In this paper, an analysis of heat transfer in a porcelain-housed ZnO surge arrester is presented. Due to the fact that ZnO based surge arresters are key components for protection and reliability of electrical systems, caution must be taken regarding definition of assessment criteria. Thus, it is necessary to seek techniques to assure correct assessment and prevent malfunction. In porcelain-housed arresters, the air gap leads to an increased thermal resistance between ambient and the varistors column. Consequently, a thermal analysis must contain convection and radiation heat transfer mechanisms, besides heat conduction. This study proposes the use of computational simulations in combination with thermography as a tool for a temperature based estimative of the varistors’ state. For the analysis, thermography measurements were performed in a 69-kV ceramic-housed arrester subject to a thermal cycle and the results were compared with simulations. Furthermore, computational simulations of heat transfer in a ZnO surge arrester subject to impulsive energy income conditions. The simulations allow to obtain a relationship between varistors temperature and housing temperature regarding a given energy dissipated. The obtained results show that the employed technique can be an effective method for arresters monitoring and may allow to define assessing criteria for a porcelain-housed arrester based on housing temperature measurements.