On 7/16/2020 10:21 AM, Ralph Mowery wrote:
In article ,
says...

... at least not fiscally prudent anyway.
Since they do bury high voltage lines some places it can obviously be
done. The added bulk and weight is not necessary on overhead lines tho
and most medium to high voltage conductors are not insulated.
Without insulation, they can use smaller wire. (conductors in free
air).
This is the label off a spool of medium voltage cable that they used
for a buried 13.2kv run underground and then across the bay here.
http://gfretwell.com/electrical/DIRE...le%20label.jpg




Yes, there is insulation that will stand the veryigh voltages. I worked
with a large company that converted the main power to 13,200 volts and
fed it around the plant to about 20 of what we called substations . This
was usually ran in open wire trays. The wire is insulated,but just
layed in the trays. Some inside and some outside. There it was
converted to mostly 480 volt 3 phase and often 600 amps.

Sounds like an interesting place (but some industries are not-so-fun).


Most cars have around 10 to 20 thousand volts on the spark plugs and the
wire is insulated.

One of my ham radio ampifiers has a seperate power supply that sends
around 2600 volts DC down the wire. The wire is less than 18 guage, but
the insulation is about 1/2 inch in diameter with the wire in the
center. It is special insulation compaired to the other wires.

It is the common things around that are not very effective much above
1000 volts. So if any of the power wires that are not on the 240 volt
side of the common pole transformer are laying around, do not try to
move them unless you know what you are doing.


In the NEC thing changed to high voltage at 600 V. In the 2017 NEC in
many places that was raised to 1000 V.

Another fairly common high voltage wire is for neon signs. The largest
transformer I have seen is 15 kV, but has a grounded center tap. Wire is
5, 10, 15 kV.

If a neon sign wire is in a grounded pool of mercury the voltage
gradient from the conductor to the surface is uniform and the insulation
is happy. If the wire goes through a thin sheet metal hole and lays on
the sheet metal the voltage gradient is concentrated at the sharp edge.
Even though the voltage is within the insulation rating, the gradient
(volts per mill) may exceed the rating and deteriorate the insulation
from the surface in, causing failure. (The wire should go through an
insulated bushing.)

The U of MN had a small system in a spherical glass flask about 5"
diameter. The inside of the glass had a transparent conductive coating
with a phosphor coating on top of that. A tungsten wire was etched to an
extremely sharp point maybe 4 atoms diameter. The point was placed at
the center of the sphere, the system was evacuated to a good vacuum, and
at least 6 kV was applied between the wire and conductive coating with
the wire negative. The electric field was concentrated at the point, and
because it was so sharp the field at the point was strong enough to
strip electrons out of the tungsten atoms. The electrons accelerated to
the conductive coating and lit up the phosphor with the pattern of the
atoms.

MV (distribution voltage) cables can have the same problem with uneven
fields. It is common to have a semiconductive coating at the outside the
insulation with grounded wires on the outside the insulation (shielded
MV cable). There then is a problem terminating the stuff. In the good
old days a pothead was used (MV electricians had to work with potheads).
Now there are kits to install that give uniform voltage stress
reduction at the ends.

A few times I have driven past where wires from a large transmission
tower converted to an underground run. It is impressive.