Fail-safe protection Circuit for industrial safety applications

In industrial safety-critical applications, 24V digital line signals are often used, for example: Motor control, hydraulic/pneumatic valve control, and device control [1]. In this paper we present a fail-safe protection circuit which has the ability to cut-off the 24V supply feed of the output circuit (final elements). The Fail-Safe circuit has two input control signals and a single status-feedback signal which is used for control and monitoring purposes. The circuit test routine is applied at the system start-up or alternatively at the system shut down making the unpretentious functionality of the final elements. The feedback signal is read, and compared with the expected value. A safe condition is implemented by the control unit in case of detected fault by disabling both enable control signals, stopping the machine, sending an alarm signal, or sending an error signal to the main station. The failure mode and effect analysis has been calculated and the results show a probability of failure per hour (PFH) value of 8.5×10^-14 and a probability of failure on demand (PFD) of 9.37×10^-9, that allows us to use this design to achieve Safety Integrity Level (SIL) 4.