The paper discusses in detail the requirements for functional testing of devices and distributed functions used in SIPS. The methods for testing of such systems are proposed based on the testing of individual IEDs used in the system, distributed functions within a substation and between substations, complete SIPS and End-to-end testing of SIPS. Methods for the testing of complex SIPS, such as white and black box testing, top-down and bottom up testing are discussed. Solutions for the testing of the individual devices, as well as for the testing of distributed applications are described in detail.