OMICRON electronics' transient network simulation software NetSim allows testing of relays under close to realistic conditions using a numerical network model to simulate processes in an electric power network composed of voltage sources, lines, circuit breakers, fault instances and switching events. It is a user friendly simulation and testing tool that provides predefined network topologies with default data called test cases, such as two or three terminal lines and double circuit lines with mutual coupling. It can be used to simulate simple, simultaneous, evolving or cross-country faults of different types, locations and fault inception angles, as well as synchronous and asynchronous out of step conditions.