Capacitive Reactance

[LarryFine]

I use a solenoid tester for 99% of troubleshooting.

 

[jim dungar]

Although the two conductors separated by insulation can create a capacitive effect, I thought the effect of voltage seen on the wire not connected to power to be caused more through inductance and changing of AC field from the hot wire rather than the capacitive portion

You are correct, there are several things that come into play. It will also depend on what the de-energized conductor is connected to.

 

[kwired]

I witnessed a dead wire show voltage on it while ran within the same conduit of another line side hot switch wire. Or at least the trick tracer beeped on this wire.

Although the two conductors separated by insulation can create a capacitive effect, I thought the effect of voltage seen on the wire not connected to power to be caused more through inductance and changing of AC field from the hot wire rather than the capacitive portion

For it to be inductance wouldn't you need a ferrous core involved between conductors? Without the ferrous core you have a capacitor.

They both have similar but opposite effects. Current leads voltage with one and lags with the other, if you have a circuit for current to flow through anyway.

 

[ggunn]

It's an open neutral .

I don't understand what you mean by an "open neutral". If it isn't connected to anything it is just a wire.

 

[ptonsparky]

I don't understand what you mean by an "open neutral". If it isn't connected to anything it is just a wire.

Depends on if the other side of the load is connected to an energized conductor.

 

[ggunn]

Depends on if the other side of the load is connected to an energized conductor.

Does one normally megger conductors that are connected to loads? I have never done it.

 

[ptonsparky]

Does one normally megger conductors that are connected to loads? I have never done it.

Some where in this thread someone asked why you can get bit from the white (neutral) wire. that is what I was referring to.

But Yes, you can me with a load connected to the wires. Generally motor loads. If it shows good, no need to investigate that portion any further.

 

[ramsy]

Some where in this thread someone asked why you can get bit from the white (neutral) wire.

That was me asking why non-contact sensors wont detect current flowing on the neutral.

I believe there are a few good answers:

ptonsparky points out open neutral at empty receptacles.

LarryFine points out capacitance changes somehow on its way to earth ground.

Maybe after going thru the load, electrons are too tired to give any more static.

 

[GoldDigger]

That was me asking why non-contact sensors wont detect current flowing on the neutral.

I believe there are a few good answers:

ptonsparky points out open neutral at empty receptacles.

LarryFine points out capacitance changes somehow on its way to earth ground.

Maybe after going thru the load, electrons are too tired to give any more static.

Not a bad overall explanation. But I take issue with one point, that Stray Voltage (typically Neutral-Earth Voltage, NEV) is usually associated with dairies. It is no more common at dairies than at any other farm or rural distribution environment, where metallic neutral runs are long and ground electrodes in the supply system are fewer and farther between. And it can easily occur in residential situations too.

But dairies are much more affected by NEV since cows with their four footed stance are more susceptible to voltage gradients in the earth and even small perceived voltages in the earth or to metallic contacts like feed and water troughs will have a severe effect on milk production.

 

[LarryFine]

That was me asking why non-contact sensors wont detect current flowing on the neutral.

Now I understand your question! Non-contact testers detect voltage, not current.

They do not depend on presence or absence of load current to work, only an electrical field.

 

[LarryFine]

But dairies are much more affected by NEV since cows with their four footed stance are more susceptible to voltage gradients in the earth and even small perceived voltages in the earth or to metallic contacts like feed and water troughs will have a severe effect on milk production.

Can't cows be fitted with insulated booties?

 

[ptonsparky]

Can't cows be fitted with insulated booties?

They could. I know of a 3000 head Dairy you could try it out on.

 

[LarryFine]

They could. I know of a 3000 head Dairy you could try it out on.

That would only be 12,000 booties. Or walk the cows through a trough of Flex Seal liquid.

 

[Jpflex]

For it to be inductance wouldn't you need a ferrous core involved between conductors? Without the ferrous core you have a capacitor.

They both have similar but opposite effects. Current leads voltage with one and lags with the other, if you have a circuit for current to flow through anyway.

When I say inductive I’m referring to a “voltage” being induced from the source wire to the adjacent wire through close proximity just as a transformer does between primary and secondary wires.

Also as a not coiled wires also induce a voltage into themselves called counter EMF which resist the direction of current which created it - hence impedance.

Inductors store energy in an electromagnetic field while capacitors store energy in an electro static field

Perhaps there is also a small capacitive effect as well

 

[Jpflex]

You are correct, there are several things that come into play. It will also depend on what the de-energized conductor is connected to.

Can you describe more on what these loads types are and what effect these have? Would the load such as harmonic really need to factored in for the effect ? Perhaps if a current is to travel on second wire

 

[dkarst]

The “classic” case of ghost or phantom voltage is caused by capacitive coupling. In a typical case an energized conductor is in the neighborhood of a unconnected neutral conductor. This “neighborhood" physical proximity causes a capacitance between the conductors of ~ 5 to 20 pF per foot. For example, in 20 feet of NM cable (or conducts in same conduit) you could have 200 pF which at 60 Hz gives a capacitive reactance of 13Mohms.

Now the equivalent circuit is a source connected to two resistances (reactances) in series, the first being the virtual capacitor formed by the conductors and the second being the meter (assumed to be connected from the neutral to ground). If you have a typical high-impedance electronic DVM with ~ 10M ohm input impedance you can see half the voltage is dropped across each element which could result in a voltage reading of 40 -60V (assuming 115V source). If you use a solenoid tester, meter with Lo-Z capability, or even old VOM, the meter resistance is so low compared to the Mohm of the virtual capacitor, you get a correct reading (i.e. ~ all the voltage is dropped across the virtual capacitor).

As to inductive coupling, I guess we can argue about semantics, but for inductive coupling to occur, my opinion is you need to have time varying current flow magnetically coupled to a loop(s) which induces voltage via Faraday's law. In the case above, no current is flowing and it is simply an AC circuit formed by two impedances.

 

[Speedskater]

An Audio Engineering Society paper on AC power line current leakage by an audio transformer expert:

Ground Loops: The Rest of the Story
Bill Whitlock, AES Fellow and Jamie Fox, P.E.

https://www.jensen-transformers.com/wp-content/uploads/2015/02/AES-Ground-Loops-Rest-of-Story-Whitlock-Fox-Generic-Version.pdf

 

[kwired]

The “classic” case of ghost or phantom voltage is caused by capacitive coupling. In a typical case an energized conductor is in the neighborhood of a unconnected neutral conductor. This “neighborhood" physical proximity causes a capacitance between the conductors of ~ 5 to 20 pF per foot. For example, in 20 feet of NM cable (or conducts in same conduit) you could have 200 pF which at 60 Hz gives a capacitive reactance of 13Mohms.

Now the equivalent circuit is a source connected to two resistances (reactances) in series, the first being the virtual capacitor formed by the conductors and the second being the meter (assumed to be connected from the neutral to ground). If you have a typical high-impedance electronic DVM with ~ 10M ohm input impedance you can see half the voltage is dropped across each element which could result in a voltage reading of 40 -60V (assuming 115V source). If you use a solenoid tester, meter with Lo-Z capability, or even old VOM, the meter resistance is so low compared to the Mohm of the virtual capacitor, you get a correct reading (i.e. ~ all the voltage is dropped across the virtual capacitor).

As to inductive coupling, I guess we can argue about semantics, but for inductive coupling to occur, my opinion is you need to have time varying current flow magnetically coupled to a loop(s) which induces voltage via Faraday's law. In the case above, no current is flowing and it is simply an AC circuit formed by two impedances.

Though you may be able to do this with other materials, iron core is the best way as it has the best magnetic properties.

When I say inductive I’m referring to a “voltage” being induced from the source wire to the adjacent wire through close proximity just as a transformer does between primary and secondary wires.

Also as a not coiled wires also induce a voltage into themselves called counter EMF which resist the direction of current which created it - hence impedance.

Inductors store energy in an electromagnetic field while capacitors store energy in an electro static field

Perhaps there is also a small capacitive effect as well

single conductor run in a ferrous raceway is an inductor. Not as good of one as a transformer or a solenoid coil with several wraps around a core but it will have more inductive properties than a single conductor in free air has.

 

[kwired]

That was me asking why non-contact sensors wont detect current flowing on the neutral.

I believe there are a few good answers:

ptonsparky points out open neutral at empty receptacles.

LarryFine points out capacitance changes somehow on its way to earth ground.

Maybe after going thru the load, electrons are too tired to give any more static.

Open neutral has same voltage potential as the ungrounded conductor trying to supply a load in the circuit on the side connected to the load. The other side is a grounded conductor.

An intact neutral conductor is at same potential as the grounded source conductor, less any voltage drop at the point of measurement when there is current flowing.

If voltage drop is only a volt or so the typical NCVT isn't going to be designed to pick this up and indicate it as "hot"

A somewhat better use for NCVT is checking equipment grounding. If you forgot to connect the EGC to something, particularly with non metallic wiring methods where there is no other conductive path to pick up bonding, A NCVT works pretty good most of the time at indicating that there is no intact EGC to the item being tested as that item will have capacitively coupled voltage on it. It won't be good enough of a capacitor for you to get a shock when you touch it, as long as there is not actual ground fault involved.

Used to have inspectors that held NCVT's up to switch covers or luminaires when doing final inspections, and turn the switch on as it needs to have active load or it won't have any voltage to couple with. If you forgot to bond the switch yoke it will indicate voltage nearly every time. Only had one time that I recall getting a false positive when a luminaire was setting off the NCVT yet we checked things out and the luminaire was bonded properly and even tested with DMM. Had it not set off the NCVT we wouldn't have spent the time verifying it actually was bonded.

 

[ramsy]

Open neutral has same voltage potential as the ungrounded conductor trying to supply a load in the circuit on the side connected to the load. The other side is a grounded conductor.

An intact neutral conductor is at same potential as the grounded source conductor, less any voltage drop at the point of measurement when there is current flowing.

If voltage drop is only a volt or so the typical NCVT isn't going to be designed to pick this up and indicate it as "hot"

Now I get it. Thank you sir.

A somewhat better use for NCVT is checking equipment grounding. If you forgot to connect the EGC to something, particularly with non metallic wiring methods where there is no other conductive path to pick up bonding, A NCVT works pretty good most of the time at indicating that there is no intact EGC to the item being tested as that item will have capacitively coupled voltage on it. It won't be good enough of a capacitor for you to get a shock when you touch it, as long as there is not actual ground fault involved.

Used to have inspectors that held NCVT's up to switch covers or luminaires when doing final inspections, and turn the switch on as it needs to have active load or it won't have any voltage to couple with. If you forgot to bond the switch yoke it will indicate voltage nearly every time. Only had one time that I recall getting a false positive when a luminaire was setting off the NCVT yet we checked things out and the luminaire was bonded properly and even tested with DMM. Had it not set off the NCVT we wouldn't have spent the time verifying it actually was bonded.

Very clever. Thank you for sharing this.