Corona vs. Arc 1

[trchambe]

Hello,

I am trying to wrap my mind around what conditions will result in corona discharge, and what will result in an arc.

Is it that, if the potential gradient is high enough, corona discharge will occur? If there happens to be conductive material within the ionized region, a sustained arc will occur?

 

[waross]

An arc is from a conductor to another conductor. Corona is from a conductor into free space.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[trchambe]

Do arcs start as corona, and propogate into arcs? Or are the two unrelated?

 

[waross]

Both are the passage of electrons through an ionized gas.
In a free air arc, the electrical field strength ionizes the air between the conductors. In corona, the gradient of the electrical field ionizes the surrounding air and electrons are blasted off. The field gradient is steeper near a point than near a flat surface. Hence, the practice of using a greater surface radius to reduce or prevent corona.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Compositepro]

Corona is usually the result of an electric field strength that varies with distance, as you have around a wire near a plate. The field strength is much higher around the wire so ionization occurs there. But as the field strength drops with distance from the wire, ionization slows and stops. Ions still move in the electric field but new ones are not created.

In a corona you have ion flow. In an arc you have electron flow through the gas.

The ionization energy of most gasses is around 12 electron-volts (that is the energy imparted to an electron in moving through a field gradient of 12 volts). If the electron hits a molecule before it gains this energy it will lose energy in the collision without ionizing anything. Thus, higher pressure makes molecules closer together so it is more difficult to cause ionization.

Natural radiation or cosmic rays are supposed to create the initial ions and free electrons to allow a corona discharge to start.

 

[trchambe]

So, if I understand these responses and other readings correctly, corona is the result of a single event creating a single ion/free electron. This then accelerates through the electric field, colliding with other atoms, and this continues until a point where the electric field strength is not strong enough to impart the energy required to separate ions and electrons.

Arcing is a breakdown of the air when the voltage between two points exceeds the dielectric strength of air. Ionization of the air occurs, and a condutive path is formed.

So how does ionization occur from the arc? Is it a cascading effect similar to corona discharge? If so, what is the initial event that causes it?

 

[manning225]

There is much confusion on terminology in this area. For me a corona is a type of partial discharge. And a partial discharge is a discharge that does not bridge the electrodes. Whereas an arc does bridge the electrodes.

Our American cousins tend to use Corona and Partial Discharge interchangeably whereas in the UK we would reserve the term Corona for a partial disharge in the air (or gas) surrounding a conductor.

 

[electricpete]

All good responses. Steve summarized it well imo.

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[Compositepro]

The differences between an arc a corona discharge are pretty dramatic. In a corona, charge is transferred by ion movement (similar to current flow through salt water. In an arc you have a plasma, and current flow is due to free-electrons moving from one electrode to the other. An arc would usually be preceded by a corona discharge except in cases where the electrodes scratch together as in arc welding or opening electrical contacts.