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iv) Safety function will not work in
certain circumstances
General group:
•
Maintenance override has been configured when not permitted.
•
The precedence of two sets of two competing sets of logic incorrect.
•
Spurious setting of overrides or modes on logic solver start-up.
•
The second pulsed output to generate a general plant alarm will not work
•
Fire zone inhibits defeating the wrong signals.
•
Extraneous logic connections degrading primary SIF
Counter-measures:
•
To explicitly prohibit overrides - connect override enable an “override
prohibited” tag
•
Multiple pulses – develop a standard software function block.
•
Fire zone inhibits programmed to be self-revealing.
Neil Wakeling, August 2014