According to ISO 13849-1:2015 (Safety of machinery, Safety related parts of control systems, General principles for design)1, a machine control system is defined as “[a] system which responds to input signals from parts of machine elements, operators, external control equipment or any combination of these and generates output signals causing the machine to behave in the intended manner.”Considering the sheer breadth of this remit, it is no surprise that the control systems applied to such machines may well deploy a combination of various technologies, each reflected in their associated components. It also follows that the risk involved in operating machinery covered by ISO 13849-1 will range from negligible to highly hazardous. In order to ensure that the level of confidence is proportionate to the potential hazards associated with failure, the standard categorizes safety-related parts of machine control systems (SRP/CS) according to the demands placed upon them (“Performance Level” or “PL”).ISO 13849-1 is one of two standards that are harmonized to the Machinery Directive2, with EN 620613(Safety of machinery, functional safety of safety-related electrical, electronic and programmable electronic control systems) covering similar ground. A third standard IEC/ISO 17305 to merge the two is yet to be published. In the meantime, ISO 13849-1 suggests that SRP/CS designed to an appropriate level in any of the standards ISO 13849, IEC 62061 and IEC 61508 can be combined.