Lays down principles for testing IC systems performing category A, B and C functions, per IEC 61226, during normal power operation and shutdown, so as to check the functional availability especially with regard to the detection of faults that could prevent the proper operation of the functions important to safety. Covers the possibility of testing at short intervals or continuous surveillance, as well as periodic testing at longer intervals. It also establishes basic rules for the design and application of the test equipment and its interface with the systems important to safety. The main change with respect to the previous edition includes an extension of the scope to cover all systems important to safety, and a requirement gradation for systems and equipment performing category A, B and C functions.