In practical outdoor water jet applications serious hydroblast injuries may occur arising from the delay of non-automatic emergency switch-off. The paper presents the experiments with simultaneous pressure measurement at the input and output ends of the pipe lines with the lengths 10 and 34 m, which may help to find the most important reason for such failures. The key problem was found to be the reaction time of the machinist. Implantation of the electrical switch off into the system may significantly reduce damage. (C) 2014 Elsevier Ltd. All rights reserved.