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Introduction to Functional Safety of Control Systems
A joint report has been prepared within IEC and ISO to help users of both standards.
It explains the relationship between the two standards and explains how the equivalence can be drawn between PL (Performance level) of EN ISO 13849-1 and SIL (Safety Integrity Level) of IEC.EN 62061 both at system and subsystem level.
In order to show that both standards give equivalent results the report shows an example safety system calculated according to the methodologies of both standards.
The report also clarifies a number of issues that have been subject to different interpretations. Perhaps one of the most significant issues is the aspect of fault exclusion.
In general, where PLe is required for a safety function to be implemented by a safety-related control system it is not normal to rely upon fault exclusions alone to achieve this level of performance. This is dependent upon the technology used and the intended operating environment. Therefore it is essential that the designer takes additional care on the use of fault exclusions as the PL requirement increases.
In general the use of fault exclusions is not applicable to the mechanical aspects of electromechanical position switches and manually operated switches (e.g. an emergency stop device) in order to achieve PLe in the design of a safety-related control system. Those fault exclusions that can be applied to specific mechanical fault conditions (e.g. wear/corrosion, fracture) are described in Table A.4 of ISO 13849-2.
For example, a door interlocking system that has to achieve PLe will need to incorporate a minimum fault tolerance of 1 (e.g. two conventional mechanical position switches) in order to achieve this level of performance since it is not normally justifiable to exclude faults, such as, broken switch actuators. However, it may be acceptable to exclude faults, such as short circuit of wiring within a control panel designed in accordance with relevant standards.