In this work, we build circuit models to understand the physics of electro-thermal instability and associated thermal runway in advanced ESD protection devices under filamentation. The circuit building methodology takes into account, the instability arising out of inhomogeneous triggering of 2 − D planar bipolar and looks into inherent instability causing the 3 − D phenomenon. Subsequently, the electro-thermal coupling is analyzed to get SPICE model, as we develop a physics based methodology to comprehend the 3 − D localization in the device. Furthermore, we understand the instability through appropriate modeling of localization behavior using area factor (α) and related electro-thermal circuit models. © 2017 Elsevier Ltd