Numerous researches have dealt with public debate practices, specially on environmental issues. But few of them have studied the effects of these debates on relations and tensions between decision makers. Now, public debates in certain circumstances, may introduce conflicts and, paradoxically be used to overcome these lasts. It will be shown how PPRTs (Plans of Prevention of Technological Risks), following the Bachelot Act of July 2003, generate tensions between state and local public authorities because of two kinds of discussions, relating to, on the one side, environment and health protection and, on the other side, particular interests. In order to manage these tensions, public debates are considered by central and local authorities as a means whose control is at stake. In other terms, the test of public legitimization appears to be both a trouble maker of the state regulation process and a way of managing this perturbation. Thus the first goal of the PPRTs: reducing risks at source, has been progressively transformed into implementing protections against catastrophes.