Last edit: 17/05/2023
Components used in a Safety Function can be classified as Type A or Type B.
According to IEC 61508-2, a component can be regarded as type A if
- the failure modes of all constituent components are well defined; and
- the behaviour of the element under fault conditions can be completely determined; and
- there is sufficient dependable failure data to show that the claimed rates of failure for detected and undetected dangerous failures are met.
Electromechanical interlocking devices are examples of Type A component.
A component can be regarded as type B if:
- the failure mode of at least one constituent component is not well defined; or
- the behaviour of the element under fault conditions cannot be completely determined; or
- there is insufficient dependable failure data to support claims for rates of failure for detected and undetected dangerous failures.
A 4-20 mA transmitter is normally a Type B component.