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Current injection into solar cells may occur if cells in modules are connected in parallel without the protection of string diodes. This current injection can cause heating of the cell, which increases the recombination currents and, thus, the current injection. In this manner, a self-feeding process called thermal runaway is started. In concentrator photovoltaics modules, thermal runaway can cause substantial damage. In this paper, a model is introduced that calculates the conditions causing thermal runaway. This model is based on the two-diode model. As inputs for the model, three dependences were experimentally determined from dark IV measurements on the triple-junction cell: 1) series resistance on cell temperature, 2) saturation currents for each of the three junctions on cell temperature, and 3) cell temperature on the injected current. The model was tested by comparing the simulated and measured temperature increase in a triple-junction cell and their dependencies on the applied voltage. A reasonable agreement between the experiment and model was found whereby the voltage at which a thermal runaway occurred differs slightly by 0.04 V. The model was then applied to estimate the dependence of an expected temperature increase in a shaded solar cell on the number of cells connected in parallel and on the concentration factor of the sunlight.