While arcing during processing of food in a domestic microwave oven has been well known and has major safety implications, its mechanistic understanding is not available. A comprehensive multiphysics 3D numerical model was developed that couples electromagnetics, electrodynamics, heat transfer and moisture transfer. The model is multiscale with the electromagnetics and heat and moisture transfer at the oven cavity scale while electrodynamics at the smaller scale of the region around the food. The model was validated using calorimetric and temperature measurements and reproducibility of arcing. Time evolution of arcing, and factors affecting it, were identified by the model. In drier air, arcing occurs at a lower power level because the equilibrium between ionization and attachment is achieved at lower electric field magnitude than in moist air. Higher temperature leads to greater chances of arcing as higher temperature increases ionization more than it increases attachment. Small load of food also increases the chances of arcing. Sample geometry, orientation, and placement inside the oven are also significant factors. Conditions that lead to arcing were identified in terms of maximum electric field values and temperatures. These threshold conditions can be used to identify unsafe situations for domestic microwave ovens and other microwave devices for product and process designers. (C) 2018 Published by Elsevier Ltd.