Functional Safety Architectures
An interactive guide to 1oo1, 1oo2, 2oo2, and 2oo3 systems. Click the "Fail" buttons to simulate a component failure and observe the system's reaction.
1oo1 (One-out-of-One)
The simplest architecture with no redundancy. Any single failure can lead to a loss of the safety function.
S1
PLC
OUT
System Normal
1oo2 (One-out-of-Two)
High safety. Either channel can trip the system. Tolerant to one channel failing open, but prone to spurious (unwanted) trips.
S1
PLC A
S2
PLC B
OUT
System Normal
2oo2 (Two-out-of-Two)
High availability. Both channels must agree to trip. Tolerant to a single spurious failure, but a single dangerous failure can be unsafe.
S1
PLC A
S2
PLC B
OUT
System Normal
2oo3 (Two-out-of-Three)
The "gold standard". Balances safety and availability. Tolerates one dangerous failure OR one spurious trip. Trips if two channels vote to trip.
S1
PLC A
S2
PLC B
S3
PLC C
Voter
OUT
System Normal