Odd voltage on 14-3 conductors mystery

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Dantheelectricman said:
If this is "normal" then why have I never experienced this amount before.
Click to expand...

I don't believe we're saying "this is normal", rather, that the voltage readings have errors caused by the meters themselves. You asked "Anyone heard of this before?" And we answered, "yes" and "we've DONE this before."

As to why you've never experienced this, I simply can't say.

Dan, the high input impedance, of digital volt meters you report having used, is introducing significant error in the readings you are reporting to us. . . how much, that is yet to be determined, if you wish. A low input impedance measurement uses enough energy to make the reading that any charge is dissipated in the wiring being measured and, as a result, a reading is made that is sustainable under load.

Based upon your original trouble report from the occupants about the "Christmas tree lights were on dim", it sounds almost certain that there is something really wrong. The only thing that would change my judgement is the nameplate specs of the "Christmas tree lights." I hope you can learn the specs and share them with us. If the Christmas tree lights are a fractional Watt load (not likely, but unknown to me at this moment), then it is entirely possible that alone is the source of the "trouble."

Better yet, I hope you are able to actually use the occupant's Christmas tree lights as a test load in a trouble shooting mode that I suggest back in Post #28.
 
180330-1748 EDT

An experiment;

CREE 0.5 W 120 V 60 Hz bulb.
123 V 60 Hz power source, nominal sine wave.
Variable capacitor box.

It takes 0.19 ufd capacitancer in series with said bulb to get a low level flicker. At 0.18 ufd no light. Xc of 0.19 at 60 Hz is about 14,000 ohms.

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180330-2340 EDT

Dantheelectricman:

Another experiment.

Connect two 1200 pfd 400 V or greater voltage rated capacitors in series.
Place nominal 120 V 60 Hz sine wave across the series capacitors.
Use a Fluke 27 or equivalent meter, 10 megohms input resistance, and possibly 100 pfd input shunt capacitance.

Measure the total voltage across the two capacitors. At my bench 123.4 V.

Measure the voltage across one capacitor of the series string. My reading was 58.6 V. Then read the other capacitor voltage. My reading 59.0 V. Note: 58.6 + 59.0 = 117.6 . You tell me why it wasn't 123.4 .

You do not want to use ceramic capacitors. Paper or Mylar would be fine. 0.001 or 0.002 ufd can be used instead of 0.0012 .

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180331-0719 EDT

GoldDigger:

General run of the mill ceramic capacitors have a dielectric constant that is voltage dependent, temperature dependent, and tolerance on value that is poorer than paper, mica, and film capacitors.

I just grabbed two mica capacitors from a bag, no testing for capacitance, and by the voltages measured it can be seen that the values were fairly close.

Barium titanate. I could not find the correct words to input to Google to get to plots that we worked with in the 1950s.

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180331-0955 EDT

Dantheelectricman:

What is a simple equivalent circuit for an AC voltage source loaded with two capacitors in series where the two capacitors are of the same value?

How would this equivalent circuit simplify your understanding of what happens when a high input impedance DVM is connected across one of the series capacitors?

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180331-1409 EDT

GoldDigger:

I grabbed two 0.1 ufd 10 V ceramic capacitors from stock. Measurements using a GR 1650-A LRC bridge at 1 kHz.

First capacitor:
0.0981 ufd, D = 0.065, with Zero bias voltage.
0.0850 ufd, D = 0.060, with 10 V DC bias.
0.0970 ufd, after removal of bias.

Second capacitor:
0.0910 ufd, D = 0.045


The two mica capacitors I used in the earlier post read:

First mica:
1200 pfd, D = 0

Second mica:
1180 pfd, D = 0

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Dantheelectricman said:
To K8 the notes say there are no devices involved, connections are wire nutted, and measurements are taken to ground or neutral. And X means no connection. I thought the diagram was pretty basic and very understandable. Anyone else confused?
I will be going back to the apartment. All the talk about capacitance coupling and such is fine but doesn't explain the amount I am getting in voltage measurements. If this is "normal" then why have I never experienced this amount before. I bet you a million that if anyone else checks their own switched outlets at home you will not get these amounts. Please try it.
Click to expand...
Ever taken one lead of a DMM and touched one lead to a known 120 volt ungrounded conductor and placed the other lead in your hand while standing on a non conductive floor? You will get a reading, most the time not quite as high as you were measuring in your situation in OP, but 15-30 volts isn't too out of the ordinary either. That voltage reading is because of capacitive effects. Do same thing on a conductive floor and you get more current flow through the meter and a higher voltage is displayed. Do it with a low impedance meter, and you get enough current you feel a shock.
 
180401-1132 EDT

Dantheelectricman:

In post #47 I ask you the question ---
180331-0955 EDT

Dantheelectricman:

What is a simple equivalent circuit for an AC voltage source loaded with two capacitors in series where the two capacitors are of the same value?

How would this equivalent circuit simplify your understanding of what happens when a high input impedance DVM is connected across one of the series capacitors?
Click to expand...

An answer is:

1. The equivalent circuit is a voltage source of a value (same frequency, and phase angle) in series with a capacitor of some value.

2. The equivalent circuit voltage value is the open circuit voltage measured across the one capacitor that defines the two output terminals of the equivalent circuit. The open circuit voltage is not what a high impedance meter would read, but is probably close. The difference is because the meter is a load on the circuit.

In this case, with two equal capacitors in series, we have a voltage divider of 1/2. Thus, the equivalent circuit source voltage is 1/2 the original source voltage. 60 V from 120 V.

3. The internal impedance of the equivalent circuit is the impedance of the circuit when viewing the circuit from the above said two terminals. Ideal voltage sources have zero internal impedance. Thus, in the original circuit you replace the voltage source with a short for impedance analysis.

For this circuit this means the two capacitors are in parallel.

4. Thus, the equivalent circuit, a voltage source in series with an internal impedance, has a source voltage of 1/2 the original circuit source voltage, and the equivalent circuit series internal impedance is 2 times the capacitance of one of the two capacitors. So internal impedance of the equivalent circuit is 1/2 of the capacitive reactance of one of the capacitors.

Different 1/2 and 2 times values occur when the capacitors are of different values.

5. If the open circuit voltage was 1/4, then the capacitor ratio would be 1/3.

Do you understand the equivalent circuit concept?

..
 
180401-1216 EDT

A slight correction to my previous post. The phase angle of the equivalent circuit voltage source is not necessarily the same as the original circuit.

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180402-2112 EDT

Dantheelectricman:

When troubleshooting something, an engine, a motor, or an electrical circuit, it can be very helpful to understand how that something works. Otherwise it is hit and miss on how easily you will find the problem, or problems.

In electrical circuits this means having some understanding of electrical circuit theory, and being able to model the important components of a real world circuit into a theoretical circuit. With a useful model you can play with different values and see the effect. This should allow you to predict how the real world something works. With the model it may be easier to understand real world measurements.

If you have an electrical cable of possibly less than 1000 ft, are working at 60 Hz, and with no load on any wire, then you are primarily concerned with capacitive and conductive characteristics.

I do not have any three conductor with ground cable, thus I have run experiments on two conductor, with ground, romex cable. Romex has a very well defined geometry.

I did not test for leakage current because the cable is new and unused. Thus, little likelihood of the shunt R component being significant.

I used various instruments and methods to estimate cable capacitance. These were --- Tektronix 130 LC meter, max 300 pfd, frequency about 100 kHz --- General Radio 1650-A LRC bridge, 1 kHz --- Fluke 27 for AC voltage, and with 1 k resistor for current --- Utility 60 Hz power for 120 V.

The romex wire was CERROWIRE two conductor with ground #14 copper. The two current carrying conductors are insulated with an uninsulated ground centered between the insulated wires. Thus, the geometry is quite well defined. Note: there is not a uniform dielectric constant for the space around the wires. There is both air and insulation.

Two different lengths were tested --- 250 ft as originally coiled, and a 10 ft length cut from the 250 ft coil. The 250 ft coil produces a slightly different result, higher, because of additional capacitive coupling between different turns in the coil.


250 ft coil measured with the GR 1650-A at 1 kHz:

4250 pfd at D = 0.032, 17 pfd/ft, Xc = 624,000 ohms, Blk to EGC, Wh float
4750 pfd at D = 0.022, 19 pfd/ft, Xc = 558,000 ohms, Wh to EGC, Blk float
3600 pfd at D = 0.030, 14 pfd/ft, Xc = 737,000 ohms, Blk to Wh, EGC float

Note: 4250 in series with 4750 calculates to 2243 pfd. What appears to be two separate capacitors in series is not quite that. I attribute the difference to the way the electric fields change for the three different geometries. Which conductor is floating changes the electric field map even though the physical structure did not change.

Also the black wire has a different capacitance compared to the white wire. I suspect a different dielectric constant resulting from the black, probably carbon, coloring in the insulation.

More later.

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180403-1029 EDT

Measurement of capacitance via measured current.

If we assume a capacitor is of high quality, low losses, and inductance is insignificant at the frequency of interest, the measurement frequency, then Xc = Z.

So if we measure current and voltage to the capacitor we can calculate Z and therefore Xc.

The capacitance can be calculated for 60 Hz from the equation
C in picofarads (micro-micro-farads) = 10^6 / (377 * Xc) where Xc is in megohms. The 377 comes from 2*Pi*frequency in Hz.

Measuring Z. For measuring typical cable capacitance values at 60 Hz a 1000 ohm 1/2 W resistor and a millivolt meter will work.

An example: 100 pfd at 60 Hz has a capacitive reactance of about 26.5 megohms. At 120 V the current is 4.53 microamps. Using a 1 k resistor as a current measuring shunt the voltage reading is 4.53 millivolts.

Why not directly measure current with the meter? This method protects my meter from damage of overcurrent.

More later. Haven't proofread this.

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gar said:
180402-2112 EDT

Dantheelectricman:

When troubleshooting something, an engine, a motor, or an electrical circuit, it can be very helpful to understand how that something works. Otherwise it is hit and miss on how easily you will find the problem, or problems.

In electrical circuits this means having some understanding of electrical circuit theory, and being able to model the important components of a real world circuit into a theoretical circuit. With a useful model you can play with different values and see the effect. This should allow you to predict how the real world something works. With the model it may be easier to understand real world measurements.

If you have an electrical cable of possibly less than 1000 ft, are working at 60 Hz, and with no load on any wire, then you are primarily concerned with capacitive and conductive characteristics.

I do not have any three conductor with ground cable, thus I have run experiments on two conductor, with ground, romex cable. Romex has a very well defined geometry.

I did not test for leakage current because the cable is new and unused. Thus, little likelihood of the shunt R component being significant.

I used various instruments and methods to estimate cable capacitance. These were --- Tektronix 130 LC meter, max 300 pfd, frequency about 100 kHz --- General Radio 1650-A LRC bridge, 1 kHz --- Fluke 27 for AC voltage, and with 1 k resistor for current --- Utility 60 Hz power for 120 V.

The romex wire was CERROWIRE two conductor with ground #14 copper. The two current carrying conductors are insulated with an uninsulated ground centered between the insulated wires. Thus, the geometry is quite well defined. Note: there is not a uniform dielectric constant for the space around the wires. There is both air and insulation.

Two different lengths were tested --- 250 ft as originally coiled, and a 10 ft length cut from the 250 ft coil. The 250 ft coil produces a slightly different result, higher, because of additional capacitive coupling between different turns in the coil.


250 ft coil measured with the GR 1650-A at 1 kHz:

4250 pfd at D = 0.032, 17 pfd/ft, Xc = 624,000 ohms, Blk to EGC, Wh float
4750 pfd at D = 0.022, 19 pfd/ft, Xc = 558,000 ohms, Wh to EGC, Blk float
3600 pfd at D = 0.030, 14 pfd/ft, Xc = 737,000 ohms, Blk to Wh, EGC float

Note: 4250 in series with 4750 calculates to 2243 pfd. What appears to be two separate capacitors in series is not quite that. I attribute the difference to the way the electric fields change for the three different geometries. Which conductor is floating changes the electric field map even though the physical structure did not change.

Also the black wire has a different capacitance compared to the white wire. I suspect a different dielectric constant resulting from the black, probably carbon, coloring in the insulation.

More later.

.
Click to expand...

Would it be possible for you to run some tests using an applied 120Vac with a load connected to 2 conductors on the end of cable? Say around 5 amps or so. (You will be measuring for AC voltage from the neut, grounded conductor, current carrying conductor to the EGC conductor at the load end.

I assume the Romex would need to be uncoiled, because of the inductance created from being coiled. The inductance could/would skew your measurements.

You spoke of the geometry of the 14/2 with grd Romex cable.
If you would, please run the same test using 3 wire MC cable. MC because of the 3 insulated conductors. Again the load would be connected to the blk and wht conductors.

For a final test. Would you please run the same test where 3 insulated conductors are pulled in, say a 1/2", conduit of at least around 75ft to 100ft long. You could use maybe 1/2" flex for the conduit. The conduit is needed to hold the 3 conductors loosely randomly in close proximity to one another.

If you would in all three tests above, please run each test with the EGC floating and then with the EGC grounded.

Thanks in advance.
 
180403-1729 EDT

Jamesco:

I suggest that you run the experiments.

But you need to define the exact experiment you want to perform and what you are trying to find out in each.

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gar said:
180403-1729 EDT

Jamesco:

I suggest that you run the experiments.

But you need to define the exact experiment you want to perform and what you are trying to find out in each.

.
Click to expand...
I think he is telling you that you have better toys to play with and wants you to do it.:cool:
 
180403-2152

Jamesco:

The link you provided to me is good, but it is related to inductively coupled signals into audio systems at very much lower voltage levels than the voltages about which this thread was started.

There can be unwanted voltages on EGC from various sources.

I have spent far too much time on this thread to divert more to other somewhat related problems.

.
 
180404-1008 EDT

Continuing my measurements on previously said 250 ft roll of 14-2 with G. This roll is now 240 ft. The new measurements are using the electrical current method with 120 V 60 Hz as the applied voltage.

250 ft original package measurements, GR 1650-A at 1 kHz, from post #52:

4250 pfd, 17 pfd/ft, Xc = 624,000 ohms, Blk to EGC, Wh float
4750 pfd, 19 pfd/ft, Xc = 558,000 ohms, Wh to EGC, Blk float
3600 pfd, 14 pfd/ft, Xc = 737,000 ohms, Blk to Wh, EGC float


240 ft original package measurements using current with an applied nominal 120 V 60 Hz:

4406 pfd, 18.4 pfd/ft, Xc = 602,000 ohms, Blk to EGC, Wh float
4903 pfd, 20.4 pfd/ft, Xc = 541,000 ohms, Wh to EGC, Blk float
3789 pfd, 15.8 pfd/ft, Xc = 700,000 ohms, Blk to Wh, EGC float

Fairly good correlation for two grossly different methods at different frequencies, and different voltage levels.

The capacitance values are higher with 120 V 60 Hz. May or may not be from voltage and frequency differences.

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