Emerson Mobrey Series - Proposed Proof-test Procedure; Suggested Proof-test; Proof-test Coverage

Proposed Proof-test Procedure
15
Proposed Proof-test Procedure
April 2017
Functional Safety Manual
M310, Rev BA
Appendix A Proposed Proof-test Procedure
A.1 Suggested proof-test
According to Section 7.4.3.2.2 (f) of the standard IEC 61508-2, proof-tests shall be undertaken to reveal
dangerous faults which are undetected by diagnostic tests. This means that it is necessary to specify how
dangerous undetected faults which have been noted during the Failure Modes, Effects, and Diagnostic
Analysis can be detected during proof-testing.
The suggested proof-tests (Table A-1 and Table A-2) consist of switch operation tests in-situ.
Table A-1. Suggested Proof-test (Low Level Alarm)
Table A-2. Suggested Proof-test (High Level Alarm)
A.2 Proof-test coverage
The proof-test coverage for the tests listed in the section “Suggested proof-test ” are in Table A-3.
Table A-3. Proof-test Coverage
Step Action
1 Inspect the accessible parts of the level switch for any leaks or damage
2 Bypass the safety function and take appropriate action to avoid a false trip
3
Disable any filling mechanism and drain the vessel to force the switch to the fail-safe state and
confirm that the Safe State was achieved and within the correct time. INDEPENDENT PRECAUTIONS
MUST BE TAKEN TO ENSURE THAT NO HAZARD CAN RESULT FROM THIS OPERATION.
4
Reinstate the filling mechanism so that the vessel refills and confirm that the normal operating state
of the switch was achieved.
5 Remove the safety function bypass and otherwise restore normal operation
Step Action
1 Inspect the accessible parts of the level switch for any leaks or damage
2 Bypass the safety function and take appropriate action to avoid a false trip
3
Disable any drain mechanism and fill the vessel to force the switch to the fail-safe state and confirm
that the Safe State was achieved and within the correct time. INDEPENDENT PRECAUTIONS MUST
BE TAKEN TO ENSURE THAT NO HAZARD CAN RESULT FROM THIS OPERATION.
4
Reinstate the drain mechanism so that the vessel refills and confirm that the normal operating state
of the switch was achieved
5 Remove the safety function bypass and otherwise restore normal operation
Safety function
Proof-test coverage
4-contact versions – types D and P 84%
6-contact versions – types D6, P6, H6, and B6 88%