PL is categorised into five levels: PLa, PLb, PLc, PLd, and PLe. · PLa–b: Basic Risk (Small automated equipment, simple switches) · PLc: Medium Risk (Assembly equipment, packaging machines) · PLd: High Risk (Robot units, punch presses)This chart categorizes different voltage levels and their associated risks, guiding workers and safety professionals in selecting the appropriate level of protection. Higher voltage levels generally correspond to higher energy releases during an arc event, which means more extensive PPE is. Determining the Required Performance Level (PLr) is a fundamental step in ensuring functional safety and reducing machine-related risks to an acceptable level. The process, as defined by ISO 13849-1, should be conducted early in the machine design phase, when safety functions are being specified. Introduction, Need for power system protection, effects of faults, evolution of protective relays, zones of protection, primary and backup protection, essential qualities of protection, classification of protective relays and schemes, current transformers, potential transformers, basic relay. Important transmission lines and generators have cubicles dedicated to protection, with many individual electromechanical devices, or one or two microprocessor relays. This happens because the main function of protection devices is related to operation under fault conditions so these devices cannot be tested under normal operating conditions.