in a remodel I shut off a circuit that had a kitchen GFCI and from there the line went on to power a seperate duplex for the kitchen fridge which was not shut off when the GFCI test button was pushed. (i.e. fridge was not GFCI protected)… So I shut off the breaker for the line and my new digital tester showed that there was still 5 volts AC in the line. Why was this? is this somehow normal or a cause to get an electrian in to look at it? The tester works well and resters zero volts AC on a truly dead line.
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you're not reading any "real" voltage
the sensitivity of your DMM is the reason
If it were 1 volt or 2, I might tend to agree, but 5v?? Gotta be a pretty low-end VOM. Maybe a HarborFreight special <g> If the VOM is flukey enough to read 5v on one dead circuit but 0v on another, I'd be finding a new VOM pretty dang quick.I don't have any suggestions of my own, but I would think that a bit of stray voltage must be bleeding into the circuit somewhere.Poet, does the VOM read 5v hot to neutral and hot to ground? What about neutral to ground? Try a half-dozen other known-dead circuits. Do you get the mysterious 5v anywhere else? If not, I'd be even more strongly inclined to suspect the circuit, not the multimeter...
5V is nothing....touch the wires and the volts go away
What is surprising is that a it was not higher.Acutally a LOW END VOM is less likely to have this "problem". But the better VOM's and specially the DVM are have a very high input impednence.A DVM will often read 20-80 volts on an "open circuit".They may have an 100 megohm impedence and it does not take much in the way of capacitive coupling to get a reading.However, some as SMALL as 5 volts does make me question it.And asking what the reads are (L-N, L-G, G-N) are important clues along with what kind of load is on the circuit at the time and place the measurement is being made.
Interesting - thanks, Bill...
If a meter reads it it is there. It my not be the Line Volts.
.>A DVM will often read 20-80 volts on an "open circuit".
>and it does not take much in the way of capacitive coupling to get a reading
Do You mean <inductive coupling > ?
Or thermocouple effect. Two dissimilar Metals. but not 80V. Or leakig.
Hilmar
Edited 2/26/2006 2:13 pm ET by h12721
No, I ment capacitive coupling.It has been a LONG, LONG time since I studied field theory so I don't remember if inductive or capacitive coupling is the stronger.However for inductive coupling you need to have current flowing in the "source circuit" which may or may not be turn at anytime.But you have many feet of wires running next to each other in a house and you can easily get capacitive coupling from that.
On a 60 Hz line and using a 12 megohm impedance meter (typical), only 10 picofarads (pF) of capacitance coupling is needed to get a reading of 5 volts.
It takes less than 1 meter of 14 or 12 AWG in the same bundle to get 10 pF Like Bill said earlier, if there is an energized line and unenergized line running many 10s of feet together, an 80 volt open circuit reading is not uncommon.
On high impedance circuits (ie 12 megohm meter input) capacitive coupling shows up more, on low impedance circuits, inductive coupling has more effects.
Inductive coupling favors low frequency energy sources. High frequency energy sources generally use capacitive coupling (capacitive coupling: more facts about this subject) .
<It has been a LONG, LONG time since I studied field theory so I don't remember if inductive or capacitive coupling is the stronger.
<However for inductive coupling you need to have current flowing in the "source circuit" which may or may not be turn at anytime.
The same in the capacitive coupling.
My mistake on the thermocouple, Yes it is DC of course.
I was not thinking on measuring heat but of the crystal radio set. As two dissimilar metals.
"<However for inductive coupling you need to have current flowing in the "source circuit" which may or may not be turn at anytime.The same in the capacitive coupling."No, you don't need to have any current flow for capacitive coupling. you only need the circuit to be energies.http://www.electrical-installation.merlingerin.com/guide/pdf_files/Ap15-17.pdfCapacitive coupling depends on dv/dt and inductive di/dt.
Hey, I'm back on earth.... I was out there on the tangent of someting I didn't understand so I'm back... I don't check the responses to my questions quickly but I do check them... well., the voltmeter is a new Fluke model...and if a line is dead it says zero... the line in question is downstream of the GFCI and I get the 5 volt reading L-N and L-G. I'll check it again and get back on in a couple days. Thanks all.
by the way, my next handle will be Nail Salon.. that'd be a cool one.
A thermocouple is a part used to send temperature info to a controller. Galvanic reaction between suitable materials is what causes voltage to be present (like a battery) and it's never AC voltage, only DC.Capacitive coupling occurs, but mostly when a low voltage circuit is near a high voltage circuit and they are connected to a common piece of equipment, as in an audio system when a 70V speaker wire or bundle is near a line audio line or bundle.Inductive coupling is when a wire or bundle with a load is near another line that is connected to an inductive load (something with a coil attached) and the magnetic field thrown off by the energized wire induces a current in the other wire. This is the basis of AC transformers and many other transducers, where an inductor moves near a magnetic field or a magnetic field varies near an inductor, causing current flow. Phono cartridges and guitar pickups are good examples of this."I cut this piece four times and it's still too short."
Edited 2/26/2006 4:55 pm by highfigh
A thermocouple is a part used to send temperature info to a controller. Galvanic reaction between suitable materials is what causes voltage to be present (like a battery) and it's never AC voltage, only DC.
That is incorrect.
A thermocouple functions according to the Peltier effect - a voltage is generated when two dissimilar metal junctions are at different temperatures. Galvanic reaction is like a battery, but that is NOT how a thermocouple works. No electrolyte need be present for a thermocouple to operate, in fact, the dissimilar metal are often spotwelded together. The below is the 2nd google hit on Peltier effect and will explain the physics.
http://www.uni-konstanz.de/physik/Jaeckle/papers/thermopower/node2.html
Thanks for the clarification. "I'm sorry- never mind!"
"I cut this piece four times and it's still too short."
I always thought that anything downstream of a GFCI was also under the GFCI influence, am I completely off?
" I always thought that anything downstream of a GFCI was also under the GFCI influence, am I completely off?"If you wire the downstream outlets using the LOAD terminals of the GFCI then all the downstream outlets will be GFCI protected, but if you continue from the LINE terminals, then the downstream outlets will be normal, no GFCI protection.Frank DuVal You can never make something foolproof because fools are so ingenious.
that is great frank, i love it. foolproof indeed!
i used to belong to local 34 piledrivers in san fran bay area. several attempts were made to "pilebutt proof" things but they were never successful. i have a hard time attributing that to ingeniousness however.
do you know whats long and hard on a piledriver?
nope, wrong, its the third grade. get your minds out of the gutter
and if you like that one, do you know what happens if you give a pilebutt three steel balls?
he will lose one, destroy another one, and the third one will be in his lunch bucket!
for a couple of years after i heard that joke i carried around a 2 and 1/2 inch diameter heavy equipment bearing in my lunch bucket just so it would be true! just how you destroy a 2&1/2 inch steel ball is beyond me but i think it has something to do with the pilebutt proof thing not working, and therein lies the joke.
I would first start off by going to the panel,pull the cover and take readings. Check all the stuff you did and confirm the readings. Look to see if you can determine any discrepancies.
Then follow the ground? check to see if it is carrying some of that v. Also important to do all this at the same time so if a motor or such is cycling you get consistent info.
Crossed ground neutral or a homie miswire receptacle crossing ground neutral hot.
I had a buddy Almost lost his tongue when he got shocked working on a chimney and grabbed the cold water line for support. Half the receptacles had been replaced and whoever didn't know that different wires go to different sides of the receptacle. The water line was energized with 240v.
There is another type of tester out there....one brand name is "Wiggy" that uses line power to operate a solenoid, that in turn indicates your voltage.
The advantage of this type of tester is that it requires some power to operate- and will not be foled by those "ghost" voltages, which may come from nothing more than two wires running next to each other.
reno, did you get the e-mail I sent you?