For every 1000Volts an arc jumps 1 inch??

[WB82]

I was once told for every 1k Volts (AC I'm guessing) an arc is capable of jumping 1 inch....Does this sound accurate, if not, does anyone have any knowledge on this??? If so, the power we generate is 11kV, so your saying it could jump 11 inches?? Thanks

 

[mivey]

Approxinately 3 million volts/meter to ionize the air to strike the arc (76,200 V/in). Another approximation is 75kV/in.

Either way, a long way from the 1 kV/in you had. I can find this in a text book somewhere if you want a real number.

 

[mivey]

3MV/m is indeed the value listed in the EE Handbook as the breakdown voltage for air.

But there are many factors such as the shape of the conductor at the arc, the air density, temperature, moisture, etc but I looked up a 6.25 cm sphere-gap at 25?C and 760mm and here are some of the values for different gaps (positive/negative impulse kV crest values given):

0.5 cm: 17.0/17.0
1.0 cm: 31.3/31.3
1.5 cm: 44.5/44.8
2.0 cm: 57.0/57.4
2.5 cm: 68.8/69.3
3.0 cm: 78.8/79.4
3.5 cm: 86.6/88.0
4.0 cm: 93.6/....
4.5 cm: 99.8/....
5.0 cm: 105.5/....

From A.I.E.E Standards June 1940 as tabled in "Gaseous Conductors", J.D. Cobine, 1958. These are close to the values given in IEEE Std 4-1995 "IEEE Standard Techniques for High-Voltage Testing".

 

[broadgage]

I have heard "1KV per inch" mentioned, but suspect that this is not the actuall breakdown voltage under normal conditions, but the minimum clearance required under worst case conditions, and allowing a safety margin
Insulating bushings on outdoor 11KV transformers look to provide about 11 inches of clearance betweeen live parts and earthed metal.
Heavy fog or driving rain or clouds of insects greatly reduce the insulation strength of air and this must be allowed for in outdoor conditions.
Clearances on overhead lines are much greater in order to allow for conductor sway in the wind.
Smaller clearances would be allowable in indoor equipment.
The contact gap in a 250 or 277 volt relay, switch or contactor is sometimes less than a qaurter of an inch.

 

[mivey]

The 1kV/inch is the insulation creepage length in a high pollution environment. The values for low, medium, and high pollution levels are 16mm/kV, 20mm/kV, and 25mm/kV. It is >31mm/kV for very high pollution levels. Insulators get conductive junk on the surface when pollution is present.

But that is not the same as jumping an air gap which is generally 3MV/m. You have to determine is you are talking about clearance (distance through air or insulating medium) or creepage (distance along a common surface).

At any rate, there will be a safety standard to go by to maintain safe distances: NESC, NEC, etc. In HV circuit design, the general guideline is about 7.5-10.0 kVdc/inch, or about 2.5-3.5 kVac/inch.

 

[Volta]

Approxinately 3 million volts/meter to ionize the air to strike the arc (76,200 V/in). Another approximation is 75kV/in.

Either way, a long way from the 1 kV/in you had. I can find this in a text book somewhere if you want a real number.

Ok, this seems to be linear.

So what allows lightning to bridge the gap? Say a mile, do we need 4 billion volts, or does (most) lightning just occur in the most humid air at a lower voltage, lower altitude?

 

[a.bisnath]

it is an avarage value

it is an avarage value

I was taught this rule by my seniors in field work,it was meant to be a working value and to be used in dry conditions,it is an old rule,to be used practically

 

[mayanees]

electrical tape

electrical tape

I recall a 1kv/wrap insulatory value for electrical tape. Maybe that's what the OP was thinking???
John M

 

[K8MHZ]

Ok, this seems to be linear.

So what allows lightning to bridge the gap? Say a mile, do we need 4 billion volts, or does (most) lightning just occur in the most humid air at a lower voltage, lower altitude?

Before a bolt of lightning occurs, 'feelers' or 'stepped leaders' form and ionize the air, making it much more conductive.

Check out this video and you can see it happen in super slow motion:


http://www.wired.com/thisdayintech/2010/02/gallery-lightning/

 

[Volta]

Before a bolt of lightning occurs, 'feelers' or 'stepped leaders' form and ionize the air, making it much more conductive.

Check out this video and you can see it happen in super slow motion:


http://www.wired.com/thisdayintech/2010/02/gallery-lightning/

Good stuff. Turns out that I'm not qualified to work with lightning.