SUMMARYAn approach to modelling of electrical and thermo‐physical properties in small cylindrical volumes of cable insulation, that is, insulation spot samples is presented in this paper. Apart from small leakage currents, high currents can flow through these spots under fault conditions, at the same time heating up and destroying the cable insulation in them as well. An introduction of models for electrical and thermo‐physical properties of the insulation to the nonlinear thermal finite element method (FEM) analysis of faults is demonstrated on the examples of short circuits that occurred in buried power cables. Nonlinearity of the thermal FEM model is based on the heat sources, electrical and thermo‐physical properties of polyvinyl‐chloride and cross‐linked polyethylene insulation as functions of temperature and/or time. According to the DIN VDE standards, the cable structures considered herein correspond to the NAYY 4×120 SM 0.6/1 kV and NA2XY 4×120 SM 0.6/1‐kV cables. The proposed models for insulation properties are applicable to the fault thermal FEM analysis of all multi‐core power cables laid in the soils, air or water. Copyright © 2011 John Wiley & Sons, Ltd.