The thermal safety of chemical processes requires knowledge of the safety parameters that quantify the probability, such as time to maximum rate under adiabatic conditions (TMRad), and the severity, such as adiabatic temperature rise under adiabatic conditions (ΔTad). The zero-order approximation is used to ease the determination of TMRad values at different process temperatures; but how can one be sure that this approximation is acceptable, compared to the use of an intrinsic kinetic model? In the literature, there are no such studies that compare the values of TMRad by using zero-order and intrinsic kinetic models. For that, decomposition of hydrogen peroxide in the presence (and in the absence) of copper sulfate was studied in an advanced reactive system screening tool (ARSST) unit. This calorimeter operates under near-adiabatic conditions, based on heat loss compensation principle, and by using a background heating rate (β). In a first stage, a kinetic model was built to estimate the intrinsic kinetic constants. Then, a comparison between the values of TMRad from the zero-order and the intrinsic kinetic model was performed. It was found that the difference of TMRad values obtained by these two models can be significant. The influence of β and reactant concentrations were found to play an important role in this difference. As good practice, in the case of missing kinetic and thermodynamic data, a user should test different background heating rates to verify their influence on TMRad values obtained from the zero-order model.