Type A and Type B Components

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.