# High Voltage Powerline 750 KV arc

Discussion in 'Electronic Basics' started by Jack Jet Williams, Jan 5, 2004.

1. ### Jack Jet WilliamsGuest

Does anyone know how far 750 kv ac could arc once it got started ?

Just wanted to know.
Thanks.
Jack Jet Williams
Hesperia,Ca.

2. ### DarkMatterGuest

14 feet. That is the minimum safe distance for workers on such
lines.

Air ionizes along a path, so a set length is impossible to state. A
path could propagate several feet further than "thought".

It also differs for DC and AC with AC less likely to arc.

3. ### Don KlipsteinGuest

Actually, for a given nominal voltage, AC sparks through a greater
distance than DC does since AC has a peak voltage about 41% higher than
nominal.
But a DC arc is more "stretchable" than AC one is once it gets started.

Spark gap distance, at 760 mm pressure of dry air at 25 degrees C, is
13.3 cm between 25 cm spheres and 54.7 cm between needle points at 300KV
peak (300 KV DC or aprox. 212 KV AC).
Between electrodes that can form corona due to concentration of electric
field towards the electrodes, breakdown voltage is approx. 1100 volts per
meter plus something like 320 volts at lower voltages, but less per unit
distance when the peak voltage increases past 30-40 KV or so. Between
large spheres or wherever else the electric field is even throughout the
spark gap, the breakdown voltage is approx. 3 KV per millimeter plus
something like 320 volts.
Where the air density is different from that of 25 degrees C 760 mm,
breakdown voltage varies directly but maybe not quite proportionately.

(As far as I know, mostly from a spark gap table in the "CRC Handbook".)

Humidity has less effect than many would expect, and I am guessing its
main effect is to cause surface conductivity on insulating materials near
electrodes which can affect electric field distribution in the spark gap
in some cases.

- Don Klipstein ()

4. ### DarkMatterGuest

NOpe. AC crosses zero too many times to propagate an ion path.

DC starts one, then finishes it.

Long before the arc, an ion path must be established.

5. ### Eric R SnowGuest

I dunno. But I saw a photo of a high voltage line which had been
struck by lightning and was arcing to the ground. The ground was
exploding. The arc must have been at least 25 feet long. What a
picture.

6. ### Don KellyGuest

--------------------
What you are effectively saying is: "As AC cannot propagate an ion path
there cannot be ionic conduction between electrodes supplied by AC and that
big fat and hot arc on a AC system is a figment of imagination. Flourescent
lights also will not work on AC as they depend on establishment of an
ionized path."

Do you want to rethink this?

AC will produce an ion path and a true arc will follow if the supply
impedance is low enough to allow this. That nice big AC arc in the photo
initiating this thread is likely carrying a few KA- this is well past the
path initiation stage. Sure the arc goes out at the end of each half cycle
but if the arc products don't dissipate sufficiently near current 0, there
will still be an ionised path and the arc will restrike.
Even a relatively high impedance neon transformer will run a Jacob's ladder
and that flame going up the ladder is past the ion path initiation (Townsend
discharge) stage and well into the arcing regime (negative impedance ,
plasma, or however you want to describe it).
Do we need DC for a fluorescent lamp? It's voltage/current characteristic
puts it into the arc regime where external impedance is needed to limit the
current.

Don Kelly

7. ### Don KellyGuest

--------------

.. The actual initiation gap depends on a lot of factors such as humidity,
air pressure, shape of the "electrodes" so there is no single answer.
Typical conductor to tower flashover (50% probablity) will be in the order
of 4-5 ft for a 750KV line. At 14 ft- there is a factor of safety to
account for pointy objects, stupidity, weather conditions, insurance
requirements, etc. This is meaningless once an arc has been initiated.

However the original question was "once it started"

Again there is no single answer. The arc could go well over 100 ft if the
supply impedance is low. Effectively the current is limited mainly by the
supply impedance unless the arc is very long. and as long as the supply
dumps energy into the arc at a higher rate than it is being removed by
convection, etc, the arc will lengthen. Since it is AC, the current will go
through 0 every half cycle and, IF the arc products and ionized path
dissipate sufficiently near current 0 , the arc will go out at this time.
Needless to say, the length of the arc can be quite large depending on
current, wind and other factors.
I have seen a picture of a deliberately struck arc ( wire thrown over the
line) on a 240KV line and this arc was an estimated 70 -100 ft long.

8. ### JoGuest

Jack.
Check out these movies on Bert Hickmans site at the URL below...

http://205.243.100.155/frames/longarc.htm

The 500KV one is awesome and will give you an idea of how far an arc can
reach.
Cheers.
Mark H.

9. ### Jack Jet WilliamsGuest

Mark,
I AM IMPRESSED ! That arc was awesome.............thanks !

Jack Jet Williams

10. ### William J. BeatyGuest

Also remember than positive electrodes can launch arcs too, even
though the massive positive ions in air travel much slower than the
freed electrons do.

The "spark stuff" reaches out through the air at a much higher velocity
than the actual charges travel. A growing spark is more like a wave
moving through dominos, or a forest fire spreading across trees.
One group of ions becomes a conductive substance, which causes the
e-field to distort and concentrate, which ionizes the neighboring dry
air, and so on in a wave. Also, one group of ions emits UV light,
which as before can ionize the adjoinging air in a travelling wave.

I suspect that the physics greatly resembles the growth of thin
metal-crystal "trees" from solution, where the tips of the crystal
filaments grow much faster than the sides of the filaments, leading
to long narrow tree-like shapes.

With AC, the main issue is whether the ion path "cools off" during
the zero crossing. If it did, a new spark would have to leap out
every 120TH of a second, and each spark would take a different
path. Also, previous sparks could not add new length to existing
spark filaments, so the sparks would be much shorter. Clearly the
ionized path does not cool off with hot 60Hz AC sparks, since they
appear as a single glowing filament which slowly crawls around, not
as a collection of shorter sparks which flicker around maniacally.

((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty Research Engineer
UW Chem Dept, Bagley Hall RM74
Box 351700, Seattle, WA 98195-1700
ph206-543-6195 http//staff.washington.edu/wbeaty/