Understanding the mechanism of plasma build-up in vacuum arcs is essential in many fields of physics. A onedimensional particle-in-cell computer simulation model is presented, which models the plasma developing from a field emitter tip under electrical breakdown conditions, taking into account the relevant physical phenomena. As a starting point, only an external electric field and an initial enhancement factor of the tip are assumed. General requirements for plasma formation have been identified and formulated in terms of the initial local field and a critical neutral density. The dependence of plasma build-up on tip melting current, the evaporation rate of neutrals and external circuit time constant has been investigated for copper and simulations imply that arcing involves melting currents around 0.5 − 1 A/μm 2 , evaporation of neutrals to electron field emission ratios in the regime 0.01 − 0.05, plasma build-up timescales in the order of ∼ 1 − 10 ns and two different regimes depending on initial conditions, one producing an arc plasma, the other one not. Also the influence of the initial field enhancement factor and the external electric field required for ignition has been explored, and results are consistent with the experimentally measured local field value of ∼ 10 GV/m for copper. c