This standard also applies to actuators, although they are not included within the scope of IEC Publication 231 A. Its object is to lay down principles for testing such systems during normal power operation and shut down, so as to check complete availability especially with regard to the detection of unsafe faults. It covers the possibility of testing at short intervals or continuous surveillance, as well as periodic testing at longer intervals, to establish basic rules for the design and application of the test equipment and its interface with the protection system. Further, the effect of any test equipment failure on the reliability of the reactor protection is considered.