Introduction of Vacuum Interrupters

Historically, the idea of vacuum interrupters dates back to 1920s when Sorensen and Mendenhall at the California Institute of Technology started research on the concept of short circuit current interruption in a vacuum medium. They conducted an experiment by placing two fixed contacts and a C-shaped moving contact in a glass envelope and used a magnetic actuator to drive the moving contact, tested at ~15kV. But it was in the 1950s that its commercial manufacturing started when General Electricals USA developed the Spiral Petal Arc Control Contact Vacuum Interrupter. Since then a lot of research has been done in the field of vacuum interrupters to produce low cost low maintenance vacuum circuit breakers, switches and reclosers to be employed at medium and high voltages. In the following sections, we will study the basics of vacuum arc formation and its interruption, design, and applications of vacuum interrupter technology and its limitations.

The Phenomenon of Vacuum Arcing and its Interruption

Arc Formation

As the contacts separate a sequence of events takes place that eventually builds up the arcbetween the gaps. We can explain this as follows. As the contacts separate, the actual area of contact between the electrodes decreases so that the contact resistance rises which increases the heating of the contact spot. The spot melts and an unstable molten metal bridge gets formed between the contacts and eventually the bridge ruptures. At this moment an arc is formed with the metal vapor and is maintained between the contacts. The arc modes are of two types:

Low Current Diffuse Arc

For currents up to 5kA, the vacuum arc is characterized by a diffuse collection of currents.

High Current Columnar Arc

For currents above 5kA, a single high pressure arc column is formed similar to arcing in air medium.

Arc Interruption

At the natural current zero the metal vapors condense rapidly and recombine on contacts as well as shield surfaces and start to regain vacuum condition. Full recovery can be obtained within microseconds of current zero. The circuit is successfully interrupted if the dielectric strength exceeds the circuit applied voltage.