Model-based analysis of an automated changeover switching unit for a busbar: MODSAFE 2009 work report

Verification of digital instrumentation and control (I&C) systems is challenging, because programmable logic controllers enable complicated control functions and the state spaces (number of distinct values of inputs, outputs, and internal memory) of the designs become easily too large for comprehensive manual inspection. Model checking is a promising formal method that can be used for verifying the correctness of system designs. A number of efficient model checking systems are available offering analysis tools that are able to determine automatically whether a given state machine model satisfies the desired safety properties. Model checking can also handle delays and other time-related operations, which are crucial in safety I&C systems and challenging to design and verify. The system analysed in this research project is called "automated changeover switching unit for a busbar" and its purpose is to switch the power feed to stand-by power supply in the event of voltage breaks. The system is modelled as a finite state machine and some of its key properties are verified with the NuSMV model checking tool. The time-dependent components are modelled to operate in discrete fixed-length time steps and the lengths of the timed functions are scaled to avoid state explosion and enable efficient model checking.