Proof test
Periodic test 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.The time for detecting the failure, planning tasks as well as
operating resources is also included. It should be reduced to a minimum.
MRT (Mean Repair Time)
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.The MRT should
preferably be small.
Device type (type A and
type B)
Actuator controls 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 behaviour 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 behaviour 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.
6
Part-turn actuators
Terminology with AC 01.2-SIL/ACExC 01.2-SIL
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