Digital snap-action automatic switch board firmware design for SIL4 railway applications

Starting from the '90s, the traditional analog devices deployed throughout the entire Italian Traditional Fundamental railway Network (RFT) are being gradually replaced in favor of digital devices. The benefits coming from this decision are multiple and among others: the smaller overall size with respect to traditional systems, the faster reaction in case of failure, the remote management and the distributed diagnostic. To ensure the same grade of reliability in terms of security though, it is necessary to use high-reliability SIL4 electronic equipment and verify the correct functioning of the system components by the means of continuous testing, securing the system after a failure occurs. Tekfer is a company belonging to the railway signalling sector, producing digital systems capable to substitute the traditional analog devices ensuring the same certified SIL4 security. In its upcoming products line there are some solutions designed to replace very widely used traditional components: this is the case of the snap-action automatic switch. In fact these switches can be used to safely toggle the power supply of several kind of devices such as railroad switches and level crossing barrier machines, while protecting the circuit. If a short circuit occurs or a current is detected to be out of thresholds, the system must safely interrupt the supply, leading the involved devices to a secure state. The security of these devices was originally granted by the intrinsic safety provided by the analog components but with the introduction of digital devices it is necessary to address the potential security issues coming from the added complexity. This thesis aims to illustrate the firmware design of a snap-action automatic switch board compliant with the SIL4 certification, starting from the specifications of an existing Tekfer product of the same family, proceeding with the subsequent software implementation and concluding with the results. The results given by the measurements performed on a hardware prototype of the board, give evidence of a fast response to failures as defined by the specifications.