I was working last night on an electrical circuit that was shut off at
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?
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 :-)
On 13 Mar 2005 14:02:55 -0500, email@example.com 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. ;)
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.
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.
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.
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
Fill us in, on what you found.
tom @ www.MedicalJobList.com
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,
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.
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.
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.
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.
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.
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?
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.
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.
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
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