PFHD, E-stop + PFHD, Sentry + PFHD, Contactors = 4.3x10-8 + 4.9x10-9 + 4.9x10-9 + 2.47x10-8 = 7.75x10-8 PL ePLr=cInputOutputSafety function 3PLr=cSafety function 1B1 MKey5 (PL c)S1 E-stop (PL e)InputLogicPFHD, MKey5 + PFHD, Sentry + PFHD, Contactors = 1.14x10-6 + 4.9x10-9 + 2.47x10-8 = 1.17x10-6 PL cK2Sentry (PL e)Q1/Q2 Contactors (PL e)Q1/Q2 Contactors (PL e)OutputThe reason for not achieving more than PL c with Safety function 1 is that only one key switch is used per door, and a key switch is mechanically a Category 1 device. For e-stop devices though, a fault exclusion for the mechanical parts is allowed according to EN ISO 13849-2 if a maximum number of operations is considered. For this solution to reach a higher PL, EN ISO 14119 and ISO/TR 24119 need to be consulted.Step 3 - Calculate the safety functionsThe output subsystem that is composed of double moni-tored contactors has been calculated at 2.47x10-8. The safety functions are represented by block diagrams.Safety functions 1 and 2 are identical. Therefore, only safety function 1 is shown.Safety functions 3 and 4 are identical. Therefore, only safety function 3 is shown.* Monitoring of contactors with K2How safe is a mechanical switch? Mechanical switches have a tendency to break if misused.Manufacturer instructions must be followed, e.g. no excessive force or dirty environment. For interlocking switches in general EN ISO 14119 must be considered. It handles e.g. the possibility to defeat a switch and require-ments on key switches. Connecting key switches in series gives a significant risk of masking errors, as stated in the technical report ISO/TR 24119, which limits the maximum achievable DC depending on the number of frequently used doors connected in series.S1 E-stopK1Sentry SSR10S2 E-stopB2 Key switchQ1ContactorB1 Key switchK2Sentry SSR10K1Q2ContactorResultResultPL cPL e*LogicK1Sentry (PL e)LogicK2Sentry (PL e)1INTRODUCTION1– 17
