Allen-Bradley 1794 - Publication 1794-RM001 G-EN-P - December

Publication 1794-RM001G-EN-P - December 2011
SIL Policy 1-11
Table 1.4 shows an example of a PFD calculation for a safety loop involving
two DC input modules used in a 1oo2 configuration and a DC output module.
1794-IE4XOE2XT 4 Input/2 Output Analog Combo Module 11,800,802 8.47E-08 5.99E-10
1794-IE8XT 8 Input analog Module 14,041,000 7.12E-08 5.03E-10
1794-OE4XT 4 Output Analog Module 11,381,744 8.79E-08 6.22E-10
1794-IF2XOF2IXT 2 Input/2 Output Isolated Analog Combo Module 6,317,918 1.58E-07 1.13E-09
1794-IF4IXT 4 Isolated Input Analog Module 7,297,140 1.37E-07 9.75E-10
1794-IF4ICFXT 4 Isolated Input Analog Module 7,297,140 1.37E-07 9.75E-10
1794-OF4IXT 4 Isolated Output Analog Module 5,493,902 1.82E-07 1.30E-09
1794-IJ2XT 2 Ch. Frequency Counter Module 11,714,128 8.54E-08 6.04E-10
1794-IRT8XT 8 TC/RTD Input Analog Module 8,204,792 1.22E-07 8.66E-10
(1)
MTBF measured in hours.
(2)
Calculated using field-based values for components
(3)
λ = Failure Rate = 1/MTBF
Table 1.3 FLEX I/O Product Probability of Undetected Dangerous Failure per Hour (PFH) Calculations (T
1
= 1
yr
)
Catalog Number Description Mean Time
Between Failure
(MTBF)
(1)
λ
(3)
Calculated PFH
1oo2 architecture
Table 1.4
Catalog Number: Description: MTBF: Calculated
1oo2 PFD:
1794-ACNR15 ControlNet Dual Media
Adapter 1.5
3,259,605 1.56E-06
1794-IB16 24V DC Input Module 6,409,846 4.34E-06
1794-IB16 24V DC Input Module 6,409,846 4.34E-06
1794-OB16 24V DC Output Module 4,284,857 9.62E-06
1794-OW8 Relay Output Module 1,312,973 1.20E-06
1756-L63B
1
ControlLogix Controller 2,460,065 2.33E-04
1756-CNB ControlNet Bridge Module 3,596,087 1.15E-04
1756-CNB ControlNet Bridge Module 3,596,087 1.15E-04
Total PFD calculation for a safety loop consisting of these products: 3.70E-04
1 See Publication 1756-RM001
for more information.
AB Drives