Circuit Breaker Altitude Relation

Here's a sort of hand-waving explanation:

As the air pressure reduces, the distance an ion can travel before bashing into something increases (the mean free path increases), meaning that the distance an arc can jump increases. Keep reducing the pressure, tho', and you come to a point where there aren't enough molecules in the air to ionize & form the arc.

it means that as we get higher the arc possibility gets lower???

NO - arc formation is more possible - until you get so high up that the air pressure (& hence density) is so low that you run out of anything to form an arc with^1,2. That's why HV power lines have wider separation at higher altitudes (and why contactors are derated at high altitudes).

¹ But see http://en.wikipedia.org/wiki/Vacuum_arc

² Way up here ...

could u explain what "you run out of anything to form arc" phrase means here ???

Arcs in gasses are generally formed by ionizing the gas molecules - if there aren't enough gas molecules there (because the pressure is so low and the air is so 'thin') - you can't form an arc.

Note: arcs can be formed in vacuum (did you read the link I gave you?) - but they are made of ions which have boiled off the electrodes.

pls tell me about insulation levels, what about vacuum insulation feature compared to other insulator such as glass mica plastic etc? vacuum has a 1 relative dielectric constant and others have 8 10 times higher dielectric constant but the least dielectric of them is vacuum and even worse than air????? how can air be better insulator if vacuum has no any material to conduct the arc and air has many one ??? these concepts will kill me:(

The break down voltage of a gas at different pressures is a non-linear relationship. (The Wiki link, while accurate, is difficult to follow.) I've always liked this explanation and use it often at work.

At atmospheric pressure, the electron cloud of each gas molecule regularly bump into each other (mean free path as you point out JohnDG). So as electrons get normal knocked off of their molecule for any reason, the repulsion of neighboring molecule's (but not adjacent molecules, this is a gas not a liquid or solid) electron cloud repel the electron back towards the positively charged ion the electron came from. With a voltage gradient externally applied there is a direction that the free electron can be drawn to before it runs into a repelling cloud. So one can either have a high enough voltage to attract any and all electrons knocked free or one can have less uncharged (blocking) molecules in the way. However, this trend has a limit. For if there are no gas molecules to be ionized (the mean free path exceeds the gap of the contacts) no conduction can occur.

Now at the region of a nominal one atmosphere of pressure, as the pressure drops the breakdown voltage drops also.

it makes no sense if an air density grows smaller and it means more possibility of arc, why ??? if there is fewer molecules in the air it means no medium for arcing isnt it?

High voltages + low atmosphere = corona

what I wonder is if air is better insulator than vacuum so why dont they use air circuit breakers instead of vacuum circuit breakers as a higher technology?

Air is not a better insulator than a vacuum. But it is cheaper to manufacture an ACB than a VCB. VCBs are used because as the voltage gets higher, the AMOUNT of air spacing must increase to the point of becoming impractical. VCBs can do the same job in a much smaller space.

To summarize the above postings that you seem to have trouble grasping, air insulation value (dielectric strength) of a contact gap decreases with altitude only as long as there is still air at all. Once you remove the air altogether, i.e. a vacuum, then the dielectric of a contact gap goes back up dramatically. So if you have a set of "air gap" contacts at 5km above sea level, the rarefied air in that gap has less insulating capability than at sea level, but at 1000km, essentially "outer space", the insulation level would be much greater than at sea level.

what ı cannot grasp in fact is not clear to all of you still:) i want to learn the relation between dielectric strength and dielectricity, namely insulation level and having a higher relative dielectric constant ok??? vacuum has 1 and air has 1,007 so which one is better insulator do ı ask??u can understand the paradox (not so but seems so to a man who doesnt know the real fact in this case) here? it says everywhere vacuum is the best insulator but it has the lowest dielectric constant, so dielectricity ann insulating capability is not proportional true???

Let me try to explain by starting with some definitions. All dielectrics must be an insulator. The dielectric strength (E_ds) of any insulator is the breakdown voltage per thickness, for quartz this is 30MV/m. The Relative Permittivity (ε_r) (A.K.A. dielectric constant) is a dimensionless constant that must be multiplied by free space permittivity (vacuum) (ε₀=8.854*10^-12 F/m) to obtain the permittivity. It's the permittivity of the insulator along with the geometry between two conductors that determine th capacitance between two conductors. Now the relative permittivity of air at sea level (dry) is about 1.0006, just above unity, not one thousand and seven.

Now what actually determines these values for a substance is the chemistry of the material. For the dielectric strength, the valence electrons must be well bound to the molecule. The permittivity though involves the concept of a dipole moment. If a molecule has a more positive and a more negative side then this is a dipole. Water, pictured here from Wikipedia, has a more negative side (red) and a more positive side (blue). An important thing to remember, dipoles are electrically neutral. In an electric field this will polarise and align in a certain direction. Now how easily alignment happens and how much more positive that end is determines how much of a relative permittivity this insulator has.

So in essence, these two concepts can only happen with an insulator but they refer to different aspects of an insulator.

I hope that helped.

pls just tell me the best insulator in the nature and the dielectric constant of perfect vacuum it had had a chance to exist. also how can u make a generalisation of dielectric losses ? for example air has zero loss but why? what about ceramic for example? it has a high loss??

so a perfect vacuum will have the highest dielectric constant?? the vacuum word in the dielectric tables means under 1 athmospere air or somewhat less dense air but not perfect? can u tell me the theoretically true dielectric constant value of perfect vacuum?

The theoretically true dielectric constant for a vacuum is 1.00000... by definition. The dielectric constant for Polystyrene 2.6, Bakelite 5, Mica 5.4-6.0, Quartz 3.8-5 and glass has a large range listed of 4.5-10.

pls tell me if there is a relation between dielectric constant and insulation capability of the materials?? if perfect vacuum has a 1 constant (namely lowest of all) how can we say perfect vacuum is the best insulator?

I cannot tell you what is the best insulator for I don't know the application. Not all insulators have a dielectric constant different from 1. (None are less than 1.) The dielectric constant determines the capacitance value created not the resistance. So if you are trying to fabricate a capacitor you may want a high dielectric constant so that your capacitor doesn't have to be all that physically large. This like all engineering choices will create trade offs. Possibly your tiny capacitor won't tolerate large voltages, or be thermally stable.