User manual
13
Functional Safety Manual
IP101/FSM, Rev AA
January 2012
Horizontal Float Switch
PROPOSED
PROOF-TEST
PROCEDURE
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.
Suggested Proof-test The suggested proof-tests (Tables 7 and 8) consist of switch operation tests in-situ.
Table 7.
Suggested Proof-test
(Low Level Alarm)
Table 8.
Suggested Proof-test
(High Level Alarm)
Proof Test Coverage The Proof Test Coverage for the tests listed in the section “Proposed Proof-test
Procedure” may be considered to be 100%, covering all components of the Mobrey
level switch.
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