We propose a unified model for the three branches of the explosion limits of an H-2-O-2 mixture. By using different expressions for the ignition delay time at each limit, and by utilizing the Le-Chatelier rule, we obtain an analytical expression for the ignition delay time of the mixture. We show that by solving the time delay equation for a selected typical ignition delay time we obtain a single expression for the Z-shape explosion limits. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.