Last edit: 11/01/2024
THE DOUBT
Why are well-tried components important in Machinery Safety?
CONSIDERATIONS
The concept is defined in both ISO 13849-1 and IEC 62061. Those types of components are compulsory for Category 1 (ISO 13849-1) and in both Basic Subsystem Architecture A and B (IEC 62061). In all other Categories and Architectures non well-tried components can be used, provided they have Reliability data.
Well-tried components are needed in those cases since the Safety-related Control System has a single channel and no Diagnostic. Here the definition.
[ISO 13849-1] 3.1 Terms and definitions
3.1.50 well-tried component. component-successfully used in safety-related applications
A well-tried component for safety-related applications is a component which shall be
- Either widely used in the past with documented successful results in similar applications. This aspects opens the door to the “proven in use” concept of IEC 61508-2.
- Listed in the informative annexes A to D of ISO 13849-2:2012, or
- Made, verified and validated using principles which demonstrate its suitability and reliability for safety-related applications according to relevant product and application standards.
It remains as a key guideline that a particular component can be considered as well-tried only if suitable for the specific application, considering for example the environmental influences.
A switch with positive mode actuation is a well-tried component if it complies with IEC 60947-5-1, Annex K. That means if a manufacturer has a new switch in his product range and it complies with the above standard, it is a well-tried component from the first unit produced. However, if a manufacturer has a new component, whose category is not mentioned in the annexes of ISO 13849-2 and he wants to be defined as well-tried, he needs to use it extensively first.
Despite opening the door to the Proven in Use concept, both standards make it clear that complex electronic components (e.g. PLC, microprocessor, application-specific integrated circuit) cannot be considered as equivalent to “well-tried”. For all the above reasons, the concept of well-tried components is not the same as Proven in Use according to IEC 61508 or Prior Use according IEC 61511-1.
There are PLC manufacturers that have input, and output Digital modules with reliability data, but only if used in a redundant (1oo2) configuration. The reason is that in a single configuration it would be a 1oo1 safety subsystem or Basic subsystem Architecture A according to IEC 62061. In which case, only well-tried components can be used: again, complex electronics cannot be considered well-tried.
Examples of well-tried components are:
- Power contactors built in conformity to IEC 609474-1
- Auxiliary contactors built in conformity to EN 50205 or IEC 609475-1 and IEC 60947-4-1:2001, Annex F
- Relay but only built in conformity to IEC 618103
- Pressure switch built in conformity to IEC 609475-1 for the electrical part
Conclusions
Well-tried components are compulsory to be used in single channel safety systems without diagnostics. They are “simple” components having predictable failure modes. Complex electronics, as well as Inductances, Resistors, Capacitors, Discrete semiconductors and Optocouplers, cannot be considered well-tried components.