A zone model for the developmeii~ of a fire in a single compartment with one opening is studied to obtain explicit formulae for the critical conditions for occurrence of flashover and its temporal characteristics. In addition to previous studies, generalisation is presented for walls of arbitrary thermal inertia. The correlation between the model and previous models is also examined. Under reasonable assumptions all models are shown to be described by the same mathematical problem. This suggests that the formulae obtained for critical conditions and temporal characteristics can be used for any of the models observed. Results are illustrated for an experimental fire box used in many experiments.