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Application manual for PSRclassic and PSRmultifunction, and the modular safety relay system
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PHOENIX CONTACT 102597_en_03
2.2.5 Determination of the achieved PL for the overall safety
function
For subsystems with integrated diagnostic functions such as safety relay modules and
safety control systems, the achieved PFH
D
and PL are provided by the manufacturer with
the specification of the category.
For subsystems consisting of discrete components (e.g., switches, contactors, valves, etc.),
the PFH
D
value is determined from the category, DC, and MTTF
D
. For components that are
subject to wear, the MTTF
D
is determined based on the number of operating cycles using
the B10
D
value provided by the component manufacturer.
In addition, for category 2 or higher the effect of common cause failure (CCF) must also be
considered.
2.2.6 Verification of the achieved PL
Each individual subsystem and the entire safety chain must both meet the requirements of
the necessary PL
r
. This includes both the quantitative evaluation and the consideration of
systematic aspects, such as proven components and safety principles.
The systematic aspects include:
– Correct dimensioning of components
– Consideration of expected operating conditions and ambient conditions
– Use of basic and proven safety principles
– Avoidance of specification errors and software errors through testing
2.2.7 Validation
The last step should check whether the selected measures achieve the necessary risk
reduction and therefore, the protection objectives of the risk assessment. The result of the
validation process is included in the final risk assessment.
The purpose of the validation process is to confirm the specification and level of conformity
of the design of safety-related parts of the control system (SRP/CS) within the overall spec-
ifications for the safety requirements of the machinery. Before validation of the design of the
SRP/CS or the combination of SRP/CS that contains the safety function, the specification
requirement for the safety function must be confirmed. Validation involves performing anal-
ysis and function tests under normal conditions in accordance with the validation plan.
EN ISO 13849-2 contains detailed requirements and describes the basic procedure for the
individual validation processes.