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A safety-related system can comprise stand-alone systems dedicated to perform a
particular safety function or can be integrated into a plant.
Periodic test is performed to detect dangerous hidden failures in a safety-related system so
that, if necessary, a repair can restore the system to an "as new" condition or as close as
practical to this condition.
MTTR (Mean Time To
Restoration)
Mean time to restoration once a failure has occurred, indicates the expected mean time to
achieve restoration of the system. It is therefore an important parameter for system
availability for the safety function. The time for detecting the failure, planning tasks, as well
as operating resources is also included. This should be reduced to a minimum.
Mean repair time indicates the mean time required to repair a system. The MRT is crucial
when defining the reliability and availability of a system for safety operation. The MRT should
preferably be small.
Device type (type A and
type B)
Actuator controls version can be regarded as type A devices if all of the following conditions
are met for all components required to achieve the safety instrumented function:
• The failure modes for all constituent components involved are well defined.
• The behavior under fault conditions can be completely determined.
• There is sufficient dependable failure data from the field to show that the claimed rates of
failure are met (confidence level min. 70%).
Actuator controls shall be regarded as type B devices if one or several of the following
conditions are met:
• The failure of at least one constituent component is not well defined.
• The fault behavior is not completely known.
• There is insufficient dependable failure data to support claims for rates of failure for
detected and undetected dangerous failures.
PTC (Proof Test Cover-
age)
Proof test coverage describes the fraction of failures which can be detected by means of a
proof test.
Multi-turn actuators
SA series with Control 3.XX /NI/SIL
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