Voting (1oo2, 2oo3)

Instrumentation & Control

How many of several devices must agree before a trip acts, written M-out-of-N. It trades dangerous failures (not tripping when you should) against spurious ones (tripping when you need not). 2oo3 keeps both low, which is why it is the usual answer at SIL 2 and 3.

Explained in depth

Safety instrumented systems and SIL — the layer that acts on its own

Related terms

Cause and effect chart
SIF
SIL
SIS
4–20 mA
Alarm flood
Cv
Direct and reverse acting
Ex i
Fast loop
H and HH
Hook-up drawing