# Thread: Insulating properties of air

1. Senior Member
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Nov 2008
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106

## Insulating properties of air

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?

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

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

4. 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.

5. Originally Posted by 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?
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.

6. gar
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7. Senior Member
Join Date
Nov 2008
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## 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 = ?"

8. ## 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
Last edited by ohmhead; 05-22-10 at 06:22 AM.

9. Originally Posted by 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?
Check out NEC Table 408.56 for some info.

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?

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