Retention containers of obligatory use for all the fuel storage installations with simple wall tanks with capacity higher than 1.000 litres, according to "Real Decreto 1523/99 of October 1st, 1999, modifying the "Reglamento de Instalaciones Petrolíferas" (Petroleum Installation Regulation) and "las Instrucciones Técnicas Complementarias MI-IP03 y MI-IP04" (MI-IP03 and MI-IP04 Technical Complementary Instructions).