Arching horn and Corona Ring

An arcing horn is used on a breaker to "stretch out" the arc and break the current flow on a zero crossing. Arcing horns are typically a straight light gauge rod.

A corona ring is used to reduce the electric field intensity around the energized end of an insulator, breaker hardware, connectors, any place there are electric field intensities that would cause the air to break down in corona. Corona rings are bent tubing to conform to the electric field around a specific hardware.

Arcing horns are also commonly used to protect insulation from impulse and other overvoltages. The horn gap (distance between arcing horns) is set to ensure that flashover occurs across the gap rather than along the insulation surface thereby protecting the insulation surface and preventing arc termination and associated damage to the end terminals or line and ground end hardware. They may also be used to connect a surge arrester to protect transformers and other equipment from overvoltage surges (gapped arrester). A gapped connection is one method of preventing line lockout in the event of arrester failure.

At transmission line voltage the arcing horns, when the breaker is closed normally have nothing except corona from the tips and arc marks, the instant the breaker begins to open an arc is established across the gap between the arc horns, when the gap is long enough the arc breaks. The plan is to keep the sliding contacts from getting arc metal removal so the contacts maintain low resistance, arcing horns are sacrificial.

At switchgear voltage, there are arc chutes and usually puffers to extinguish the arc during breaker opening, the arc chutes may be of a sand-crystal cast material (like space shuttle heat tiles), asbestos layers, and electrical insulating board to protect the works during an explosive event when temperatures get hotter then the sun. There is specific NFPA training for arc flash exposure.

Corona rings are meant to distribute the electrical field and neither the hardware protected or the corona ring should have corona, the typical line voltage that corona rings are applied is 150KV and higher, altitude or high temperatures can reduce the voltage to 138KV lines. Properly designed corona rings do not have corona.

Corona can appear to start and stop at essentially the same voltage, there are other variables. Corona produces light (from UV thru visible and into the infrared), sound (thru all wavelengths), ozone, and nitric acid (in the presence of moisture).

Arcing arrestors were used long ago, some of the old-old transmission lines in New York state still have them. They were opposing arcing fingers mounted in parallel with the insulators, the gap determined the flash-over voltage. The intent was to protect insulators from lightening surges. I don't know if those old lines are energized anymore. You don't see arcing fingers on modern (post WWII war) transmission lines.

To break an arc the voltage must be decreased below about 60% of the voltage an arc starts at, thus if a transmission line insulator arcing arrestor flashes over and maintains an arc the line is going to be shutdown. Thus arcing arrestors (without an arc extinguishing capability) decrease the reliability of a transmission line.

Thanks for the excellent post! That's just the kind of answer I was looking for.

There is another point that I heard,

The corona ring installed in 230 kv line and above

Arching horn installed in les than 230 kv line

What is the reason for that?

There are lines that have non-ceramic insulators (NCI) at 138kV that must have corona rings. Almost all NCI need corona rings.

Arcing horns can be at all voltages, as they cause the arc -when the breaker is opening- to travel further like a jacobs ladder until the arc extinguishes, and it saves the contacts from the arcing which would cause surface roughness and problems with the breaker seating, causing high resistance and leading to the breaker burning up.