# Cybersecurity Requirements for Industrial Machine Control Systems
## 4 Relationship Between Safety Categories and PLs and Security Levels
With the assumption that machine control systems are connected to the communications network and may be vulnerable to cyber-attacks, the following guidelines were proposed for the PLr column with the parameter O = 100% (O--the probability of occurrence of a hazardous event):  
Category B allows us to obtain safety levels PLa or PLb. An SF with PLa/b level reduces the risk with a low level (S1, F1, P1/P2; S1, F2, P1). Category B can be realized as a one-channel electromechanical or programmable system, for example, based on a standard PLC. In the case of a system built based on electromechanical parts, there is no possibility of a cyber-attack, so SL-T 0 (no protection) is assumed. In the case of a system built based on programmable components (e.g., a standard PLC), there is a possibility of a cyber-attack, but the risk minimized by the function is low (at the PLa/b level), so SL-T 1 (low level) is assumed. Category 1 achieves safety levels of PLb or PLc and is mainly applied at the PLc level. An SF with PLc level reduces the risk with a medium level (S1, F2, P2; S2, F1, P1). Category 1 is implemented as a one-channel system based on well-tried components, which according to ISO 13849-2 [25] includes electromechanical parts. Programmable electronic circuits are not used, as they are not classified as well-tried ones. In the case of a system built based on well-tried components (electromechanical components), there is no possibility of a cyber-attack, so SL-T 0 (no protection) is assumed. Category 2 achieves safety levels from PLa to PLd, with the most common use for the PLc level, and usage is less common for PLd level. An SF with a PLc level reduces risk with a medium level (S1, F2, P2; S2, F1, P1), while one with a PLd level reduces risk with a higher level (S2, F1, P2; S2, F2, P1). Category 2 is implemented as a one-channel electronic system with monitoring, so there may be a loss of an SF between testing and the result of not detecting all faults. Category 2 is typically used for safety systems containing a type 2 light curtain according to IEC 61496-1 [26], which is tested by a programmable module. In the case of a system built with programmable elements, there is the possibility of a cyber-attack. When the minimized risk is low, which corresponds to PLa/b, then SL-T 1  
Figure 6: Auxiliary graph for determining the relationship between PL/Cat and SL—determination of the achieved PL, among others, using categories (own elaboration). Figure 7: Auxiliary graph for determining the relationship between PL/Cat and SL—parameters S, F, and P for determining the PLr (own elaboration). (low level) is assumed. When the minimized risk is medium (at PLc level) and high (at PLd level), SL-T 2 (medium level) and SL-T 3 (high level) are used, respectively. Category 3 allows for safety levels from PLa to PLe, with the most common use for the PLd level. An SF with a PLd level reduces risk with a high level (S2, F1, P2; S2, F2, P1). Category 3 is implemented as a redundant two-channel system with low- or medium-level diagnostics, so there may be a loss of SF as a result of not detecting all faults. Category 3 is often implemented from two different channels, namely programmable (stopping machine actuators) and electromechanical (cutting off electrical/pneumatic/hydraulic power). However, its typical application is also inverter drives with an STO (safe torque off--see ISO 13850 [27]) stop function, characterized by PLd and category 3. For systems realizing PLb/c levels and built on the basis of one programmable channel and another electromechanical channel, SL-T 2 (medium level) is adopted. For high-risk minimization (at PLd level), SL-T 3 (high level) is adopted. Category 4 allows for PLe levels of safety. An SF with a PLe level reduces the risk with the highest level (S2, F2, P2). Category 4 is implemented as a two-channel system with high-level fault detection diagnostics. High diagnostics are provided by programmable electronic systems.