Verification and validation 487
Proof test intervals during operation
Proof tests are intended to ensure that the safety integrity of a safety system is
maintained continuously and does not deteriorate over time. Proof tests are often
required for mechanical brakes, for example. Proof tests are used mainly for parts of
the system that cannot be automatically diagnosed.
The proof test interval is the interval between two proof tests. When the proof test
interval has elapsed, the safety system has to be tested and restored to an "as new
condition". The proof test must also be part of the regular maintenance plan.
For some of the components (electronics), the proof test interval is the same as the
expected life time of the system.
A specific safety calculator software can assist in determining the requirements for
the proof tests.
Residual risks
The safety functions are used to reduce the recognized hazardous conditions. In
spite of this, it is not always possible to eliminate all potential hazards. Therefore the
warnings for the residual risks must be given to the operators.
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