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6.3.1 Safety Circuit Integrity and ISO 13849-1 Safety Circuit PrinciplesSafety circuits involve the safety-related functions of a machine that minimize the level of risk of harm. These safety-related functions can prevent initiation, or they can stop or remove a hazard. The failure of a safety-related function or itsassociated safety circuit usually results in an increased risk of harm.The integrity of a safety circuit depends on several factors, including fault tolerance, risk reduction, reliable and well-triedcomponents, well-tried safety principles, and other design considerations.Depending on the level of risk associated with the machine or its operation, an appropriate level of safety circuit integrity(performance) must be incorporated into its design. Standards that detail safety performance levels include ANSI B11.19Performance Criteria for Safeguarding and ISO 13849-1 Safety-Related Parts of a Control System.Safety Circuit Integrity LevelsSafety circuits in International and European standards have been segmented into Categories and Performance Levels,depending on their ability to maintain their integrity in the event of a failure and the statistical likelihood of that failure.ISO 13849-1 details safety circuit integrity by describing circuit architecture/structure (Categories) and the requiredperformance level (PL) of safety functions under foreseeable conditions.In the United States, the typical level of safety circuit integrity has been called "Control Reliability". Control Reliabilitytypically incorporates redundant control and self-checking circuitry and has been loosely equated to ISO 13849-1 Category3 or 4 and/or Performance Level “d” or “e” (see ANSI B11.19).Perform a risk assessment to ensure appropriate application, interfacing/hookup, and risk reduction (see ANSI B11.0 orISO 12100). The risk assessment must be performed to determine the appropriate safety circuit integrity in order toensure that the expected risk reduction is achieved. This risk assessment must take into account all local regulations andrelevant standards, such as U.S. Control Reliability or European "C" level standards.The safety controller inputs support up to Category 4 PL e (ISO 13849-1) and Safety Integrity Level 3 (IEC 61508 and IEC62061) interfacing/hookup. The actual safety circuit integrity level is dependent on the configuration, proper installation ofexternal circuitry, and the type and installation of the safety input devices. The user is responsible for the determination ofthe overall safety rating(s) and full compliance with all applicable regulations and standards.The following sections deal only with Category 2, Category 3, and Category 4 applications, as described in ISO 13849-1.The input device circuits shown in the table below are commonly used in safeguarding applications, though other solutionsare possible depending on fault exclusion and the risk assessment. The table below shows the input device circuits and thesafety category level that is possible if all of the fault detection and fault exclusion requirements are met.WARNING: Risk AssesmentThe level of safety circuit integrity can be greatly affected by the design and installation of the safetydevices and the means of interfacing of those devices. A risk assessment must be performed todetermine the appropriate level of safety circuit integrity to ensure the expected riskreduction is achieved and all relevant regulations and standards are complied with.WARNING: Input Devices with dual contact inputs using 2 or 3 terminalsDetection of a short between two input channels (contact inputs, but not complementary contacts) isnot possible, if the two contacts are closed. A short can be detected when the input is in the Stop statefor at least 2 seconds (see the INx & IOx input terminals Tip in Safety Input Device Function onpage 71).WARNING: Category 2 or 3 Input ShortsDetection of a short between two input channels (contact inputs, but not complementarycontacts), if they are supplied through the same source (for example, the same terminal from theController in a dual-channel, 3-terminal hookup, or from an external 24 V supply) is not possible, if thetwo contacts are closed.Such a short can be detected only when both of the contacts are open and the short ispresent for at least 2 seconds.XS/SC26-2 Safety Controller68