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25 Proof testing should be end to end so far as is reasonably practicable including the detector
at the liquid interface and the final element. The test period should be determined by calculation
according to the historical failure rate for each component or the system and the probability of
failure on demand required to achieve the specified SIL. Records of test results, including faults
found and any repairs carried out, should be kept. Part 1 of BS EN 61511 provides appropriate
guidance on this issue.
26 Safety systems which operate only infrequently may remain dormant for long periods and may
suffer failures which are unrevealed. Proof testing is required to reveal such failures, exercise the
system and demonstrate that the system functions as intended.
Test coverage
27 A proof test or a number of tests should cover, where practicable, all dangerous failure
modes. The test interval will be that determined in the PFD calculations.
Part tests
28 A full function test should be carried out, where practicable. Where not practicable, and
more than one test is used to demonstrate the function operation, then there should be sufficient
overlap such that no parts of the function are not tested.
29 Proof tests (part or full) should be carried out before and after any calibration, corrective,
remedial or intrusive action carried out. For example, proof tests should be carried out before and
after maintenance.
Proof test method
30 This should be carried out, where practicable, using wetted process conditions to operate
the sensor. Where this is not practicable then a simulated test of the sensor (eg radar, vibronics
or radio frequency admittance) may be acceptable where it can be demonstrated that the wetted
contact cannot be prevented from operating the sensor on genuine high-level condition.
31 Final element (Isolation valves, pump) should be tripped for a full proof test.
32 Testing should cover the testing of any diagnostic features.
33 Further guidance is in the HSE research report CRR428 Principles for proof testing of safety
instrumented systems in the chemical industry.
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Documentation
34 The requirements of BS EN 61511 concerning documentation should be met in full for new
systems. For existing systems, the documentation requirements should be complied with as far as
is reasonably practicable.
Recommended data sources for SIL calculations
35 Where a company does not have their own failure data, paragraph 38 lists typical data
sources that could be used to establish the recommended parameter values for the SIL
calculation of SIFs and the architectures of the SISs.
36 Users should consider the effect of the installed and process environment on the data used.
37 Manufacturers’ reliability data can be used where it can be shown to be appropriate and the
type, duty and environment are similar to that specified.