  Although used in the design and costing of large projects such as linear
colliders and tokamaks, the theory of vacuum arcs and gradient limits is not
well understood. Almost 120 years after the isolation of vacuum arcs, the exact
mechanisms of the arcs and the damage they produce still being debated. We
describe our simple and general model of the vacuum arc that can incorporate
all active mechanisms and aims to explain all relevant data. Our four stage
model, is based on experiments done at 805 MHz with a variety of cavity
geometries, magnetic fields, and experimental techniques as well as data from
Atom Probe Tomography and failure analysis of microelectronics. The model
considers the trigger, plasma formation, plasma evolution and surface damage
phases of the arc. Our data clearly shows surface damage produced by
differential cooling capable of producing local high field enhancements $\beta
\sim 200$, and arcing in subsequent pulses. We update the model and discuss new
features while also pointing out where new data would be useful in extending
the model to a wider range of frequencies.
