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!

Reply to
blueman
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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.

Reply to
m Ransley

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.

Reply to
SQLit

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

Reply to
Mark

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 @

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Reply to
The Real Tom

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

Reply to
Jeff Wisnia

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

Reply to
Bob Urz

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

Reply to
nicksanspam

Yes.

Reply to
Joseph Meehan

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

Reply to
TURTLE

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

Reply to
Jeff Wisnia

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.

Reply to
Joseph Meehan

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?

Reply to
Doug Miller

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

Reply to
stretch

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.

Reply to
blueman

I verified this with 2 different digital voltmeters, albeit none of them very professional ones.

Reply to
blueman

Good point! I did indeed find 30 volt both hot-to-ground and neutral-to-ground.

However, I believe I can explain the neutral-to-ground voltage very simply. The reason I was opening up the junction boxes in the basement and checking voltages was that I was having some wacky behavior with most of the circuit out but a few of the compact flourescents dimly lit. It turns out that one of the neutrals had slipped out of the wire nut. This could explain the induced current in both the neutral and the hot since with the breaker off they were both effectively "floating".

Let me know though if you think I am missing something.

Thanks.

Reply to
blueman

Roger that, I think you're on track now. If you reconnected that neutral wire nut and left the breaker open I'd bet dollars to donuts that you'd still measure "something", but now less than 30 volts, with that meter between hot and neutral or hot and ground, but nada between neutral and ground. (Because the capacitance between the hot and neutral in that run would form the lower leg of a capacitive voltage divider and shunt some of that stray current to ground.)

Glad you got it fixed though!

Jeff

Reply to
Jeff Wisnia

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.

Reply to
PrecisionMachinisT

No question that magnetic fields can induce voltages Doug, but they are unlikely to be anywhere near as large as 30 volts in house wiring because the outgoing and return current carrying conductors are so physically close to each other that their magnetic fields cancel almost completely and don't leave much of a net ac magnetic field to induce a voltage in another nearby conductor. And, I would expect that since the ground lead is also in close proximity to the hot and neutral leads in typical house wiring that magnetic field would induce a near equal voltage in it too, bucking out the voltages induced in the other two wires, leaving little voltage to measure between them.

It's genberally capacitive coupling which causes those "phantom low current voltages" on disconnected conductors in houses.

Things can be different in industrial applications where currents can be much higher and the wire runs a lot longer. For example, running a three phase circuit between two boxes in three separate pieces of metal conduit instead of a single larger conduit because a hack installer didn't have any large enough pipe with him (and didn't know any better) might not seem dangerous at first thought. But, as soon as some significant currents are put through those conductors they induce currents in the three conduits, which, since they're electrically onnected together at both ends by the walls of the boxes, act like a shorted 3 phase transformer secondary. The three conduits can get hot and often will start arcing where they join onto the boxes.

At any event, the OP found the problem, an open connection in the neutral lead at a wire nut. The degree of "seriousness" of that defect in his house wiring is debatable.

Peace,

Jeff

Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public schools"

Reply to
Jeff Wisnia

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