I never believed anyone would be that stupid to do what we found.
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blueman wrote:

at
Depends what you measured the 30 Volts with.
The modern digital electronic meters are very high impeadance and can easily pick up 30 volts from induced voltage as you call it.
A neon test light is also pretty sensitve to induced voltage.
Use an older mechanical volt meter (Simpson 260) or make a test lamp with a night light bulb.
Mark
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IHMO:
Good question! I've run into this alot, and at the power plant I worked at, when you opened a breaker our electrictions would verify no induced voltage. We had lots of high amps cables running side by side. Many times we were required to put in grounding straps/bars to cancel out the induced voltage.
Now as for 30v's that is kinda high, but around the definition of low-voltage(24v) of the NEC, I would verify your volt meter is working correctly, and check for any potential voltage leakage into the dead circuit.
Fill us in, on what you found.
later,
tom @ www.MedicalJobList.com
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blueman wrote:

Measuring 30 volts from hot to ground with a high impedance voltmeter can easily be caused by capacitive coupling to that that wire from another wire in close proximity carrying full voltage. It's a common question posed here. It could also occur if the breaker had popped under overload many times and had a very high resistance leakage path inside it through deposited vaporized contact material.
But, if you really measured 30 volts from neutral to ground (and didn't intend to say that the measurement was from hot to ground.) then there's something seriously wrong because the neutral should be at ground potential, and there's no way that 700 microamps of capacitive coupling or leakage is going to create 30 volts between two wires which are connected together.
Better measure again to be sure, and if you still get 30 volts between neutral and ground, my guess is that the ground lead ISN'T really grounded and you're seeing an induced voltage on it too, relative to neutral. THAT could be sinister.
Let us know what you find,
Jeff
--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)
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blueman wrote:

Plug in a common household lamp to the dead circuit. Make sure its on. Now take your meter reading when its under a load. If your still getting voltage you DO have a problem. My guess is you will show close to zero once you take the measurement with a load connected to it.
Bob
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blueman wrote:

Yes.
--
Joseph Meehan

26 + 6 = 1 It's Irish Math
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This is Turtle.
He did state that his compact flourescent light would cause it to light up dimly. Mill-Voltage does not light up flourescent lites to light up. I thought that at the first but the flourescent lite took that ideal out.
TURTLE
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Doug Miller wrote:

You weren't bothered at all by his saying he measured 30 volts between neutral and ground?
If you weren't, then please riddle me this Doug. Why would ANY kind of voltmeter, digital or otherwise, indicate that much voltage between neutral and ground (That's what the OP stated.) unless there was a serious defect in the home's wiring.
Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)
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blueman wrote:

SQLit did bring up a good point. A floating neutral could be a possibility. Try putting a small load on the circuit and that 30V should drop to almost zero. It it holds at about 30, then you have a potentially dangerous wiring problem. However your measurement of 0.7 ma would tend to rule the neutral problem out.
--
Joseph Meehan

26 + 6 = 1 It's Irish Math
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Nonsense. He's probably using a digital multimeter, which is exquisitely sensitive to very low amperage induced currents. If he uses an analog multimeter, the "problem" will very likely disappear. There is no reason to suspect wiring problems of any sort unless these readings are seen with an analog meter as well.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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He said it lit a compact flourescent bulb also. That does not sound like a meter problem or induced voltage. The floating ground sounds more likely.
Stretch
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snipped-for-privacy@milmac.com (Doug Miller) writes:

Yes, I am using a digital voltmeter and when I "short" the conductors to ground the total currrent is just 0.75mA which was what made me think induced current. However, this being electrical, I wanted to check to make sure I wasn't missing something.
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I verified this with 2 different digital voltmeters, albeit none of them very professional ones.
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wrote:

Serious this time........take your reading and then take several more readings, turning on and off some different lights, 110v breakers etc.......
If your measured voltage goes up to near 220v in some instances and then in other instances down to near or at zero volts then you have a floating neutral and you really should have someone with experience trace it down and fix it for you.
Usually its just the main neutral lug screw in the service has come a bit loose and needs tightened--generally it will be discolored and will have kinduva darkish 'staining' to it...
BUT the problem could also be in the meter panel or even at the utility co. transformer.....
Best to use an analog meter for this--( in case this isn't clear to you as of yet ), these are typically the older meters, or in any case ( generally ) they are meters that have an actual needle rather than having an lcd display.
--
SVL

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