arcing distance for 13.2 kv conductor to ground
arcing distance for 13.2 kv conductor to ground
(OP)
Let's say I have the geometry as shown attached. It is a 2AWG 13.8 kv conductor 4" away from outside corner of a grounded metal structure. Assume the air is dry, the conductors uninsulated, no surges on the system. Would you expect that an arc of this distance (4") along the path shown is possible or expected ? What would be the basis for a conclusion (facts, analysis, standards)?
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RE: arcing distance for 13.2 kv conductor to ground
For 13.8 kV, I think it would be more like 1 or 2 inches rather than 4 inches. So at 4 inches, I wouldn't expect it to breakdown or flashover.
RE: arcing distance for 13.2 kv conductor to ground
I don't know of any code that would allow this, so if it exists, you would need to immediately modify the construction to meet the applicable codes.
Alan
Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin
RE: arcing distance for 13.2 kv conductor to ground
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: arcing distance for 13.2 kv conductor to ground
http://en.wikipedia.org/wiki/Peek's_law
ev = mv * gv * delta * r * ln(S/r)
where
ev = voltage to begin corona discharge
mv is conductor roughness factor 0.8 to 1. Say 1 for simplicity.
gv = 30 kv/inch = breakdown strength for air
delta = 1 for air at STP
S = spacing
r = condutor radius
Letting delta = 1, ev=1, dividing by r we have
ev/r = gv * ln(S/r)
ln(S/r) will be higher than 1. So the threshhold voltage/distance ev/r will be higher than gv = breakdown strength of air. That doesn't make sense... the stress concentration factors associated with non-uniform field should make ev/r < gv, should they not? I guess I must be missing something.
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RE: arcing distance for 13.2 kv conductor to ground
racobb - the question arises during analysis of an actual failure. More details will surely bring the thread in a different direction than the simple initial post question that I am focusing on... I will save full discussion of the failure for another thread.
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RE: arcing distance for 13.2 kv conductor to ground
ev/r = gv * ln(S/r)
Let gv = 30 kv/cm = 3e4V/cm = 3e6 V/m
S = 4". R = 0.15" for 2AWG
ev/r = 3e6 V/m * ln(4"/0.15") = 1E7 V /m = 10000 kv/m (again doesn't make sense since it is higher than gv)
If I had done ev/r = 3e6 V/m / (ln(4"/0.0.15") = 900 kV/m
That is closer to your answer, but I had to divide instead of multiply to get there. How did you come up with 600 kv/m?
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RE: arcing distance for 13.2 kv conductor to ground
Can't believe it is overhead conductor.
Alan
Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin
RE: arcing distance for 13.2 kv conductor to ground
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: arcing distance for 13.2 kv conductor to ground
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: arcing distance for 13.2 kv conductor to ground
Without more info, you are wasting alot of peoples time here.
Alan
Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin
RE: arcing distance for 13.2 kv conductor to ground
I think I did figure out how to apply the wikipedia formula again to find voltage:
ev = r* gv * ln(S/r)
Let gv = 75 kv/inch
S = 4". R = 0.15" for 2AWG
ev = 0.15* 75 kv/inch * ln(4"/0.15") = 37kv.
I see they call this critical value of line to neutral voltage.... are they meaning
So would seem if my geometry were two AWG 2 conductors separated by 4", then there is a margin factor of 37 kv / [13.2 kv/sqrt3] ~ 4.8 between actual voltage and voltage required for corona. Or if I put in the factor mv=0.8 then a margin of 3.8. Assuming dry air, no surges etc.
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RE: arcing distance for 13.2 kv conductor to ground
racobb - I promise, I will present the full failure scenario in another thread. As I said, it will surely lead in much different directions, but I just wanted to explore this question first for my own education to try to understand it as best I can.
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RE: arcing distance for 13.2 kv conductor to ground
http://home.earthlink.net/~jimlux/hv/paschen.htm
Perhaps the same thing as Peek's law?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: arcing distance for 13.2 kv conductor to ground
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RE: arcing distance for 13.2 kv conductor to ground
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RE: arcing distance for 13.2 kv conductor to ground
I looked at this formula for Corona discharge and allowing the air density factor and surface condition factor of the conductor to be unity I obtained an RMS value 25.98kv for the onset of Corona discharge to occur in your set up.
http://w
It goes on to say how the air density varies with location at sea level etc which may bring this figure down.
Under the conditions you describe I would not expect a flashover to occur unless some small insects or rodents have got in, if they have I don't suppose there is much evidence of that left.
regards
desertfox