Insulating properties of air

[Red Wiggler]

Is there a "rule of thumb" concerning the insulating properties of air? In other words is there something to go by such as 1000 volts needs 1" of clearance to ground? 480 volts = 1/2" to ground, etc?

 

[George Stolz]

Can you be more specific about the particular installation being considered?

 

[kwired]

I would think that the conductors movement ability and likely hood of foreign object contact would need to be considered in any clearances.

 

[kingpb]

If I understand correctly as to what you are looking for, you need to review the NESC (National Electric Safety Code) otherwise known as IEEE C2.

 

[drbond24]

Is there a "rule of thumb" concerning the insulating properties of air? In other words is there something to go by such as 1000 volts needs 1" of clearance to ground? 480 volts = 1/2" to ground, etc?

I seem to remember that it takes 10,000 volts per inch to arc through normal air. That probably doesn't help, but if I had 1,000 volts I'd want a lot more than an inch of clearance. Things like humidity will change the properties of the air and decrease the arc threshold. I don't know of any official distances to follow, but the NESC would be a good place to look like king said.

 

[gar]

100521-2138 EST

See:
http://en.wikipedia.org/wiki/High_voltage
http://www.physicsforums.com/showthread.php?t=140393
http://www.pupman.com/listarchives/2003/November/msg00428.html

Then maybe do some of your own experiments.

.

 

[Red Wiggler]

Insulating properties of air

The reason for the question is just for my personal benefit. I was thinking of a situation where a conductor (Medium Voltage) improperly routed with in a section of gear in such a way that a section of the conductor is in contact with a of bus bar of a different phase. (Which I know is not an acceptable practice).

I was hoping that I could find information (table) that says a conductor of whatever voltage should maintain X" from busing (sections of bus bar).

1000 volts = 2"
5000 volts = 5"
xxxx volts = ?"

 

[ohmhead]

Dielectric breakdown

Dielectric breakdown

Well read page 4-133 art 250. in the Standard Handbook for Electrical Engineers . There is a chapter on Paschen,s air density effects on dielectric strength . Read this very good and a chart and graphs !

But hands on 1 inch can handle a lot more the 1000 volts but its all about what voltage at what pressure and whats between the charge points .

This is just personal information below do not use this for you work i play with high voltage as a hobbie !

Take 15000volts AC 1/4 inch gap it will cross at 1/2 of a inch it will not cross at 60 ma if you increase the freq it will be less and start to pass the gap at less distance between the points .

When we test high voltage cables we rap the terminal ends with sandwich rap its very thin in mills but 70 000 volts ACwill not pass thur 4 raps of this thin layer .

Now in free air at 30000 volts AC it will bleed of into the air you can hear it hissing you can see it spark the gap at 50000volts ac but it depends on many different conditions .


Actually a air blast is used to stop a flow or spark in high voltage heat makes a good path for flow but a cool blast of air will put it out

Take care

 

[iwire]

Is there a "rule of thumb" concerning the insulating properties of air? In other words is there something to go by such as 1000 volts needs 1" of clearance to ground? 480 volts = 1/2" to ground, etc?

Check out NEC Table 408.56 for some info.

 

[mcclary's electrical]

Well read page 4-133 art 250. in the Standard Handbook for Electrical Engineers . There is a chapter on Paschen,s air density effects on dielectric strength . Read this very good and a chart and graphs !

But hands on 1 inch can handle a lot more the 1000 volts but its all about what voltage at what pressure and whats between the charge points .

This is just personal information below do not use this for you work i play with high voltage as a hobbie !

Take 15000volts AC 1/4 inch gap it will cross at 1/2 of a inch it will not cross at 60 ma if you increase the freq it will be less and start to pass the gap at less distance between the points .

When we test high voltage cables we rap the terminal ends with sandwich rap its very thin in mills but 70 000 volts ACwill not pass thur 4 raps of this thin layer .Now in free air at 30000 volts AC it will bleed of into the air you can hear it hissing you can see it spark the gap at 50000volts ac but it depends on many different conditions .


Actually a air blast is used to stop a flow or spark in high voltage heat makes a good path for flow but a cool blast of air will put it out

Take care

I was once told that 10,000 vac would not pass through two wraps of 33. Okay Myth busters,,,is that for real?

 

[ohmhead]

I was once told that 10,000 vac would not pass through two wraps of 33. Okay Myth busters,,,is that for real?

We use HVBT tape on medium voltage buss bars when we splice switch gear together one full rap at 1/2 lap does it . Its rated for 5 kv to 15 kv one rap and they have 35 kv tape that does the same .

We had a 3M Rep come out to our shop this year and he shows us a new 3M tape out that will seal itself to a insulated conductor meaning it not only raps it but it chemically bonds into the insulation on the conductor over time .

Dont ever use semi con tape on a extension cord !

 

[LarryFine]

We use HVBT tape on medium voltage buss bars when we splice switch gear together one full rap at 1/2 lap does it . Its rated for 5 kv to 15 kv one rap and they have 35 kv tape that does the same .

You're a real rap artist. :grin: