Induced voltage? Stray?

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crossman

Senior Member
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Southeast Texas
One topic in electrical classes that occasionally arises is digital meters and "stray" or "induced" voltage. What is the actual explanation of these voltages?

As an example, say we have several wires in a conduit or cable. Some of the conductors are feeding loads, but there are also some spare wires that are not connected on either end. With a digital meter, sometimes we can measure a substantial voltage from the spare conductor to ground.

Would this actually be an induced voltage caused by the magnetic field of one wire inducing voltage into the spare wire? I'm thinking it would actually be a capacitively coupled voltage.
 
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crossman

Senior Member
Location
Southeast Texas
Perfect. Sorry I did not think to google for this myself. I appreciate the link, it explains the situation. Another interesting fact: Fluke sells a "low impedance" attachment for their digital meters to essentially convert it into a wiggie!
 
76nemo

76nemo

Senior Member
Location
Ogdensburg, NY
Confused

Confused

Issue #1 - I still say check it with a wiggie.

Issue #2 - I'm fairly sure this can be done without conflicting with 300.3(B). See diagram. (EGC omitted for clarity)


crossman, you posted that. If you are unfamiliar with stray voltage, what do you call what happened in that scenario where he had 41V on the neutral?
That was from "Backfed or something".
 
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crossman

Senior Member
Location
Southeast Texas
76nemo said:
crossman, you posted that. If you are unfamiliar with stray voltage, what do you call what happened in that scenario where he had 41V on the neutral? That was from "Backfed or something".
Click to expand...

Just because I am familiar with "stray" voltage doesn't mean that I understand it. I was going to ask the same question in the "backfeed" thread but figured that would be off-topic, so I started a new thread. I hope this is acceptable.

As for the question, the majority of people say it is an induced voltage. I was sitting here thinking about that last night, and it dawned on me that in the case of the conductor which is not connected on either end, it can't be induced voltage that we are measuring. I pictured it in my mind and decided that it must be capacitance. See diagram that I pictured in my mind.

So again, the question was posted in a new thread because it would have led the other thread off topic, and I wanted to get people's thoughts on "stray" voltage so that I would have a better understanding of it.:smile:
 
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crossman

Senior Member
Location
Southeast Texas
celtic said:
Crossman....have you seen this scientific study?

Post #17 has all sorts of pictures.
Click to expand...

That is beautiful! Experiments like that are priceless! We need more stuff like this.:smile:

But, I am going to go on record and say that it wasn't inductance causing the DMM voltage reading. What you had there was a very low-value capacitor in series with a very high resistance meter and 120 volts applied across the series circuit. The capacitance drops some of the voltage, and the meter sees the rest.

The only way to get an induced voltage would be to involve BOTH ends of the red wire in the measurement.
 
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ronaldrc

Senior Member
Location
Tennessee
Hello

I have a little different opinion of whats happening.
I don't think it is capacitance or induction.

The way I look at it is I see the insulation around the copper wire as a resistor and in series between the black hot wire and the red wire.

And don't forget this resistor would be in the megaohms. :)
 
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crossman

Senior Member
Location
Southeast Texas
That is an interesting point to think about.

The definition of a capacitor is "two conductors seperated by an insulator" which is certainly what we have with the red wire and the black wire. I am going to guess that the stray voltage reading has more to do with the actual capacitance value of the two wires than electrons leaking through the insulation and air between the two wires.
 
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ronaldrc

Senior Member
Location
Tennessee
Capacitance is theory, I never did completely buy the theory that electricity doesn't flow through a capacitor.I always seen the insulation as a resistor.Just my opinion.

I think this needs more investigation and we do not have the right tools as of yet to see or measure these events.
 
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crossman

Senior Member
Location
Southeast Texas
Well, you have me there. But since it is possible to store electrons on one plate of a capacitor and to have a deficiency of electrons on the other, the theory certainly makes sense to me, not to mention that the theory also makes sense to hundreds of scientists who know way more about it than me.

An experiment could be performed to see if the wires in question have a capacitive effect. Take a 120v DC source. Connect the + terminal to the black wire. Connect the - terminal to the red wire. After awhile, remove the wires from the voltage source. Then use a sensitive DC volt meter to determine if there is any voltage still on the wires. I'm thinking that there will be a certain amount of potential difference between the wires.
 
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ronaldrc

Senior Member
Location
Tennessee
Crossman

I can not argue with you there,but it is theory and I don't think or don't know who is right. But I have my doughts about the theory.

I don't no how old you are but I don't think in my life time I will ever know. :)
 
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polk

New member
Change the frequency if the load voltage

Change the frequency if the load voltage

What would happen if you changed the frequency of your load voltage?
If inductance we should see a change...if resistance no change.
 
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Wes G

Senior Member
Electrical Technology Instructor

Electrical Technology Instructor

A number of years ago I was working on the replacement of a small 460V 3phase motor. All motor leads had been disconnected in the MCC about 250 feet from the motor, and I was disconnecting the motor. When I got a nasty shock from the disconnected wires, I got my Fluke 87 and checked for voltage and read 430V. Thinking I must have a short between the motor leads and other wires in the conduit, I spent the next 4 hours troubleshooting the other two three phase motor loads contained in the same conduit to no avail. Finally I figured that if there was a short, connecting the original leads to ground would trip the OCPD of the offending source. So I put on my PPE and very gingerly grounded my conductors. There was no spark that I could detect but neither was there any offending voltage left.

I wrote it off at the time as induced voltage, but I still ponder the exact cause. With all 3 phase wires from each of the other motor circuits in the same conduit, I would have anticipated a canceling of the effects of induction or of capicitance.

Any ideas out there?
 
76nemo

76nemo

Senior Member
Location
Ogdensburg, NY
Low vs. High

Low vs. High

That was one of the main reasons for the invention of low-impedance meters, to be able to see ghost voltage, ....Safety. If you see 85V on a conductor with a high impedance meter, there's cause for concern. Use a low impedance meter to check the same line and see just mV, hence "Ghost voltage", and no threat.
This is a very handy feature to have. "Is it there, or not?" Some like the Wiggy's, some like Flukes new T+, but I'd rather have a dual impedance DMM so I can see a LCD readout, not an LED range.
 
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nakulak

Senior Member
I've heard of rubber tire backhoe operators hitting high voltage line, then getting electrocuted when they climbed down off the machine because the machine retained a charge (don't know if its true or not)
 
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