The SIP-1 apparatus (Sistema di Iniezione Passiva) was conceived, designed, numerically simulated and tested by the SIET company as an innovative depressurisation and make-up device for the New Generation LWRs. In particular it is suitable to cope with those accidents where pressure in the circuit must be dumped to allow low pressure injection systems to intervene. ENEA sponsored all the research activity, starting from the SIP-1 design, its numerical simulation with the Relap5 code, the realisation of an experimental facility up to the test execution and post-test calculations. An experimental campaign on the GEST-SIP1 facility was performed in July 2000. The facility is mainly constituted by an U-tube Steam Generator simulator (1:1 elevation and 1:50 volume scaling factors), utilised as pressure vessel, which a proper model of SIP-1 apparatus has been connected to. A series of LOCAs was simulated by varying the break size and location and different steady conditions were investigated to verify the stability of SIP-1, the lack of unexpected interventions and the actuation modalitie. This paper deals with the description of the GEST-SIP1 experimental facility, the SIP-1 operating principles, the most meaningful results of the tests and the capability of the Relap5 code in reproducing phenomena and events in the post-test analysis of a test.