In article , Floyd L. Davidson wrote:
bud-- wrote:
Floyd L. Davidson wrote:
(Don Klipstein) wrote:

As for capacitive - the capacitance between adjacent conductors in a
cable is in the 10's of pF per foot ballpark.
True for communications cables, but much lower for
power cable.

Measured capacitance for 14/2 Romex - 15 pF/ft hot-to-neutral, ground
open. I would call that 10's of pF per foot *ballpark*.

I wouldn't. That's 1.5 * 10, not even 2x.

You do realize how much reactance 15 pF/ft is at 60 Hz, right?
How many feet of cable does it take to even become slight, much
less significant? A 100 foot run, at 15 pF/ft simply isn't
much.

120V 60 Hz across 1500 pF will result in 69 microamps of current. Full
scale reading of many common analog meters is 50 microamps.

Regardless, that is nothing like the capacitance
between adjacent _cables_, which is what we would be
interested in.

May be what you are interested in, but I am interested in "wires in the
same cable or pipe". For example a 2 wire run to a switch with "only hot
feed and switched return" or 3 wire run with hot, neutral, swiitched.

Even so, it is less than typical induction.

Guessing velocity factor of 2/3 at low frequencies, that 15 pF/foot for
14/2 romex works out a characteristic impedance of 102 ohms and an
inductance of .155 microhenry per foot (current going out along the hot
and back along the neutral). Put 15 amps through a 100 foot run of this
and the EMF around that loop is about .09 volt. Any secondary 1-turn loop
not perfectly coinciding with the primary loop in this example will have
less than .09 volt induced in it.

You can get more inductance ang greater induced voltage with more
current or if the current going one way and its return take paths farther
apart than the hot and neutral in a 14/2 romex cable are.

Multiply that by 2 times pi
times frequency times voltage and that is how much current can flow
capacitively from one conductor to another - a few microamps per foot!
Enough current flows through the plug of a switched-off floor lamp to make
a neon test lamp visibly glow. An unconnected wire adjacent to a hot one
in a cable easily picks up enough capacitively to generate a fairly high
voltage reading on high impedance meters, and even an impressive reading
on many analog meters.
But that does not deal with the cited case, which would
be capacitive coupling between different cables, not
between the conductors in a single cable.

It deals with my cited case exactly. You eliminate capacitive coupling
by eliminating the places where it occurs.

I'm just pointing out that capacitive coupling is very
slight, compared to inductive coupling, which is
virtually always relatively significant.

Try connecting the black wire of a 20 foot run of 15 pF/foot romex to
120V hot, have ground open, and have an analog meter on an AC voltage
range go from neutral (neutral is otherwise open) to something grounded.
Let's say the meter has a sensitivity rating of 10K-ohms per volt on AC
voltage ranges and you are using a 250V range, for a meter resistance of
2.5 megohms. That 300 pF will allow enough current through to get a 33
volt reading.

- Don Klipstein )

In article , Floyd L. Davidson wrote:
bud-- wrote:
Floyd L. Davidson wrote:
bud-- wrote:
Floyd L. Davidson wrote:
(Don Klipstein) wrote:
As for capacitive - the capacitance between adjacent conductors in a
cable is in the 10's of pF per foot ballpark.
True for communications cables, but much lower for
power
cable.
Measured capacitance for 14/2 Romex - 15 pF/ft hot-to-neutral, ground
open. I would call that 10's of pF per foot *ballpark*.
I wouldn't. That's 1.5 * 10, not even 2x.
You do realize how much reactance 15 pF/ft is at 60
Hz, right?
How many feet of cable does it take to even become slight, much
less significant? A 100 foot run, at 15 pF/ft simply isn't
much.


Gee - the reactance could actually be calculated.

And a 100 ft cable at 15 pF/ft gives a reactance of 1.77meg ohms.
Assume you have one cable wire connected to hot and you are measuring
from the other wire to neutral -- classical measurement of "phantom
voltage" on a switch run. A good digital meter would have an input
impedance of at least 10M. The voltage measured would be over 100V.

With 20 ft - common to have that much cable - the reactance would be
8.8M and the voltage would be over 55V [over 90V if you add the
resistance and reactance correctly].

A reality check is in order. Since those numbers are *not* what
people actually read, something is wrong with your calculation.

When people get voltage readings from capacitive coupling, it is usually
from something with less coupling than hot to neutral of same cable with
the grounding conductor open or absent and length 10's of feet.

- Don Klipstein )

In article , Floyd L. Davidson wrote:
(Chris Lewis) wrote:
According to Floyd L. Davidson :
bud-- wrote:

With 20 ft - common to have that much cable - the reactance would be
8.8M and the voltage would be over 55V [over 90V if you add the
resistance and reactance correctly].

A reality check is in order. Since those numbers are *not* what
people actually read, something is wrong with your calculation.

Some people _do_ see voltages as high as that.

And that's why they report seeing millivolts?

I have heard people report volts and even occaisionally quite a few
volts and 10's of volts before - for example, voltage from a switched hot
to ground with the switch off, when the switch is at the end of cable
having a switched hot and an unswitched hot.

In rooms that are especially bad for "electrostatic hum", I have seen a
long wire, over a foot from any wall/floor/ceiling and not connected to
anything but an oscilloscope input pick up about a volt. More common is
several millivolts (usually 10's) to a tenth or two of a volt. The
waveform was a distorted 60 Hz sinewave.

Keep in mind that most AC analog meters will not give a reading at all
on just a few millivolts. It takes at least one or two tenths of a volt
to get even a fraction of a microamp through most silicon diodes.

- Don Klipstein )

In article , alt.home.repair,
says...
According to Twayne :

An analog meter cannot have a high input Z? They can easily be high
enough to read phantom voltages, as they're usually called.

They can, but as analog meters are usually 50K ohms (or less) per
volt, and digitals are often in the 10s of megaohms, the effect is
usually a lot more drastic with the latter.

Ever hear of a "Vacuum Tube Volt Meter"? ;-)

--
Keith

krw wrote:
In article , alt.home.repair,
says...

According to Twayne :


An analog meter cannot have a high input Z? They can easily be high
enough to read phantom voltages, as they're usually called.

They can, but as analog meters are usually 50K ohms (or less) per
volt, and digitals are often in the 10s of megaohms, the effect is
usually a lot more drastic with the latter.


Ever hear of a "Vacuum Tube Volt Meter"? ;-)

Hmmm,
If you know VTVM, you must be an old fart, LOL!
I still have one and it works 100%.

On Sun, 13 Jan 2008 10:18:16 -0500, krw wrote:
In article , alt.home.repair,
says...
According to Twayne :

An analog meter cannot have a high input Z? They can easily be high
enough to read phantom voltages, as they're usually called.

They can, but as analog meters are usually 50K ohms (or less) per
volt, and digitals are often in the 10s of megaohms, the effect is
usually a lot more drastic with the latter.

Ever hear of a "Vacuum Tube Volt Meter"? ;-)

I once tried a neon test meter and it was even worse. It *constantly* read
voltage when it was just stray fields through insulation. I ended up setting
up a test light using the light fixture I was in the process of installing.
A 60W test light.

A better solution would be to slap a 5K resistor across the probes of the meter.
Do the math to make sure that it won't smoke at line voltages. (W=E^2/R)