Initiation and the math behind Arc Discharges

[akansh_karthik_1]

TL;DR Summary

How do arc Discharges form and how do I calculate the required voltage and current

Arc discharges are characterized as being high current discharges. I have done some research and there are some conflicting answers on how ar discharges start.

Thermionic / Field Emmision Explanation:

Arc discharges can start in two different ways thermionic emmission, where intense heat causes thermionic emmision of charge to initiate the arc, and field emmission, where a strong electric field causes the initiation of the arc.

I also found some places that say arc discharges are caused by acheiving breakdown voltage and then reducing voltage and increasing current(since after breakdown is acheived the air is conductive) to create the high current arc.

Which one of these explanations is correct? I also would like to know if there are any laws that can calculate "Arc inception voltage" or "Arc inception current" (If that's even a thing).

 

[chimay]

I think this is what you are looking for: https://en.wikipedia.org/wiki/Paschen's_law

 

[akansh_karthik_1]

I think this is what you are looking for: https://en.wikipedia.org/wiki/Paschen's_law

If electrical breakdown uses high voltages to breakdown air how can an arc discharge be a high current discharge?

 

[Baluncore]

If electrical breakdown uses high voltages to breakdown air how can an arc discharge be a high current discharge?

Because the voltage needed to sustain the current through ionised air, is significantly lower than the initial breakdown voltage required to initiate conduction.

 

[DaveE]

If electrical breakdown uses high voltages to breakdown air how can an arc discharge be a high current discharge?

Basically it's the difference between making ions when they don't exist yet (high impedance, high voltage) and conducting current through a bunch of ions that you've previously made (lower impedance, higher current).

 

[Rougesang]

While high voltage is required to initiate electrical breakdown and arc discharge, the resulting discharge can involve high current flow due to the conductivity of the ionized path and the formation of plasma.

 

[akansh_karthik_1]

How do you calculate how much you can increase current before the air stops being ionized? Or is there no limit?

 

[Baluncore]

How do you calculate how much you can increase current before the air stops being ionized? Or is there no limit?

There is no limit to the damage that can be done by an uncontrolled current. The air is heated very rapidly, so expands and causes an explosion. The ionisation will continue so long as current is available. The UV radiation from the arc will damage organic insulation, resulting in a carbon arc and fire.

 

[DaveE]

How do you calculate how much you can increase current before the air stops being ionized? Or is there no limit?

I don't know how to calculate it, but I can pretty much guarantee it's on the web. Maybe some research and reading would help you?

Yes there is a limit to how many ions you can make in a confined gas or region. In the curve below, this is the region between points J and K, which has locally resistive behavior because there aren't more ions available. The negative resistance regions (E-G & I-J) are where ions are created. More ions => lower resistance. The I-J region is an avalanche effect and typically happens very quickly (hence the dashed line in the graph).