Programmable Safety Controller SCR P
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Figure 7: SCR P Input and output terminal locations
6.3.1 Safety Circuit Integrity and ISO 13849-1 Safety Circuit Principles
Safety 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 its
associated 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-tried
components, 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 B11.19
Performance Criteria for Safeguarding and ISO 13849-1 Safety-Related Parts of a Control System.
Safety Circuit Integrity Levels
Safety 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 required performance
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 Reliability
typically incorporates redundant control and self-checking circuitry and has been loosely equated to ISO 13849-1 Category
3 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 or ISO
12100). The risk assessment must be performed to determine the appropriate safety circuit integrity in order to ensure that
the expected risk reduction is achieved. This risk assessment must take into account all local regulations and relevant
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 IEC
62061) interfacing/hookup. The actual safety circuit integrity level is dependent on the configuration, proper installation of
external circuitry, and the type and installation of the safety input devices. The user is responsible for the determination of
the 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 solutions
are possible depending on fault exclusion and the risk assessment. The table below shows the input device circuits and the
safety category level that is possible if all of the fault detection and fault exclusion requirements are met.
WARNING: Input Device and Safety Integrity
The Safety Controller can monitor many different safety input devices. The user must conduct a Risk
Assessment of the guarding application to determine what Safety Integrity Level needs to be reached in
order to know how to properly connect the input devices to the Safety Controller. The user must also
take steps to eliminate or minimize possible input signal faults/failures that may result in the loss of the
safety functions.
WARNING: Risk Assessment
The level of safety circuit integrity can be greatly affected by the design and installation of the safety
devices and the means of interfacing of those devices.
A risk assessment must be performed to
determine the appropriate level of safety circuit integrity to ensure the expected risk reduction is
achieved and all relevant regulations and standards are complied with.
